Part 1: Why Improve Weed Management in Young Trees and Vines?

The investment in a new orchard or vineyard is large, and poor weed management can reduce the performance of a new planting for many years. Planning your weed management strategy is an important step in getting the most out of your investment.

Although most growers have a large dislike for weeds in general, it is important to understand the "enemy" and to focus efforts and expense for the biggest return.

Planting new orchards or vineyards requires time and labour when growers are busiest - usually the spring - and controlling weeds after planting may become a low priority. We've all heaved a sigh of relief as the last vine or tree is tamped in, rushing off to catch up on other work. But this is exactly when efforts should be focusing on keeping weed competition down.

The Goal: Weed scientists have been working to identify the Critical Weed-free Period for each crop. For annual crops this time is early in the season and surprisingly short. This concept is easy - if a crop is kept weed-free during its Critical Period, no yield reduction from weeds will result. Weeds growing within the crop before and/or after this Critical Period will not affect yields.

Critical Weed Free Period in Fruit Trees: At Harrow, the effect of weed competition for 3 months after planting was measured. For all tender fruit species, the best growth (measured in trunk diameter) occurred when herbicides gave complete weed control through the whole time. Hand weeded checks had small weed growth removed by hoeing every couple of weeks, but even that little competition reduced trunk growth. And where weeds were allowed to grow (from May to July), growth was severely reduced.

Research by Dr. Ian Merwin at Cornell University has shown the effects of weeds in new orchards. In a Gala/M26 planting, first cropping 2 years after planting, yields were much larger when weeds were controlled early (May) in the planting year, and as well, when weeds were controlled longer (May to July) in the planting year. Weed competition occurred even under a full trickle irrigation schedule. Note that these yield effects were 2 years after the weed competition occurred!

Early weed competition will also have an effect on fruit size. In Dr. Merwin's research with Gala (where size can be a concern), controlling weeds during May and June, as well as during the pre-harvest month of August, gave better fruit size in the first crop year - again, 2 years after the weed competition occurred.

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Critical Weed Free Period in Grapevines: Research by Dr. Tom Zabadal at Benton Harbour, Michigan showed a large increase in growth (measured by cane prunings) by controlling weeds in the planting year. This effect was still measurable by the end of the 2nd season. Adding extra nitrogen or irrigation could not produce as much growth as removing the weeds. (If you have good weed control, adding nitrogen and irrigation gave extra growth as well, but not when weeds were present). Dr. Zabadal found that wider weed-free strips under the vine promoted significantly greater vine growth - up to 2.5 m (8 ft) wide!

Economic Effects: In perennial crops like fruit trees and grapevines, the "yield" effect may not be
apparent in the first year. But economic studies of new plantings show that yields early in the vineyard or orchard life are much more valuable than yields in later years.

As growers, we recognize that there may be reasons other than yield to control weeds at other times e.g. to facilitate harvest or to attract PYO customers. But to maximize your dollar return in yield, weeds need to be controlled only for this short period.

The strategy: Once we recognize the importance of controlling weeds during the Critical Weed-free Period, we can begin planning how to achieve the weed management needed. Watch for a series of articles in this season's newsletters to outline practical steps to achieve the weed management you need.

Related Links

• Part 2: Improving Weed Management in Young Trees (Preplant/early spring)
• Part 3: Improving Weed Management in Young Trees (Spring/early summer)
• Part 4: Improving Weed Management in Young Trees (Late summer/fall)
  

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