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YOUR GAR­DEN­ING QUES­TIONS THIS MONTH

NZ Gardener - - News - Bar­bara Smith

Your ques­tions an­swered.

QGRASS ROOTS

I have been watch­ing some Bri­tish gar­den shows which are great. They said that if you don’t re­move turf from around new trees (at least for the first three years) you can slow their growth by a third. Does this ap­ply to wild­flow­ers planted un­der fruit trees as well? ALIX LATTEY, TAUPO

AFour mem­bers of the NZ Tree Crops As­so­ci­a­tion (treecrops.org.nz) put their heads to­gether to an­swer your ques­tion.

Derek Craig, the Na­tional Re­search Co­or­di­na­tor, said that he too would re­move grass cover around newly planted trees and mulch to sup­press grass re­growth. The mulch would also shel­ter the tree roots from sum­mer heat and help re­tain mois­ture. Over time, the de­com­pos­ing mulch would also feed the tree. The rea­son for re­mov­ing the grass is to elim­i­nate root com­pe­ti­tion. Fruit tree roots are very shal­low grow­ing, and the grass roots are vo­lu­mi­nous and very good at strip­ping wa­ter at the same soil level as the fruit tree roots.

The ad­di­tion of a flower herbal ley has some of the same prob­lems as grass cover though stress from root com­pe­ti­tion for wa­ter and nu­tri­ents would be al­le­vi­ated by ir­ri­ga­tion through sum­mer. How­ever, a more holis­tic view would coun­ter­bal­ance the wa­ter com­pe­ti­tion with sys­temwide ben­e­fits such as of­fer­ing in­sect shel­ter, nec­tar, pollen and al­ter­na­tive food hosts plus act­ing as liv­ing mulch to shel­ter soil and roots.

Fel­low Tree Cropper Eric Cairns added that grasses (mono­cotyle­dons) ex­ude al­lelo­pathic sub­stances into the soil that in­hibit the growth of trees. Most fruit trees (ex­cept cit­rus) aren’t all that shal­low rooted, but will send their feeder roots to where the nu­tri­ents are, so they will be at the sur­face if a mulch is used. He says wild­flow­ers will com­pete for mois­ture and nu­tri­ents, but would not be as in­hibit­ing as grasses.

Tree Cropper ed­i­tor Britt Coker re­ferred to the food for­est regime of lay­er­ing plants to max­imise space and out-com­pete weeds. The ben­e­fits of draw­ing in pol­li­na­tors out­weighs the com­pe­ti­tion, she sug­gests.

And fi­nally, our Waikato colum­nist Sh­eryn Cloth­ier: “Com­pet­ing roots can be detri­men­tal in the first sea­son or two of a tree get­ting es­tab­lished, so clear­ing or mulching the area within 30cm of the trunk has ben­e­fits. But after, an un­der­storey of wild­flow­ers is ben­e­fi­cial to the soil’s ecosys­tem and there­fore, ben­e­fi­cial to the roots.”

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