- Protect Your Trees From Freeze
- Trees most susceptible to damage:
- Acute action before freezing weather:
- Advanced planning for freeze
- Action after a freeze
- Jan 25, 2013Protecting your fruit from frost and freeze
- How to protect blooming trees from hard-freeze
- Strawberry Plants And Frost: How Do You Protect Strawberry Plants In Cold
- Strawberry Plants and Frost
- How Do You Protect Strawberry Plants from Frost?
- How to Protect Fruit Trees from Frost
- Understand Critical Temperatures for Blueberry Plants
- Use Row Covers and Nursery Foam to Combat the Cold
- Invest in Wind Machines to Properly Protect Plants
- Turn to Sprinkler Irrigation for Increased Protection
- Don’t Forget to Protect Blueberry Crops from Birds with Avian Control!
- Blueberry Winter Damage: Care Of Blueberries In Winter
- Care of Blueberries in Winter
- Blueberry Bush Winter Care
Protect Your Trees From Freeze
Temperatures below 32 degrees over a sustained period of time are cold enough to freeze your trees’ buds/blossoms, fruit, leaves, and/or twigs.
Trees most susceptible to damage:
Citrus, Jacaranda, Catalpa, Oleander, Eugenia, and other tropical/sub-tropical plants are most likely to sustain damage. Tender, new growth is also easily injured by freezing temperatures.
Acute action before freezing weather:
Protect your trees and plants:
- Cover susceptible trees and plants with burlap, sheets, tarps, etc., that extend to the ground to trap in the earth’s accumulated warmth. Use a frame or stakes to minimize contact between the cover and the foliage.
- Bring potted plants and trees to more protected locations.
Keep plants well-watered:
- Moist soil will absorb more solar radiation than dry soil, and will re-radiate heat during the night.
- If you have a large tree that needs protection, running sprinklers at the coldest time of the day (usually between 4:00AM and 6:00AM) can give it a slight edge. The strategy makes use of latent heat released when water changes from liquid to a solid. When ice crystals form on the leaf surface they draw moisture from the leaf tissue. The damage from this dehydration will be less severe if the plant is not already drought-stressed.
Advanced planning for freeze
- Remove turf/weeds from under trees’ canopies—bare soil absorbs and reflects heat best.
- Wood chip mulch prevents soil moisture loss and insulates roots.
- Plant frost-sensitive plants near sources of reflective heat (like buildings, walls, etc).
Action after a freeze
Help trees recover:
- Do not prune anything off immediately. Wait to see what sprouts in the spring; the damage is often not nearly as bad as it initially looks, and new growth may come out of tissue that you thought was dead.
- If dieback is severe enough and your tree has lost “shade,” protect the now-unshaded portions of the trunk/branches from the sun, with a physical cover or with whitewash (1:1 ratio of latex paint and water).
- Remove frosted/mushy fruit while still salvageable, for snacking on or juicing.
Jan 25, 2013Protecting your fruit from frost and freeze
After the damaging frosts and freezes many endured in 2012, growers are looking at frost prevention methods coming into 2013. This led to a packed house at the 2012 Great Lakes Fruit, Vegetable & Farm Market EXPO in Grand Rapids, Mich., for a session covering weather risk management.
There are a few things growers need to know to effectively initiate frost protection. One thing is the temperature at which catastrophic damage will occur. This is different for different types of crops, but is usually between 22˚ F and 32˚ F.
Another is the location of the inversion layer. Cold air sinks below surrounding air, and when a frost event occurs there is a layer of cold air hugging the ground. The inversion layer is the point where the coldest air at ground level meets warmer air above it, much like a fog bank. Knowing where the inversion layer typically forms is a vital part of protecting crops from frost, Evans said. This can typically be achieved with weather monitoring equipment.
According to Robert Evans, a research agricultural engineer with USDA’s Northern Plains Ag Research Lab in Sidney, Mont., there are two basic types of frost avoidance strategies growers can incorporate: passive and active.
Passive frost protection can minimize risk, decrease the probability or severity of frosts and freezes, or cause the plant to be less susceptible to cold injury. These practices include site selection, variety selection and multiple cultural practices, Evans said.
“The best time to protect a crop from frost is before it is planted, and good site selection is the most effective passive risk-avoidance strategy,” he said.
Good site selection for frost protection includes air drainage. The availability of natural heat sources, such as large water bodies and rivers, can help make the site a good choice. Quite often, the most limiting factor is the price of land, Evans said.
“Obtaining a good site with good air drainage, especially in a premier growing area, can be very expensive, but it is often an investment with a very high rate of return.”
Active frost protection is getting a lot of attention now, Evans said. Active management comes in three basic areas: the addition of heat, the mixing of warmer air from the inversion layer under radiation frost conditions, or the conservation of heat from the plant. The amount of time, energy and money spent on one of these techniques, however, doesn’t always equal the reward, Evans said.
The use of sprinklers or heaters adds heat.
Over-tree sprinkler systems can provide the highest level of protection, Evans said. The key to using water is to continually use it to form clear ice. Clear ice means that an endothermic reaction is taking place and the warmth of the plant is being trapped inside it. If the ice starts to become cloudy, the plant is losing heat and damage can occur. Spraying water must continue the entire time the freeze event is taking place, and the sprays must keep going from before there is a freeze event that would damage the fruit until the ice is completely melted from the tree after the event, Evans said. If the water stops spraying on the clear ice, it goes from being endothermic to exothermic, and the heat loss and ice will damage the fruit.
“Because of the large amounts of water required for over-tree sprinkling for frost protection, some growers have attempted to spread overhead water applications by wide spacing of sprinkler heads, cycling of water applications on and off, or misting techniques to reduce the total water supply needs across a block,” Evans said. “However, these techniques do not apply adequate water directly to the plant canopy to account for evaporation, and these systems are not recommended because of the high level of risk.”
Under-tree sprinkler success is influenced by how strong the temperature inversion is and where it is located in the air column, the amount and temperature of the water applied, the volume of air flow through the orchard, the release of latent heat from the freezing of the applied water and the radiant heat from the soil, Evans said. The air flowing through the orchard can be affected by fans and other measures and can actually rob heat from the water and trees, Evans said. The two systems work very well with each other, as under-tree sprinklers are less likely to have massive heat losses due to air movement.
Heaters placed around the orchard are another way to add heat. Heat guns, small gas-powered heaters or even the burning of organic materials such as wood or hay have been used for some time with varied success, Evans said. Using a heater alone, however, is one of the least effective frost prevention methods. This is because heat rises, and heaters can’t cover a large area.
Evans said it is more efficient to use many smaller heaters instead of large, central heat sources. Contrary to belief, clouds of smoke offer no protection whatsoever from frost. Still, it takes anywhere from 40 to 60 heaters per acre to survive a frost event. If heaters are used, Evans suggests adding other elements, such as sprinklers or fans.
Frost fans get air from the inversion layer down into and mixing with the freezing air at ground level. They have gained in popularity lately, because they can be used at any time and are energy efficient, Evans said. Frost fans, on average, cover 10 to 13 acres per fan.
Amy Irish-Brown, an Extension educator with Michigan State University (MSU), said that in the past year she has seen the number of permanent frost fans in her area increase by a factor of 10 or more.
“Where there used to just be a few, now they are going up everywhere,” she said. “They aren’t cheap, running anywhere from $16,000 to $35,000 each, but for many growers it beats the alternative.”
The problem with fans, Evans said, is that if the inversion layer is higher than the height of the fan, it can’t pull down the warmer air and is ineffective. Site selection is a key element when placing a fan. Portable fans can be used effectively too, he said, especially when used with other methods like sprinklers.
Another method of frost prevention is using helicopters to hover over the orchard during a freeze event. The rotors pull the air down from the inversion, Evans said.
The use of helicopters is a last resort kind of measure, Irish-Brown said. The cost is extremely prohibitive and there are many safety and noise factors to take into account. Renting a helicopter can run an average of $1,600 per hour, she said. They can cover a large area, up to 40 acres per helicopter, but the cost can quickly reach astronomical levels for a single use.
Perhaps most importantly, growers need to know the weather conditions and pay careful attention to what’s going on outside, said Beth Bishop, the coordinator of MSU’s Enviro-weather program.
“Minimizing weather risk requires knowledge,” Bishop said. “Growers need to know the current weather conditions and have an idea of what they will be in the near future. Growers must also understand the cumulative effects of previous weather conditions.”
For more information about Enviro-weather, visit www.enviroweather.msu.edu.
For more on what happened during the spring of 2012, .
By Derrek Sigler, Assistant Editor
Tags: Crop Management, Equipment, Water
How to protect blooming trees from hard-freeze
Q. My peach tree is in full bloom and I just saw a weather report that hard frosts are expected next week. Will this damage my tree?
A. You are not alone. Many of our trees here at the college are in full bloom as well.
The 70-degree weather we had in January fooled many of the trees into thinking it was spring. It is not uncommon for us to get a killing frost while our fruit trees are in bloom. In the Redding area we can have hard frosts well into April and even later in the mountains and surrounding foothills.
Seasoned gardeners can all tell you horror stories about a late frost leveling their spring plantings. This is why early blooming trees, such as cherry and apricots, are unreliable producers of fruit in our area.
What is unusual about this year is that trees are blooming so early and we could easily have another couple of months were temperatures can get well below freezing. The freezing temperatures will defiantly damage the flowers on the tree and any potential fruit production for this year will be lost.
If the temperatures are low enough it may also cause damage to the leaf buds and branches as the tree is no longer dormant.
To protect the trees from the cold there are several things you can try depending on the number of trees you have, their size, location, and the severity and length of below-freezing temperatures.
You can build tripods out of light lumber or PVC pipe around each tree and cover them with frost cloth, blankets or tarps. I have used the same structure I use to support my bird netting to cover my tree with an old tarp. Do not use plastic or sheets as they do not provide enough insulation.
Another option is to hang the trees with the old-fashioned Christmas lights, however these are getting harder to find. They need to be the big bulb type, the newer LED Christmas-tree lights won’t work since they’re cool burning and won’t give off sufficient heat. You can improve the protection by also covering with a blanket or tarp.
Another option is to turn a sprinkler on your trees just as the freeze begins, to coat them with ice. Although it seems counterintuitive, the ice will protect the tree because the temperature beneath the ice will not drop below 32 degrees. For this method to work you will need to keep the sprinkler on until the temperatures rise above freezing. If you plan to use this method of frost protection and the cold temperatures last for a while, monitor to ensure that you are not creating a flood somewhere else.
You may have noticed that some of the big orchards and vineyards use big fans to create air turbulence to protect trees from freezing. I do not like to recommend use of the fan for residential yards for a couple of reasons. First, most of you do not have a large enough fan to create enough air turbulence to be effective in protecting the tree. And if you do just happen to have a giant fan at your disposal you can cause increased damage to surrounding areas by moving the cold air into neighbor’s yard.
The Shasta Master Gardeners Program can be reached by phone at 242-2219 or email [email protected] The gardener office is staffed by volunteers trained by the University of California to answer gardeners’ questions using information based on scientific research.
Strawberry Plants And Frost: How Do You Protect Strawberry Plants In Cold
Strawberries are one of the first crops to make their appearance in spring. Because they are such early birds, frost damage on strawberries is a very real threat. Strawberry plants and frost are fine when the plant is dormant during the winter, but a sudden spring frost when the plants are blooming can wreak havoc on the berry patch. Protecting strawberry plants from frost is of paramount importance, but HOW do you protect strawberry plants?
Strawberry Plants and Frost
Frost can decimate an entire berry crop, especially if the berries have been exposed to warming temperatures. A freeze following warm spring weather can be devastating. And strawberries are particularly susceptible to frost damage since they are often in bloom before the last frost free date.
Strawberry blossoms are most sensitive to frost right before and during opening. At this juncture, temperatures below 28 F. (-2 C.) will damage the blossoms, so some frost protection of strawberries is integral to the harvest. Frost protection of strawberries is less important when the flowers are still in tight clusters and just barely peaking from the crown; at
this point they will tolerate temps as low as 22 F. (-6 C.).
Once fruit begins to develop, temperatures below 26 F. (-3 C.) may be tolerated for very short periods, but the longer the freeze, the higher risk of injury. So, again, it’s important to be prepared to protect the plants from frost.
How Do You Protect Strawberry Plants from Frost?
Commercial farmers do a couple of things to protect the berries from frost and so can you. To protect them from winter temps, mulch over the strawberries in the fall to early winter with straw or pine needles. In the spring, move the mulch between the plants after the last frost. This will help retain soil moisture, retard weeds, and prevent dirty irrigation water from splashing on the fruit.
Overhead irrigation is another popular method for protecting strawberries plants from frost. It sounds crazy, but it works. Basically, the farmers are encasing their entire field in ice. The temperature of the ice remains at 32 F. (0 C.) because as the water becomes ice it releases heat. Since strawberries aren’t injured until the temperature falls below 28 F. (-2 C.), the berries are saved from frost injury. The water must be constantly applied to the plants, though. Too little water can cause more damage than if no water is applied at all.
Another interesting fact on protecting strawberries from frost is that soil retains heat during the day and is then released at night. Wet, thus dark soil, retains heat better than dry, light colored soil. So a wet bed serves yet another purpose.
Also, row covers can provide some protection. The temperature under a cover may equal that of the air, but this takes a while and may just buy the berries enough time. Water can also be applied directly over the row cover to protect the flowers inside with a layer of ice.
Where your berries are located can also provide them with some protection. Our strawberry patch is on the south side of a garage with a significant overhanging eave, which serves to protect the berries.
How to Protect Fruit Trees from Frost
How can I prevent a late spring frost from damaging our fruit crop?
You can take several fairly simple steps to reduce the risk of frost damage to buds, blossoms and fruit without using heaters, commercial wind machines or overhead sprinklers, according to the University of California, Davis’ article Principles of Frost Protection. First, before planting fruit trees of any kind, choose the location carefully. Avoid planting at the bottom of a slope — where frost accumulates — or on cold hilltops. If possible, plant on a north-facing slope to help delay blooming and thus avoid frost damage. The Alabama Cooperative Extension System (ACES) suggests checking seed catalog descriptions and choosing fruit varieties less susceptible to frost damage in order to find varieties that bud and bloom later, when frost is less likely to occur.
For existing fruit trees, ACES recommends putting off pruning until late winter to early spring to stall budding and blooming. If frost is in the forecast when trees are in bloom and the soil has been dry, water the soil a day or two beforehand to a depth of 1 foot (wet soils radiate more heat than dry soils do). To trap extra warmth, the UC Davis article says to cover the wet soil around the bases of the trees with clear plastic until the danger of frost has passed. If you’re using a cover crop, mow it and remove vegetative mulch (at least temporarily). Bare soil — or soil covered with clear plastic — stores and radiates more warmth.
ACES also notes that frost blankets can provide frost protection for fruit trees and small fruits. When you place frost blankets around tree trunks, be sure to anchor them on the ground to trap the soil’s radiant heat.
— Vicki Mattern, Contributing Editor
Vicki Mattern is a contributing editor for MOTHER EARTH NEWS magazine, book editor and freelance magazine writer. She has edited or co-authored seven books on gardening, and lives and works from her home in northwestern Montana. You can find Vicki on Google+.
When I woke up yesterday morning, it was misty. We’re approaching the middle of October, which is the usual time for the first frosts of autumn in my part of the UK. People in different areas are already reporting the arrival of the frosts on Twitter. This means it’s time for me to pop out into the garden and bring in my lemon tree (which I grew from a pip, several years ago). It has been enjoying the summer weather in the garden, but it’s only really hardy down to -10°C. I’ve nearly lost it a couple of times, and it has died right back to nothing, but somehow it always manages to come back. It’s time to think about which fruit trees are truly hardy, and which will need some help.
Lemons are one of the hardiest citrus fruits, and sadly none of them are really hardy in our climate. When you’re talking about frost and fruit trees and bushes, there are really two points at which you need to worry. Now is one of them – the first frosts will kill anything tender, and damage anything not fully hardy. It’s time to bring it indoors, move it into the greenhouse, or wrap it up nice and cozy in horticultural fleece for the winter.
Of course, there are fruits that are tastier after a frost (such as sloes), fruits that are fully hardy (such as sea buckthorn) and fruit trees that need a period of cold to develop properly (apples are one such example). Plants that go dormant in the winter, which includes most top fruit and soft fruit, are generally hardy – dormancy is their way of protecting themselves. For these plants it’s not the cold that will bother them, but wind rock damaging their roots, or problems with water logging or even drought (which can be an issue when the ground freezes solid).
What is frost? Frost happens on clear nights, when the temperature drops below 0°C (32°F). Cold air sinks to the ground, flows according to the local contours, and pools in any locations it can’t escape – creating frost pockets. As you get to know your garden you’ll find out where the frost pockets are, where the cold bites deepest and which areas take the longest to warm up again during the day. You may find that strategic gaps in fences and hedges will allow the frost to flow away from your garden, or discover that removing the lower foliage from some trees and bushes will do the same thing. Learn to plant smaller fruit trees and bushes on higher ground, where they will be lifted above the frost.
The second period of the year in which you need to worry about frost damage to fruit trees is in the spring, when late frosts can kill early blossom and wipe out an entire year’s fruit crop.
Look at the hardiness rating of fruit plants, the lowest temperature to which they are hardy. Plant anything susceptible in sunnier, sheltered spots. The secret weapon in walled gardens is the walls – not only do they create shelter, but they heat up during the day and radiate that heat at night. You can recreate that effect by planting fruit trees that bloom early in spring against south-facing walls.
Dwarf trees in pots can be moved indoors temporarily, or to a more sheltered spot. Soft fruit and small plants in the ground can be protected with cloches when frost is forecast. For larger plants it’s a case of horticultural fleece or hessian sacks. However you choose to protect plants, you need to uncover them during the day so that beneficial insects can pollinate the flowers for you and ensure a good crop of fruit. It’s easier to protect fruit trees with fleece/hessian if they’re trained up a wall or a fence, and almost impossible to do for for larger trees.
For larger fruit trees, water well when frost is forecast – wet soil radiates more heat upwards into the tree than dry soil. If you can, cover soil (temporarily) in clear plastic, to trap more heat during the day. Bare soil radiates more heat too, so clear away any weeds and ground cover plants, and pull back any mulches until the risk of frost has passed.
If you live in a cold spot then look for frost-tolerant fruit varieties, cultivars that flower later in the spring and should escape the frosts, and traditional varieties that have been grown in your area for decades/centuries. All reputable fruit tree suppliers, such as Pomona Fruits, have hardiness details in their plant descriptions, and will be happy to help customers choose the right varieties for their location.
Gardeners can protect fruit trees from all but the most unexpected of frosts – the freak weather conditions that happen occasionally. To ensure a fruit harvest during those conditions, you need to take a leaf out of Carol Deppe’s book, The Resilient Gardener, and plan for diversity. If you have the space you can plant several different varieties of each fruit, which will bloom at different times, so that one or two may fruit if the others take a year off. For most of us it’s more a case of not putting all of our fruits in one basket, and ensuring a diversity of different fruit types. So if the apple/plum/pear harvest doesn’t arrive this year than it’s sad but it’s not the end of our fruit hopes, because we’ve got strawberries/raspberries/quince or even sea buckthorn to look forward to!
What’s your strategy for protecting fruit trees from frost?
You can find more seasonal (and unseasonal!) gardening advice in my section on gardening basics 🙂
This post was produced in association with Pomona Fruits, but the words are mine, as are the frosty photos. Are you feeling cold yet? Where did you leave your sweater??? 😉
Frost damage to plants is the single biggest cause of economic loss to crops in the United States – far surpassing any other weather related issue. While frost doesn’t impact the home gardener on the same scale, it can be disheartening to lose plants in a sudden freeze.
Citrus, succulents, newly planted or tender perennials, and many tropical and subtropical plants all are vulnerable.
Frost damage occurs when the water inside the cells of a plant freeze, causing damage to the cellular walls, which degrades the overall health of the plant. Affected plants will wilt and in severe or prolong periods of frost, die.
Shoots, buds and flowers will wither and turn brown or black as if they have been scorched. Even bark can crack or split and die off.
Young, newly planted, specimens are especially vulnerable.
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We already have had several days of frost and freeze here in the Bay Area, and it looks like more is on the way. So, how do you protect your prized citrus, succulents, rhododendrons and azaleas?
If a severe freeze, or multiple days of below freezing weather is expected, water 2 to 3 days ahead of time. This will increase the soil’s ability to retain and give off heat.
Wrapping trunks of young trees with blankets, towels or piping insulation will provide added protection.
If you are doing container gardening and are able to pull pots into the garage, shed or other enclosed area, that would be ideal. Otherwise, move them up against the side of the house or garage, preferably beneath an overhang.
Stringing your plants with old-fashioned, incandescent Christmas lights –not LEDs — can be very helpful. Covering the lighted plants with frost cloth, sheets or blankets will add 4 to 8 degrees of protection, enough to keep most plants alive.
Frost cloth is lightweight enough to leave on for several days, however heavier covers should be removed each day once the temperature has warmed up, and then reapplied each night before sunset.
Make sure the cover goes all the way to ground in order to capture the radiant heat from the soil. Also, stake heavier covers so that the weight won’t break branches, damage the leaves or suffocate the plant. You can also use inverted boxes, buckets and plant pots.
Adding a 2- to 3-inch layer of mulch around your plants can also help, but be sure to stay several inches away from the trunk or stem; mulching too close will cause the plant to rot.
There are many varieties of foliar sprays available today that claim to protect against frost. Although recent field trials have shown no real harm in using them, they also show have found little to no actual protection from these products. You are better off using other methods to keep your plants safe.
Wait until all chance of frost has passed before trimming off any damaged or diseased branches. Pruning too soon can cause significantly more trauma, even death, to a young, vulnerable plant that might otherwise have recovered in the spring.
Rebecca Jepsen is a Santa Clara County Master Gardener.
- Learn about harmful temperatures
- Cover up bushes
- Use wind machines
- Try sprinkler irrigation
- Trust Avian Control
Early spring is the prime time for growing and cultivating blueberry plants and bushes, but it is also a reminder that the last hints of winter could pop up at any time. Sudden drops in temperature are bad news for blueberry growers, as frost can severely damage blueberry bushes and buds. Learn about five helpful tips and tricks for blueberry bush frost protection in the months of spring to start your harvest at its best.
RELATED: How to Grow Berries in the Backyard
Understand Critical Temperatures for Blueberry Plants
One of the first steps to protecting blueberry bushes from frost is to understand the temperatures at which they can be damaged or destroyed. The extent of frost damage depends on how mature your blueberry plants are. The more mature the blueberry plant, the more easily the fruit can be damaged by cold.
- Unopened blueberry buds are damaged at 21°F
- Blueberry buds that have ruptured are damaged at 25°F
- Fully opened flowers are damaged at 29°F
- Fully formed blueberries are damaged at 30°F
Use Row Covers and Nursery Foam to Combat the Cold
In the event of a frost, nursery foam or floating row covers can help to insulate blueberry bushes and crops. When installed properly, they can potentially offer up to 2°-3° of additional protection to the plants. When a row of nursery foam is double-layered, the heat gathered around the blueberry plants can offer up to 10° of additional protection. Row covers and nursery foam can be found at most gardening supply and home improvement stores.
SEE: Spring into Blueberry Growing Season with Bird Control
Invest in Wind Machines to Properly Protect Plants
In the event of a less-severe frost that occurs later in the spring, wind machines can be used to heat up the air around blueberry plants. Wind machines can provide a few degrees of warmth to blueberry bushes by mixing warm air with colder air floating above low-level plants. It is important to note, however, that wind machines are not useful in the event of especially cold temperatures or freezes.
Turn to Sprinkler Irrigation for Increased Protection
One of the tried-and-true methods of protecting blueberry crops from frost is with sprinkler irrigation. Sprinkler irrigation works by coating blueberry crops in a thin layer of water, heating them up as the water goes from liquid to solid form. When used as directed, sprinkler systems can beat the frost to the punch and allow you to control the temperature on your own terms.
ALSO SEE: Protecting Berries from Birds
Don’t Forget to Protect Blueberry Crops from Birds with Avian Control!
While frost and freezes are certainly enemies of a blueberry grower, so are birds! Remember to prevent your budding blueberry bushes from becoming a bird’s next meal with a liquid bird repellent like Avian Control. To order commercial or residential sizes of our proven bird control solution, contact us at 888.868.1982, today!
5 Blueberry Bush Frost Protection Tips and TricksLearn some tips to protect your blueberries against damaging frost with Avian Enterprises. Brand: Avian Enterprises 5 Blueberry Bush Frost Protection Tips and Tricks
Most rabbiteye blueberry varieties require 400 chill hours to 600 chill hours (hours below 45 F) to break dormancy. Until the cold requirement is achieved, an extended period of warm weather usually will not cause floral budbreak. Once the chilling hour requirement has been satisfied, extended periods of warm temperatures will initiate flower bud growth. Susceptibility to cold damage in rabbiteye blueberry is directly related to the stage of floral development. As flower development progresses, susceptibility to damage becomes greater. Swollen, unopened flower buds can withstand temperatures as low as 21 F. Buds in which bud scales have abscised and individual flowers are distinguishable are killed at 25 F. Flowers distinctly separated with corollas unexpanded and closed are killed at 28 F. Fully opened flowers are damaged at 29 F and fruit are severely damaged at 30 F.
Certain varieties seem to be more cold tolerant than others due to the extent of floral development achieved when a killing freeze occurs. Earlier blooming varieties are more prone to freeze injury because they will have the greatest number of advanced blooms.
The common method of determining if buds have been damaged by frost is to cut through the bud several hours after a freeze and look for browning that indicates injured tissue. Sometimes the freeze injury is not severe enough to kill the fruit or flower completely but may affect some individual part, such as the pistil, stamen or seeds, which may result in a reduction in fruit set or size. Blueberry fruit can develop and mature after a portion of the ovaries are damaged; however, because fruit size is highly correlated with seed number, fruits from seed-damaged flowers are usually smaller and often misshapen.
Freeze damage is also the cause of outward scarring on the fruit, which results in reduced quality. The area of the fruit exposed to cold temperatures will desiccate, resulting in a brown necrotic ring around the calyx. Because this tissue is dead or dry, it is more brittle than surrounding tissue and may be the site of splitting during a wet harvest. At best, it will cause a discolored ring and possibly some disfigurement of the fruit. At worst, freeze injury can promote secondary fungal infections that can spread to and destroy healthy blooms.
Methods of Protecting Blueberry Crops from Frost and Freeze
Freeze protection of blueberry fields is not an exact science. It is difficult to make recommendations about freeze protection because every freeze event is different. Factors that affect the efficacy of freeze protection include:
- Weather conditions
- Temperature before the freeze
- Length of freeze period
- Growth stage of the plant
Floating row covers are especially useful for small acreages of low-growing crops or when water for overhead irrigation is not available. The amount of frost protection obtained varies with the weight and fiber arrangement of the row cover. Usually the amount of protection increases with the weight, though differences in texture make this correlation less than perfect. Row covers weighing 0.6 ounces per square yard typically can give 2 or 3 degrees protection during a radiational freeze, while nursery foam covers or a double layer of row covers can give more than10 degrees of protection. Weather conditions before the frost affect the amount of protection obtained from row covers because little or no heat may accumulate under the row cover on cloudy windy days. When row covers are used for frost protection, they should be pulled over the crop during midafternoon to allow heating to take place. Row covers also can be used in conjunction with sprinkler irrigation on top of the row cover. Row covers used in this way typically cut the amount of overhead irrigation needed for frost protection by about 50 percen,t on average.
Wind machines have been used successfully to protect tender blueberry blooms. Most devastating spring freezes are radiational freezes, and wind machines are effective in this type of freeze. In a radiational freeze, there is no wind and the heat at ground level is lost to the atmosphere. A wind machine causes air turbulence that disrupts the inversion layer by intermixing warm and cold air. Often an inversion layer of warm air is 50 feet to 200 feet above the surface and, if one is readily available, a wind machine will pull down this warm layer and mix it with the air in the field. Growers sometimes use helicopters to gain the same effect. A helicopter will find the inversion layer, push down the warm air and mix it with colder surface air. The air currents mix the air and prevent the escape of the warm air back into the atmosphere.
Overhead water sprinkling is another effective method of frost protection. However, it is somewhat expensive to install and requires a large volume of water. Water volume is critical! It is necessary to apply 1/2 inch to 1 inch of water per hour and at least one sprinkler rotation per minute. Water must be applied constantly because ice is a poor insulator. Protection comes from the constant application of liquid water, which is above 32 degrees F, and the release of heat when water turns to ice. This keeps the plant tissue at or above 31.5 degrees F. The water must be applied constantly until the air temperature rises above 32 degrees F. If the water is turned off too soon, the entire crop may be lost.
Braswell, John. Establishment and Maintenance of Blueberries. 2009. Retrieved 01 June 2010.
Blueberry Winter Damage: Care Of Blueberries In Winter
Most perennials become dormant during the late fall and winter to protect themselves from the cold temperatures; blueberries are no exception. In most cases, blueberry plant growth slows as dormancy develops and cold hardiness of the plant increases. However, in some instances, dormancy has not been established and protecting blueberries over winter to mitigate any blueberry winter damage is of primary importance.
Care of Blueberries in Winter
Specific care of blueberries in winter is usually not necessary, as fully dormant blueberry plants are generally very cold hardy and rarely suffer any severe blueberry winter damage. There’s the caveat, however, the plants must be fully dormant and Mother Nature doesn’t always cooperate and allow the gradual cold hardening necessary to prevent potential winter damage of blueberry plants.
Also, a sudden return to warm temps after a period of cold, especially in warmer climates, can cause injury to the berries if they begin to bloom
early followed by a sudden cold snap. Usually when this occurs, the plant will be in various stages of budding and only buds that are emergent suffer damage. Generally, winter damage of blueberry plants occurs when temps are below 25 degrees F. (-3 C.), but this is in correlation with the relative dew point as well as the amount of wind.
Dew point is the temperature at which water vapor condenses. A low dew point means the air is very dry, which makes the flowers several degrees colder than the air making them susceptible to injury.
Blueberry Bush Winter Care
When faced with the prospect of a cold snap, commercial growers turn to overhead irrigation systems, wind machines and even helicopters to assist in the protection of the blueberry crop. I would venture to suggest that all of this is impractical for the home grower. So what blueberry bush winter care can you do that will protect your plants during cold weather?
Protecting blueberries over winter by covering the plants and mulching around them can be beneficial. It is important when covering the plants to trap heat much like a small greenhouse. A frame of PVC covered and securely anchored can accomplish this purpose. Also, keep your plants moist. Moist soil absorbs and retains more heat.
Of course, ideally you will have planted late flowering cultivars if you reside in a region where the possibility of freezing exists. Some of these include:
Be sure to select your planting site with care. Blueberries prefer full sun, but tolerate partial shade. Planting in a partially shaded tree canopy will protect the plants from drying out, thus aid in thwarting freeze injury.