Weed Management in Young Orchards
Orchard weed management is significantly more difficult in a young orchard, compared to one that is well established. The young tree casts very little shade, and the weeds thrive in the sun, irrigation and fertilizer that the grower is applying to optimize tree growth. Weeds grow rapidly under these conditions; especially during late spring and early summer, when small weeds can become larger than you can easily spray every two or three weeks. Zero, or very few weeds present in the band treated under the tree row for the duration of the season is not necessary and should not be expected.
Young trees have small, shallow root systems that are easily out-competed by weed roots. The bark on the young trees is green and vulnerable to damage by most contact herbicides, and the shallow root system may pick up some common soil residual weed control materials that are perfectly safe in an older orchard.
Therefore, the orchard weed manager is required to give extra time and effort to the control of weeds in the young orchard, especially the first season. The options expand as product choices and efficacy increase in the second and third season of growth.
First year: Protection of the young tree trunk is a very good practice. You can use tree wraps sold for this purpose, but I have also seen milk cartons or a stout paper products work well. These tend to break down and lose their protective abilities after about a year. Cheap, white latex paint is often applied, both to shield the trunk from herbicides and the effects of weather. The paint often flakes and cracks as the tree trunk grows, making it less effective after the first season. Since the trunk remains vulnerable for three to six years, or until a layer of corky bark has formed, the paint or wrapping is almost certain to need replacement before contact herbicides are entirely safe when sprayed directly on them. The smaller tree structure volume of modern dwarf apple trees and their shallow fibrous root systems make them more vulnerable to both contact and residual herbicide damage. This is especially true with glyphosate (Roundup, many other trade names).
So, the first season, after protecting the trunks, spray the weeds every time their growth approaches 4-6 inches in height or width. The young weeds are growing rapidly during this vegetative stage, especially in the spring, so don’t delay application. If you spray these young, vulnerable weeds on time, the lower rates of contact herbicides work well, and tree safety is more certain. Use adequate spray water volume (usually 40 – 50 gal./A) and a wetting agent to assure complete wetting of the weeds. Rarely, but too often, the herbicide contacting the young tree may penetrate the thin, green bark and kill the cambium layer, setting up a “canker” about 4 to 12 inches above the soil line. This damage is more common when low volumes of water carrier are used. The concentration of product sprayed is a key factor in the safety of contact herbicides that may contact the lower trunk. Rate per acre is stated clearly on the herbicide label, but guidance about concentration of the spray material is less obvious. The label often states a minimum volume of water per acre, but using even a moderate label rate of the herbicide in the minimum gallons water per acre may result in a concentration of spray material that will damage young tree bark. Applying contact herbicides mixed with liquid fertilizers will also increase the chance of trunk damage. This is especially true with glyphosate, which may penetrate the green bark and crown suckers of young trees, and cause serious damage to their growth, which may appear the spring after a fall application. Trees rarely recover from over-exposure to this very effective product.
Weeds, having been exposed to many applications of glyphosate in the past 30+ years, are beginning to show some resistance to this product, a situation similar to what we have been seeing for years in insects and disease pathogens. This has become a very serious problem in weed control in several areas of the world, especially the USA, where glyphosate resistant crops have encouraged the use of this herbicide exclusively for repeated applications on the same crop site over the past 10-20 years. This crop scenario is disturbingly similar to what we do in many orchards of Washington: use of glyphosate repeatedly as the sole weed control product. It is a wonder that serious glyphosate resistance has not developed, (if it hasn’t already.) It would be wise for the industry to use no more than one glyphosate a year, and then only when perennial weeds such as grasses or bindweed are present. That single application should be applied in mid-summer to early fall, so as to treat the perennial weeds while they are above the soil surface and vulnerable. Other times of the season, other contact herbicides can be used as necessary, as well as an ever-growing number of registered residual products. By rotating residual and contact herbicides of different herbicide classes, resistance to any product would be greatly slowed.
Good choices of contact herbicides that you may use in young orchards include Venue (pyraflufen-ethyl), Rely (gluphosinate-ammonium) or Gramoxone Inteon, Firestorm (paraquat). Other contact herbicides that are sometimes added to the mix to improve control of broadleaf weeds include Stinger (clopyralid) or 2, 4-D (Saber, Weedar 64). Do not apply 2,4-D and sprinkle irrigate within 10 days, as this product is mobile in the soil and may be picked up by tree roots, leading to tree and fruit damage. Golden Delicious, Gala and Fuji seem to be the most sensitive, but there is little experience with the many new cultivars being grown recently.
Residual Herbicides: These are herbicides that are often applied to more mature orchards to prevent the growth and establishment of weeds for a month or more after application. Many growers are reluctant to use residual herbicides in young orchards due to a bad experience they or their neighbors had in a young orchard years ago. In the past, it was risky to apply most of the available “soil residuals” in young orchards, especially those growing in light, sandy, low organic matter rocky soils, as they sometimes damaged the tree if they were picked up by the roots. Now, many new herbicides with far different modes of action are available, and many of these are quite safe when applied to young orchards. Some of these products work very well, and may greatly reduce the number of weed sprays necessary to keep the weeds under control. Most of these products work on germinating seeds, or new shoots and roots developing in the top inch or two of soil, and none of them are picked up by tree roots and transported to damage the tree fruit or shoots. Residual herbicides that are quite safe in young orchards include Prowl H2O (pendimethalin, K1), Matrix (rimsulfuron, B), Alion (indaziflam, L), Surflan (oryzalin, K1) and Goal, Goaltender, Galigan (oxyfluorfen, E). Read the label, because the years that must pass between planting and product application varies.
Page work in progress. (Oct. 2013 note) Come back for pictures of recent plot results.
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