Any gardener who has ever transplanted or repotted plants has seen first-hand how plants that have been grown in pots can sometimes get rootbound, with their roots spiralling or circling around the rootball. This is quite an unnatural way for plant roots to grow, as they normally grow outwards and downwards!
Spiralling roots cannot be left as they are when repotting, because they can strangle the rootball, impairing the growth and health of the plant. Even worse, with woody shrubs and trees, when spiralling roots are growing against the stem, near the level of the soil line where the plant emerges, they can girdle the stem.
Stem girdling occurs when one or more roots which completely or partially encircle the stem thicken as they grow, and begin to compress the bark and sapwood of the stem. Since the stem also grows wider over time, it further compounds the problem. When the stem is compressed, the flow of water and sap is restricted, which limits the transportation of water and nutrients (through internal tubes known as the xylem) from the roots to the leaves, and the transportation on sugar-rich sap (through internal tubes known as the phloem) from cells that photosynthesise to ones that don’t.
In severe cases of stem girdling, not only can sap and water flow be stopped completely, but compression can also cause death of the cells of the bark and sapwood, weakening the wood structurally, and creating an entry point for decay to set in.
How to Root Prune Rootbound Plants
The best way to deal with spiralling roots of rootbound plants is by root pruning, which is nothing like pruning the above-ground, upper-half of any plant!
Root pruning is a fast and easy process, all that is required is a sharp knife,. The serrated edge of a Japanese hori-hori gardening knife works particularly well, but so does a retractable utility knife (also known as a Stanley knife, box cutter or X-Acto knife). Thicker roots require a sturdier blade.
The traditional Japanese Hori Hori soil knife is a multi-purpose gardening tool, its curved blade works like a long planting trowel, the serrated edge makes it one of the best root pruning tools available!
To remedy the problem of spiralling roots:
- Remove the pot, and lay rootball on its side.
- Make an X-shaped cut at the bottom of the rootball.
- Continue all four cuts down the sides of the rootball.
For a potted tree or plant in a 30cm (12”) wide pot or larger, make the bottom X-shaped cut around 7-10cm (3-4”) deep, and the cuts at the sides around 2.5cm (1”) deep. For smaller pots use correspondingly shallower cuts.
The idea is simply to sever the spiralling roots on the surface of the rootball so they no longer form circles. With 4 cuts along the sides, any encircling root will be cut into four segments, and will no longer be able to cause root girdling.
The diagram below illustrates how to make the root pruning cuts to prevent root girdling of the stem and constriction of the rootball.
If an plant isn’t rootbound, but the roots are just beginning to spiral, they can be carefully lifted and unwound without pruning, keeping the longer roots intact.
The reason why we root prune is to cause minimal root disturbance, because if it’s one thin that plants dislike, it’s having their roots disturbed…
Teasing Roots is a Bad Idea…
Why not just tease out the roots by hand? The short answer is to avoid unnecessary root damage! Roots don’t just anchor a plant into the ground, they’re the plant’s major absorption organ to take up water and nutrients from the soil.
In any plant, the leaves and roots are in perfect balance, there is exactly enough roots to support the leaves that a plant is carrying, so when the leaves transpire and lose moisture to the air, the roots can take up water from the soil to replace it. keeping the leaves alive.
If the roots are damaged, they cannot supply enough water to support all the leaves, so the plant wilts and leaves drop. We see this when a plant is over-watered (causing root rot), or under-watered (causing root drying), and the roots die back.
The roots system of plants is an intricate branched network, with one thick main primary root, from which multiple smaller lateral roots branch off from.
The lateral roots are comprised of:
- Coarse roots, which are woody, they have some ability to absorb water and nutrients, but their main function is to connect the thinner roots to the plant and carry water and nutrients back into the plant.
- Fine roots, which are less than 2mm thick and heavily branched, and whose main function is to absorb water and nutrients through the root tips, which are covered in fine root hairs to increase the surface area for absorption.
The fine lateral roots are usually short-lived, being constantly turned over by the plant as it seeks to conserve its resources while extending its roots into new areas of soil in search of water and nutrients.
So what happens to all these fine lateral roots, covered with millions of fine roots hairs to absorb water and nutrients, when a gardener teases, tears and ‘massages’ the roots? A lot of these delicate vital structures are destroyed by such rough handling, resulting in a significant reduction in active functional roots which can support the leaves, and as a consequence, leaf drop and a stressed plant.
The roots are a critical part of the plant’s structure, and are easily damaged, so minimise root disturbance when transplanting, and if spiralling roots are encountered, root prune using the cross-cut method described earlier.
As a handy tip, after transplanting, it’s always a good idea to water with seaweed extract, it contains compounds called cytokinins, which are plant root growth stimulating hormones, they help the plant establish itself better and reduce the effects of transplant shock.