Tree Fruit Leader, Vol. 5(2) Nov. 1996

How Effective is Your Early Season Weed Control?

by Eugene Hogue, Pomologist, Agriculture & Agri-Food Canada, Summerland Research Centre 
and Mike Sanders, Tree Fruit Specialist, Kelowna

Replanting on old, tired soils is a very difficult task leaving very little margin of error. Very often weeds are one of the main contributors to poor tree growth.

In the last edition of the "Tree Fruit Leader" some research from New York and Summerland was referred to. This suggested that for optimum tree growth, weed control must start early in the season and that even a few weeds interfere with tree growth.

To test these contentions some weed control demonstration plots were organized. The work was done by Michael Andres, a summer student funded by the OVTFA.

Twelve growers in the Kelowna area agreed to cooperate in this project. At each site, ten trees were selected for weekly spraying beginning around May 1, and 10 trees were selected for weekly spraying beginning around June 1. These trees and 10 trees that received the growers herbicide program had trunk calipers measured at the start of the season and again on August 16. Increase in trunk caliper is considered an accurate measure of tree growth.

Four of the plantings that had predominantly whips planted had total terminal growth measured in August. The results indicate that there were no statistically significant differences in growth response due to weekly weed control from either early June or early May (Table 1). However, the data shows a clear trend of increasing vigour with trees weeded weekly from June lover grower treatments, usually 4 weeks or more between treatments, and of trees weeded weekly from May 1 over those weeded weekly from June 1.

The lack of statistical difference may have been due to several factors. First, the overall vigour in fields chosen was not generally high. The biggest differences due to weed control practices would be expected when other factors were not limiting. Secondly, in the design of the experiment, the number of trees used was too low to allow us to show the relatively small differences occurring over the abbreviated testing period. Natural variability will mask differences. Thirdly, the weed control programs in the grower plantings chosen were generally quite good, and although our weekly treatments had a clearly visible effect on the appearance of the orchard floor, most of the growers did not allow excessive competition to occur.

We are satisfied that these demonstration trials have shown that maintaining the area around newly planted apple trees on dwarfing rootstocks free of competition is clearly beneficial. The trials also demonstrated that keeping weeds out of a new planting in the early season is difficult when irregularly using only non-residual chemicals like glyphosate or paraquat. To obtain weed free conditions from planting through to mid to late summer, the critical growth period, therefore, it is worth considering the use of residual herbicides.

TABLE I: The effect of Maintaining Area around Newly Planted Apple Trees on Early Growth.

Only 4 sites where whips were planted were used for these ^omeasurements. o NS - not significant at 5% using ANOVA.

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