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WEED CONTROL METHODS AND TREE GROWTH IN APPLE ORCHARDS: PART 2
by Sandra Salmins and Deborah Buszard
Much research work has been carried out to assess ground management systems for apple orchards. The emphasis of previous research has been primarily on finding ways to maximize production. The goal of this multi-disciplinary study was broader, and stemmed from a need to evaluate the impact of several organic ground management systems for orchards, not only on production efficiency, but also on the soil environment, in terms of soil structure and nutrient content, soil fauna activity, and thus, overall sustainability of the system.
To recap quickly, in the spring of 1991, six ground management systems (cultivation, straw mulch, geotextile "fabric" mulch, manure mulch, red fescue cover and an insectary plant cover consisting of lupin and wild carrot) were set up as one-meter-wide strips along newly planted rows of scab-resistant apple trees on dwarf rootstocks. (For details, see Summer 1992 issue of COGNITION, p. 8.) Tree growth was assessed by comparing trunk growth, shoot length and number, and average leaf area.
After only two years of observation, trends in tree growth and vigor have emerged. Results indicate that tree growth is inhibited under the red fescue and insectary plant treatments, while that in the geotextile and straw mulch treatments is most vigorous. Consideration of a red fescue or an insectary plant cover system is largely based on their beneficial influence on the stability of the environment. Both of these treatments provide cover for the soil, thus reducing erosion and improving soil structure over the long term. Organic matter is provided by the grass roots and clippings, which encourage earthworm and other soil fauna activity.
In measurements of growth, trees in the cultivation and manure mulch treatments fell in the mid-range, although we cannot be sure whether this trend will continue or if their growth rates will increase or decrease compared with those of trees in the other treatments. Growth data concur with those of other studies in which cultivation was found to promote more vigorous tree growth than that of trees under living plant covers.
Past research also found that trees under cultivation grew more slowly than those under a straw mulch system. While no statistically significant differences between the cultivation and straw mulch treatments were found in the first two establishment years, trees under cultivation had smaller trunk girths and less shoot growth than those under straw mulch. Repeated cultivation tends to be detrimental to soil structure in the long term and has been found to contribute to soil compaction, erosion, crust formation, and decreased water infiltration. In 1992, a very noticeable crust did form in the cultivation treatment, and although water infiltration rates were not measured, soil volumetric water content was lower under the cultivation treatment compared with the straw mulch. It might be expected that the long-term effects of compaction and reduced water availability may further inhibit growth of trees under cultivation over time.
Cultivation machinery can damage tree roots and/or the trunk and branches, leading to a greater risk of insect and disease invasion. Damage to surface roots reduces nutrient uptake from the upper portion of the soil. Similarly, cultivation machinery can kill both beneficial and harmful arthropods and earthworms, thus upsetting the natural populations and balance of the system. Increased levels of dust associated with this practice have been found to favor population growth of destructive arthropod pests in orchards and inhibit the activity of beneficials.
In 1992, weed species in the cultivated plots shifted from mixed annual and some perennial species to an increasing population of hard-to-control perennial plants, with quack grass being predominant.
Manure mulch, on the other hand, adds organic matter and provides a protective layer to the soil which reduces erosion and conserves soil moisture. Unfortunately, this type of mulch is likely to contain viable weed seeds if not composted properly and/or can provide an ideal growth medium for arriving weed seeds. In this study, these plots were characterized by heavy stands of mixed weed species which flourished and required vigilance and time to prevent their reseeding themselves. Another consideration is the risk of supplying too much nitrogen. In this study, nitrate levels were found to be highest under the manure mulch treatment. Excessive amounts of nitrogen has been found to increase pre-harvest drop, reduce fruit coloring and fruit quality, and to delay hardening.
Trees in both the straw mulch and geotextile mulch treatments were characterized by large trunk radial growth and vigorous shoot growth. While it is possible that trees in these treatments will bear fruit earlier and produce higher yields than trees in the other treatments, geotextile mulch does not add any organic matter to the soil. Results from this study concur with work currently underway at Agriculture Canadas Summerland research station in B.C. where they are finding that trees under geotextile mulch grow vigorously. Although results indicate that trees in both geotextile and straw mulch compare favorably in terms of growth, geotextiles must be further evaluated from the perspective of long-term sustainability of the environment and their ecological effects. It is not yet known how their use will affect soil quality and soil fauna activity. One must also consider proper disposal after their useful life. These factors must be evaluated in the context of the goals of organic agriculture.
It is impossible at this stage to state which system is best over the long term, since the evaluation involves so many complex factors the means, the philosophy and ideals of the individual grower, the location, the soil, and the topography. However, from the perspective of growing apples organically, the use of a straw mulch seems to come closest to the ideal. Straw mulches add nutrients and organic matter to the soil and improve soil structure. Their use reduces erosion and conserves soil moisture. This stable environment and the availability of organic matter has also been found to favor arthropod and earthworm activity. In addition to the merits of a straw mulch to the system as a whole, it was found that trees in the straw mulch were the most vigorous, along with those in the geotextile mulch. Should this trend continue, it can be expected that trees in the straw mulch will begin to bear fruit earlier and have higher yields than those in the cultivation, manure mulch, red fescue, and insectary plant treatments. Visual observation throughout this study indicated that the straw mulch provided good weed control. Among the drawbacks to the use of straw mulch, is the increased susceptibility of trees to crown or root rots caused by Phytopthora cactorum, presumably because of increased humidity around the tree. Also, this habitat is conducive to increased rodent activity and is ideal for nesting, which may put the trees in greater danger of being damaged by winter bark feeding. This problem is usually alleviated by the use of distasteful paint and/or tree guards, which is standard practice for young trees in most North American orchards under all types of ground management systems. Another concern is that elevated levels of nitrogen have been found in trees under straw mulch and are implicated in increasing pre-harvest drop.
Based on early results of tree growth, a grower establishing an organic apple orchard on dwarf rootstock might consider using a straw mulch, geotextile mulch, or cultivation as in-row ground management. These treatments all promoted tree growth, and, depending on the means and objectives of the grower, can be considered realistic methods of organic orchard ground management.
The progress of the trees over the next few years will provide valuable information about precocity of bearing, yields, return bloom of the trees and growth rates. One of the most important tasks then will be to educate and wean conventional growers from herbicides to solutions designed for the long-term.
Sandra Salmins was awarded COGs Mary Perlmutter Scholarship in 1992 and is a student in the M.Sc. program at Macdonald College of McGill University. Deborah Buszard of Macdonalds Plant Science Department is her advisor. Brice Walsh provided them with information regarding analysis of soil nutrients and volumetric water content.
Copyright © 1993.Sandra Salmins and Deborah Buszard
Reprinted with permission. All rights reserved.
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