Gar­den­ing un­der trees

Ontario Gardener Magazine - - Contents - By Ila Throck­mor­ton

We in­sist on plant­ing un­der our trees and then won­der why we have to deal with tree roots. It’s an ir­ri­tat­ing fact of the gar­den life, es­pe­cially if you are fond of shade gar­dens.

Most tree roots ex­ist just un­der the sur­face of the soil, in the top eight to ten inches – just where we want to plant our prize hosta. But if you were a tree, you’d want to oc­cupy this prime real es­tate too, be­cause it is here that air, mois­ture and nu­tri­ents are most ac­ces­si­ble. In a flowerbed, these roots can be a nui­sance for the gar­dener who may also suf­fer bouts of doubt and guilt about dis­turb­ing the tree roots and cut­ting through them.

Don’t feel guilty.

Most trees can han­dle a bit of root dis­tur­bance and prun­ing (oaks can be an ex­cep­tion) and the tree may even pros­per from some light prun­ing; just as in the branches above, prun­ing en­cour­ages more growth, pro­vid­ing more op­por­tu­nity for the tree to take up sus­te­nance. Plant in­stead of over an­nu­als to avoid hav­ing to dis­turb roots year af­ter year.

You may also be help­ing the tree by aer­at­ing the soil, amend­ing it with health-giv­ing or­gan­ics and fer­til­iz­ers. The tree is a happy host. Some trees will let you know if they re­sent this kind of com­pe­ti­tion. Wal­nut or other mem­bers of the Juglans fam­ily, such as but­ter­nut, de­fend their ter­ri­tory by send­ing out killing chem­i­cals that will soon dis­cour­age your gar­den­ing ef­forts in that vicin­ity.

Cau­tion: be care­ful about adding large amounts of top soil over a tree’s roots zone. This can smother the tree, de­priv­ing it of oxy­gen.

The herba­ceous plants you are in­tro­duc­ing may not be as lucky as the tree. In ad­di­tion to at­tacks from the

species just men­tioned, com­pe­ti­tion for mois­ture can be a real prob­lem, es­pe­cially for wa­ter-lov­ing plants such as hosta. You may find your­self need­ing the hose in these flowerbeds more fre­quently than you wish.

Still, it’s of­ten worth the ef­fort to plant un­der a tree. Herba­ceous plants can ben­e­fit from the fallen leaves and we have al­ready dis­cussed how the tree man­ages. As well, by in­tro­duc­ing more mois­ture to the feeder roots of the tree, you may pre­vent the roots from com­ing to the sur­face and in­ter­fer­ing with your lawn in dry years. If you are run­ning into prob­lems with tree roots at sur­face level, it is prob­a­bly due to a lack of wa­ter. Wa­ter more fre­quently and more deeply. Shal­low wa­ter­ing en­cour­ages all roots – trees, peren­ni­als and lawn grass – to linger near the sur­face where they are vul­ner­a­ble to drought and other dam­age.

And yes, you can in­tro­duce shade-lov­ing shrubs to the flowerbed un­der a large tree. While there will be com­pe­ti­tion for mois­ture and nu­tri­ents, there are also sym­bi­otic re­la­tion­ships. Have you ever seen a healthy for­est with no un­der­story?

Con­trary to pop­u­lar be­lief, tree roots will not force them­selves into foun­da­tions or sewer lines. They will, how­ever, ex­ploit any leaks you al­ready have in the sewer lines. When it comes to foun­da­tions, they will sim­ply turn aside and grow in an­other di­rec­tion when they hit a solid ob­ject, whether it be ce­ment or a rock. The roots are look­ing for food and wa­ter; like ev­ery­thing else, they will take the eas­i­est path to find both.

If you are con­cerned that your lawn will suf­fer from the shade of a great tree, plant the ap­pro­pri­ate grass in this area. There are shade-lov­ing fes­cue mixes on the mar­ket that thrive in shade. These fes­cues are also drought tol­er­ant.

Shrubs such as dog­wood can form part of the un­der­story for trees.

Be care­ful of raised beds which can kill a tree if the roots are sud­denly smoth­ered by top­soil.

Hostas will grow un­der trees, if there is enough mois­ture.

Berge­nia sur­rounds a tree trunk to put on a daz­zling pink show in early spring.

Browalia and im­pa­tiens un­der a tree.

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