So There are Weeds in Your Orchard Now - Are They Important?

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Many orchards have weed escapes by mid-summer, and growers want to know when and how to "get rid" of them. We have many tools to control of these weed escapes at this time, but perhaps a better question is: Should we spend much effort to eradicate weeds from the orchard floor at this time in the season?

Why control weeds?

The main concern with weeds is their competition with our crops for moisture and nutrients. Some additional concerns about weeds in orchards include:

• hampering harvest operations

• reducing fruit quality on low hanging branches

• harbouring insect and disease pests

• increasing weed seed bank for future years

• encouraging rodents

Remember that weeds can have some beneficial effects as well:

• maintaining soil organic matter (OM), to improves structure and fertility

• tying up N and releasing it for next year

• protecting soil from compaction

• reducing erosion

• providing refuge for beneficial insects

Dr. Ian Merwin at Cornell University in New York has been researching some of these questions in apple orchards for the last 15 years.

When to control weeds?

Gala apples on M26 planted in 1991 were followed for 5 years. Controlling weeds during May and June (60 days) gave the best growth and yields (both early and total). Waiting until July and/or August to clean up weed escapes reduced tree growth, early cropping and total cumulative yield.

Where to control weeds?

Previous research in the UK and US had reported that increasing the weed-free area under trees increased growth and yields in apples and peaches, peaking around 8 to 10 m² per tree. However, these were not irrigated. The trees in Dr. Merwin's experiments were irrigated, and he found that 2 m² per tree gave the same yield as trees with a weed-free area of 6 m².

How to control weeds?

Ian Merwin and Warren Stiles tried several systems to control weeds. In 1986, an orchard was planted with Empire & Jonagold/MM111 with these ground cover treatments:

1. glyphosate in May and July (Roundup)

2. preemergence herbicides in May (Gramoxone + Solicam + Karmex)

3. mowed sod

4. mechanical tillage each month

5. crown vetch cover crop

6. hay-straw mulch applied each May

Much information has been learned from this study that was continued until 1994. Here are some of the weed control observations:

• Mowed grass sod (Treatment 3) reduced yields compared to pre and post herbicides (Treatments 1 & 2). (50kg/tree compared to ~80+ kg/tree)

• Pre and post herbicides (Treatments 1 & 2) gave similar yields over the 6 years. Note that using only 2 applications of glyphosate produced many weed escapes by late summer and through the fall. (40% cover on soil by winter)

• A 1.5m (5 ft.) wide weed-free strip gave the same yields as a 2.5 m (8 ft.) wide strip

• Although weed control and yields were good in mulch plots, 25% of the trees died of Phytophthora root rot or rodent damage

• Moderate weed competition late in the season can improve fruit quality.

What can we learn from this research?

• Focus weed control efforts early in the season. Weak weed growth in late summer and through harvest has many desirable effects.

• Weed-free strips under trees can be reduced in some orchards, especially where irrigation is used. However, a weed-free area under trees is needed because mowed sod is very competitive with trees.

• Several weed management systems can achieve the desired results, including pre and post herbicides or mulch systems. The key is to minimize any negative effects e.g. rodents, diseases, spray drift, water quality.

• Weed escapes in late summer may be a sign of good management - using weeds to protect soil and water quality, harbour beneficial insects, and produce firmer and highly coloured apples while encouraging trees to harden off for winter.