One of the questions I am most frequently asked is, “How often do you root prune?” This is a tricky question and one for which I wished there were a more clear cut answer. First let’s dispel a myth. Ready? Ok, here it is. We do NOT root prune every tree in the nursery. (Take a minute and digest that one, sit down if you need to.) Now that you are over the shock, I’ll tell you why we don’t.
First, let’s talk for a moment about what exactly root pruning is. Root pruning is just what it sounds like: cutting roots just as we would cut branches, only we do it with a tree spade, a shovel, or a trenching machine. Some of the reasons to prune a tree include:
Some trees might need to be dug in a less than preferable season. For example, oaks typically do not dig especially well at any other time than when they are dormant. But let’s say that the trees might not be needed until July. We would then choose to root prune the trees during dormancy to help prepare the trees for digging outside of their optimal season.
I often like to say that trees are not multitaskers; most are only capable of doing one thing at a time. They can either put out new roots, or new leaves, but not both at the same time. Root pruning a tree will often slow down the growth of the tree. By severing their roots, it forces the tree to put energy into regeneration of new roots rather than primary and secondary growth (getting taller and putting on caliper). And more roots are good, right? RIGHT!
Because a root pruned tree will have a higher concentration of roots in a closer proximity to the trunk of the tree, we are often able to put a root pruned tree in a smaller root ball. This turns into a win-win for those projects that cannot accommodate a larger ball size or a large piece of equipment. This also ties in nicely to purposefully slowing down the growth of a tree.
The benefits of root pruning are realized for more than just one season. A tree that was root pruned in 2013 will be more easily dug in an off season for likely two to three years beyond the date of the root pruning.
In a recent demonstration, we root pruned a red maple in early spring (we actually root pruned it for a demonstration on bare-rooting that was rescheduled for a later date). The preparation work we did for this demonstration provided an excellent study in just how effective root pruning can be at creating new roots! In the photo below, you can see how many new fibrous roots were generated in just four months’ time.
Will you just look at those gorgeous roots! You can’t have lustrous hair without a trim every now and again, the same may hold true for the roots of many species of trees.
So, knowing what we do about the many great benefits that root pruning can realize, shouldn’t we just root prune each and every tree in our nursery? In a paragraph above, I mentioned that root pruning will often slow down the growth of a tree, making it a great way to control growth in fast growing species such as Liriodendron (tulip poplar) and Prunus (cherries). But what about Fagus (beech) or Gingko? Slowing down these already slow growing species would result in longer than average growth times, resulting in fewer available trees.
Another simple – and honest – reason for not root pruning is time, or more specifically the lack thereof. Root pruning is time consuming. Each year we root prune (or plan to root prune) a certain number of trees so that we can expand on the typically short harvest window. Root pruning in the spring allows us to dig throughout the less than desirable digging months of summer. We mark these trees with a special color of ribbon and a silver tag with their root prune date on them so that we can track their history and progress.
Some trees need to be prepared months and sometimes even years in advance, so knowing when your projects will be installed is a critical first step for the success of its trees. Find a nursery that you can trust and with whom you can develop a relationship and you are sure to see a difference in the quality and performance of the trees.
Ronda Roemmelt is the sales manager and representative for Mid-Atlantic sales at Ruppert Nurseries in Laytonsville, MD. In addition to 20 years of industry experience, Ronda also has a Bachelor of Science in Agricultural Science from Pennsylvania State University with a minor in Horticulture. Ronda is a Certified Professional Horticulturalist and an ISA Certified Arborist.
Ann Larle Valentine / CC BY-SA 2.0
Root pruning guidelines
Home > Root growth > Root pruning trees > Root pruning guidelines
Trenching and digging in the soil near trees can cut roots, and this can damage the tree resulting in tree decline or the tree falling over (See: fallen tree from cutting roots). This can cause liability and safety concerns. Root pruning is more injurious to old mature trees than it is for younger more vigorous trees. Cutting roots greater than about one inch diameter during trenching and digging can mean problems for the tree. In some cases roots of one to three inches diameter represent the major structural roots holding the tree upright.
The impact from pruning roots depends on several factors (see table below). Damage typically increases with more cuts, bigger cuts, and cuts made closer to the trunk. Root pruning, trenching, and other construction activities close to the trunk result in more injury on shallow, compacted soils or on soils that drain poorly than on well drained soils. This is due to the shallow roots common on sites with shallow soils or high water table. Trees that are leaning are poor candidates for root pruning. Prune roots only with sharp tools to avoid tearing behind the cuts.
See: more details on cutting roots.
Factors affecting response of trees to root pruning
- root size: larger roots may generate few new roots
- number of cut roots: more roots cut means more tree stress
- proximity of cuts to the trunk: the closer cuts are to the trunk the bigger the impact
- species: some species tolerate it better than others
- tree age: old trees are more likely to stress and die
- tree condition: trees in poor health should not be root pruned
- tree lean: leaning trees should not be root pruned
- soil type and site drainage: shallow soils mean stay farther from the trunk
How close to the trunk can roots be cut?
Well, the answer appears to depend on who you ask. For mature trees, some experts recommend not cutting roots closer than 6 to 8 inches from the trunk for each inch in trunk diameter. That means stay at least 10 feet away from a 20 inch tree! Others are more realistic and state that we should root prune no closer to the trunk than a distance equal to 3 times the trunk diameter, preferably 5 times the trunk diameter. Dr. Tom Smiley at the Bartlett Tree Research Laboratory in Charlotte showed that roots on one side of very young trees can be pruned off completely at a distance equal to 5 times the trunk diameter without any impact on tree stability. Which ever rule-of-thumb you decide to use, do so knowing that pruning roots on trees can result in trees falling over or dying. While root pruned large trees on well drained soil may not fall over because of deeper sinker roots under the trunk, they can and have. There are fewer deep roots holding the tree up on poorly drained and compacted soils.
Alternatives to root pruning
- add soil over the roots and re-sod
- curve the sidewalk around the surface roots
- elevate the walk over the roots
- suspend the footing on pilings
- re-pour the walk with steel in the concrete
- grind the concrete down
- raise the walk by injecting grout under it
- build the structure elsewhere
- dig under roots with trench-less technology
- live with the problem
- See: more details on alternatives to root pruning
Face it, it happens at least once in our gardening lives: The plant we thought would be perfect here really would look better there. So, if this is your dilemma, there is a tried-and-true method for transplanting established shrubs, and it begins with root pruning.
Fall and early spring before bud break are the best times for transplanting because cooler soil and air temperatures help spur root development. Right now, the nutrients once routed upward to support the plant’s vegetative growth are being routed back down to the root zone. In early spring, those nutrients haven’t been sent upward yet. As a result, major root development occurs right below our feet. So why not take full advantage of this growth spurt?
The root systems on trees and shrubs spread far beyond the drip edge, or outside tips of the branches. So, when we transplant, it’s next to impossible to dig the plant with all its roots intact. Root pruning encourages the plant to develop a smaller, more compact root ball with a denser root formation, which is much easier to relocate and less stressful on the plant.
If we simply dig the shrub or tree and relocate it all in one step, a large majority of the roots are severed. This causes what is called “transplant shock.” If the quantity of roots lost is minimal, the plant will recover over a few years.
Growth during this time will be slow, if at all. Also, expect some branches to die back until the ratio of roots to stems rights itself. A major root loss will result in the plant’s ultimate demise. But the effects of transplant shock can be avoided with some preplanning.
The principle behind root pruning is the same as with branch pruning. When we prune the tips of shrubs and trees, we force the development of side branches due to the redistribution of the growth hormone auxin.
The same holds true for root pruning. Only here, the hormones are primarily gibberellins. Like auxins on stems, gibberellins are concentrated at the tips, with lesser amounts on the smaller, secondary roots. When we prune roots, the heaviest concentrations of gibberellins are removed, allowing the lesser amounts in the secondary roots to become dominant, so side growth occurs.
Studies show that another responsibility of the hormone gibberellin is to play an active role in forcing a flower bud to break dormancy.
This is why it’s recommended to root prune flowering shrubs that refuse to bloom; by doing so, you are forcing the development of these secondary roots. Theoretically, the more roots the plant has, the more gibberellin hormones are present and the greater the opportunity for this hormone to act as a breaker of bud dormancy.
You want to root prune at least six months before you transplant. So, whether you root prune now for spring transplanting or in the spring for transplanting next fall, here’s a simplified way to do it:
– Attach a piece of sturdy twine to the base of the trunk and, pulling in gently, wrap it around the branches in a spiral. Then secure the twine end to the top of the trunk. This will pull the branches out of your way and help prevent damage.
– Measure the diameter of the tree’s trunk 6 inches above the soil line. A tree with a 1-inch trunk should have a root ball diameter of 18 inches; a 2-inch trunk should have a 28-inch root ball.
The root ball measurements for other size shrubs and trees can be found in the American Association of Nurserymen’s reference American Standard for Nursery Stock or on the Internet at hort.ifas.ufl.edu/woody/planting/america nstandard.htm
Shrubs are measured differently. A 3-foot-high shrub requires a minimum 14-inch diameter root ball; a 4-foot shrub, a 16-inch root ball; a 5-foot shrub, an 18-inch root ball.
– Once you have your measurement, draw the circumference of the root ball around the plant in the soil.
– Dig down around the sides as deep as you can, at least 15 inches. You’ll hear the sounds of the roots being pruned; that’s your objective. Untie the branches until transplanting time.
– Water the plant well until the ground freezes, if pruning in the fall, or throughout the summer to aid in secondary root formation. Stop at this step if all you want to do is force an unruly plant to flower.
– When it’s transplanting time, retie the branches; dig back down along the same diameter, eventually working the shovel under the root ball. Gently lift and move. Keep well watered the first year.