Best Way to Fix Root Bound Plants, Before Planting

Home » Blog » Best Way to Fix Root Bound Plants, Before Planting

Robert Pavlis

Many plants purchased from nurseries are root bound, also called pot bound, and the internet provides a variety of suggestions for dealing with this problem. Slice the sides, cut the bottom off, butterfly them or don’t do anything at all. Which method is best?

Is the best fix for this problem different for different kinds of plants? Should trees be treated the same perennials or annuals?

Let’s see what the science says.

Best Way to Fix Root Bound Plants, Before Planting
Best Way to Fix Root Bound Plants, Before Planting

What is a Root Bound Plant?

The roots of potted plants grow out in a radial fashion, similar to spokes on a bike. When these roots reach the edge of the pot, they tend to grow around and around between the root ball and the pot. After a while the surface of the root ball is covered with roots and if left in this condition the thickness of the root layer just increases.

How quickly does this happen? It depends on growing conditions, the size of the plant and the size of the pot. Try growing a tomato seedling in a small pot and it will start getting root bound in a few of weeks. Put the same seedling in a 6″ pot and it won’t be root bound by the time it needs to be planted outside.

Microbe Science for Gardeners Book, by Robert Pavlis

Why are Root Bound Plants a Problem?

Everyone seems to know this is a problem but except for trees there are very few studies to understand the problem. Some issues are known.

Girdled Tree Roots

Woody roots are different than most herbaceous roots in that they continue to get thicker their whole life – similar to the thickening of the trunk. If circulating roots grow around a trunk and then both the roots and trunk increase in thickness over time, the roots can strangle the trunk, cutting off the phloem and xylem connection between roots and leaves, eventually killing the tree. This is a well known problem in horticulture which kills many landscape trees.

You can see the girdling roots very clearly in this picture. The tree will soon die unless corrective action is taken, and for this one, it might be too late.

Girdled tree roots ready to choke the trunk
Girdled tree roots ready to choke the trunk, credit Cathy Kavassalis

Poor Establishment in New Soil

Root bound plants have trouble growing roots in the new soil after being planted. New root growth tends to remain in a ball of the same size as the original root ball. You can easily see this if you dig up your annuals at the end of the season.

Is this problem a result of being root bound, or is it due to the fact that the media in the pot is so different from the new garden soil? Roots do have difficulty crossing from one type of soil to another.

Another important question to ask, is this really a problem? Perennials do seem to eventually grow roots in the new soil so maybe it is only a short term issue that corrects itself.

Circulating Roots Keep Growing in a Circular Direction

This is a common belief, but it’s a myth.

The old existing circulating roots will not ‘straighten’ themselves out. But assuming planting is done in suitable soil, new root growth will no longer follow the curvature of the root ball. If a root is cut, as in some of the treatments I will discuss below, the new root tips will grow in all directions. Roots do NOT memorize the circular path.

However, the new soil might cause roots to keep circulating. If for example, the planting hole is in heavy clay soil and the interior of the hole is smooth, roots will have difficultly growing into the native soil and will tend to keep going around in circles.

Some people believe that making a square planting hole will prevent this, but that is also a myth.

Differences in Soil Type

The soil in your garden is probably very different from the media in the pot, which is commonly made of peat moss, coir, or composted wood products. The porosity and water holding capacity of these two medias can be very different. In heavily root bound plants, almost no water absorbs into the root ball. Even when planted they don’t absorb water and stay bone dry while the surrounding soil is quite wet which can lead to the death of the plant.

Even if you do not treat the roots, try to remove at least some of the media inside the root ball and get as many roots touching garden soil as possible.

Fixing Root Bound Plants

There are a number of methods you can use.

Do Nothing

Ignore the problem and plant. Except for girdled roots on woody plants, this method usually works but may not be the best option for the plant.

Teasing the Roots

In this method the roots are gently separated from the root ball so that some are free to be covered by the local soil. This is easy to do unless the plant is heavily root bound and it is the least invasive way to treat plants.

Slicing the Sides

This is the most common advice. Take a knife, tree pruning saw or dry wall saw and make four vertical slices, cutting through roots to a depth of about 1/2″ (1-2 cm). The cut roots form new root tips that will grow in various directions. Note: don’t use a good saw for this.

This method is demonstrated in this video.

YouTube video

The Butterfly Cut

This one is a bit more aggressive than slicing the sides. Start by slicing the sides but continue the cuts along the bottom, cutting up into the root ball about half way. Take your hands and spread the four sections. This method cuts more roots than the above slicing method and when planted, the cut root ends will be closer to the surface of the soil.

Slice Off the Bottom

Most of the roots are at the bottom of the pot, so just slice the bottom inch off. This removes a lot of roots and is easy to do, but it does not solve the circulating roots that are near the soil surface. A lot of people will combine this method with one of the other shaving methods.

Boxing the Root Ball

All the surface roots are removed by making vertical slices. Some make several slices resulting in a round root ball (round boxing) and others make 4 vertical cuts resulting is a ‘boxed’ root ball. Smaller plants are easier to do as boxes and larger trees are easier to cut round. The bottom roots are also cut off.

Boxed root ball, credit University of Minnesota
Boxed root ball, credit University of Minnesota

Washing and Pruning Roots

I have described this process in a different post. It is mostly used for woody plants to try and prevent future girdling roots.

This is a controversial method with very little scientific support. The root washing part makes sense since it removes the potting media. Severe root pruning makes less sense and can lead to plant death.

If woody plants are planted at the correct height (discussed below), girdled roots are much less likely to be a problem, making severe root pruning a poor option.

According to Science – Which Method is Best for Trees?

There have been a number of scientific studies that looked at various methods for treating trees before planting. There are two goals for such treatment; stopping girdling roots and getting better root growth in the new soil.

Stopping Girdling Roots

The washing and pruning method and the boxing method will stop girdling roots because such roots are removed before planting, provided trees are planted at the correct height. During the research on this, scientists also looked at planting depth and found that when trees are planted at the correct height, girdling roots are no longer a significant problem with any on the preparation methods. When the root flare is at or even just above soil level, roots do not grow above the soil, preventing girdled roots.

The key is to plant correctly as I’ve detailed in Planting Trees the Right Way. Remove the soil at the top of the pot until you can clearly see the root flare. Then plant so the root flare is at or even above the surface of the soil. If you do that with any of the above treatments, you eliminate most girdling problems.

Better Root Growth

Which preparation method produces the best root growth?

Four different species (two maples, an arborvitae and a crabapple) were grown for 5 years after doing nothing, using 4 vertical cuts and boxing. All three methods produced about the same root growth, but boxing had the fewest number of girdled roots.

Vertical slicing and shaving (round boxing) were used to move two tree species from 1 gal pots to 6 gal pots. Shaving produced better roots. In a second experiment, by Ed Gilman, oaks were moved from 1 gal pots to 15 gal containers and after one year, shaving and teasing produced better roots than no root pruning.

Butterfly cuts, vertical slicing and teasing were tested on severely root bound Salix and Tilia species, planted in the ground. After 2 years, none of the techniques produced better root systems than doing nothing.

London planetrees that were teased or had their outer roots shaved off (round boxing) did better than no treatment.

Shaving of oak roots resulted in fewer circling roots and better anchorage in field soil, two years after planting. Trunk caliper was unaffected.

Several of the above studies also looked at top growth and found no significant difference in the pruning method used.

The washing and pruning method has limited scientific support and it does not have industry support.

Boxing eliminates most girdling and produces equal or better root growth than other methods. Boxing is relatively quick and easy to do and it produces the best results.

According to Science – Which Method is Best for Annuals and Perennials?

I found very little research on annuals or perennials. If you find more, let me know in the comments.

Girdling stems are not as much of an issue with these plants because roots don’t keep getting thicker for the life of the plant and there is no long-lived stem to girdle. Getting new roots to grow into the local soil is still an issue.

Plumbago (Plumbago auriculata), a perennial,  was planted in the landscape using one of three methods; non root bound, root-bound, and root-bound sliced. The non root-bound plants were smaller when planted and grew faster initially, but after 8 weeks all plants were the same size. Slicing the root ball did not help the plants, but this was a short term experiment.

Three root ball conditions were evaluated for Begonia semperflorens, a annual: non root-bound (6-week-old plants), root-bound (10-week-old plants), and root-bound with the bottom 1 cm of the root ball removed. After 12 weeks the non root bound  were larger than either of the other two treatments. There was  no difference between doing nothing and slicing the bottom off. Root pruning had no effect.

Both of these were short term studies but they indicate that root pruning, slicing or the removal of the bottom, gave no real benefit. Planting non pot bound plants is of some value. Based on this limited evidence, the common suggestion to make 4 vertical slices on perennials is not supported. For both annuals and perennials, you might as well just plant the root bound plant.

It is my personal view that removing soil or media, that is very different from your local soil, before planting, is beneficial, but I can’t find any science to support this view. I also suspect it is a short term benefit for perennials.

Best Way to Fix Root Bound Plants, Before Planting

For Trees and Shrubs

Box cut the roots and remove the bottom roots. Remove the top soil/media until you find the root flare and plant so it is at or above the soil level. Planting depth is more important than root treatment.

For Annuals and Perennials

Shake off loose potting media and plant. Ignore the common gardening advice to make 4 vertical cuts.

If you like this post, please share .......

Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

16 thoughts on “Best Way to Fix Root Bound Plants, Before Planting”

  1. Robert, another excellent video. My question relates to root-bound Cucurbits. The common view seems to be not to disturb the roots. I have many in pots waiting for my peas to finish up (slow this year) so I can transplant into that garden space and they are becoming somewhat root-bound. Should I disturb these roots, or leave them alone when transplanting? Thanks!

    Reply
  2. I would love an article on whether some root bound plants are more likely to flower from being bound. I read it all the time online, often with snake plants, and now I’ve come across in a thread about a peace lily that has been repotted. Those comments always get plenty of upvotes. I know that the idea of plants liking to be “snug” is pretty much a myth but I would like to know what effect being root bound has on flower production. The claim is often that the plant “panics” and thinks it’s dying so it throws out blooms to save itself.

    Reply
  3. I have two fig trees that are about 3 years old. I transplanted them into the ground a couple months ago. They were quite rootbound in their pots before I moved them. Not having known better, I just stuck them in the ground and thought the roots would expand on their own somehow. Should I try to dig them up and prune the roots or wash and separate them or just leave them be? Thank you!

    Reply
    • If it has only been a couple of months you can dig them up, wash the roots and replant without much harm.

      Reply
  4. Should monocots be treated differently? While monocots are not true “woody” plants (therefore the “do nothing” plan can work), they regenerate roots differently. Most monocot roots, if cut, don’t branch out like dicot roots do (correct me if I am wrong on this), instead, while the root continues to function for some time, the plant ultimately has to regenerate a new adventious root from the crown. Which is something it will do anyway as it grows, so is there a benefit to “boxing” or root pruning a monocot plant?

    Reply
    • This is an excellent question and I tried to find a solid answer, but did not find one. Your logic makes sense to me.

      Reply
  5. I’ve exposed the root flare, and then cut any bends where the roots are not growing out from the center of the tree. The thinking is that bends have occurred when they hit something (like the side of a pot, or a large rock), and cutting at the bend may prevent them from continuing in the wrong direction.

    Reply
  6. As usual, excellent compilation of research which saves me a lot of time, especially when answering questions from clients and general public at garden clinics. Thank you Robert, I endeavor to remember where information comes from and to give proper credit. And I often direct people to your site.

    Reply
  7. Have you seen the Vigoroot planters? – apparently air prunes to reduce tree being pot bound… – am going to try it with a young Amalanchier! Thanks for your continuing advice around those myths!

    Reply
    • My anecdotal observation is that they do provide a benefit, the question is the cost/benefit analysis from a commerce perspective. I grow my own trees for eventual transplanting this way (using variouis root pruning pots – rootmaker, Smart Pot, generic “grow bags”, Air-Pots from the UK. They can still eventually get rootbound but it’s “different” than circling roots (the roots just get so dense it starts to restrict growth). For me they seem to establish faster.

      Again – anecdotal – but I had a 2″ caliper oak in a 15 gallon (I think?) SmartPot that I transplanted in March one year. It grew 2-3′ that season, and showed no noticeable transplant shock at all. A traditional container would have likely needed boxing or other root pruning, and while the tree would likely have done just fine, my guess is at least the first season it would not have grown much and exhibited more transplant shock (small leaves, limited growth, etc). However, my loamy native soil probably helps, too (most plant roots grow happily right into it after transplanting).

      Reply
  8. On a related question, do you have a view on the merits of root washing perennials that require excellent drainage? As you note, most plants come from the nursery in a medium that is heavy on coir, peat moss or composted wood, all of which are moisture retentive. I have become frustrated with perennials drowning over winter in my very sandy soil because they started life in a medium that made life easy for the growers but did not suit the plant at all.

    Reply
  9. The only time I’ve ever cut the roots with a knife is when I bought some Azalea plants that were so rootbound they were on deaths door. I slashed them down the sides, then pulled the roots by hand, then immersed them in a pot of water, which they desperately needed since they were so pot bound the water just rolled off the roots before that.

    All other pot bound plants I just use an old fork and tease out the roots, and if not that bad, simply by hand. Never lost a plant yet doing it this way.

    Reply

Please leave a comment either here or in our Facebook Group: Garden Fundamentals