Excerpt from the Popular Garden Series magabook Orcharding with permission from its publisher, BowTie magazines, a division of BowTie Inc. Purchase Orcharding here .
It can be useful to divide weeds according to their lifecycles: annuals, biennials and perennials. The annuals germinate, grow and go to seed in a single growing season. Annuals can be further divided into winter and summer annuals. Biennials sprout one year, grow and then go to seed in the second year. Perennials live for three or more years, either evergreen or dying back in the winter and regrowing from some vegetative structure like a bulb or creeping root each year. With all weeds, seed control is critical. You never want weeds to set seed, replenishing what is referred to as the “seed bank” in the soil. In the case of perennials, it’s also important that you not spread vegetative structures that can regrow.
Weeds do damage by competing with crop plants for resources: water, sunlight and nutrients. Weed competition is most damaging when fruit trees are young. Once fruit trees are established, weeds can be managed differently. In areas where water is scarce, weeds may always be a threat to the well-being of your fruit trees, but if summer rainfall is plentiful, you may not have to worry as much. Remember that weeds close to fruit trees can provide cover for rodents; voles in particular love to skulk through the weeds and girdle your trees.
A pre-planting reduction of weeds can be achieved by solarizing your soil. Solarization also kills some soil-borne pathogens. Solarizing should be done during the hottest time of the year. The ground must be bare and irrigated before it’s covered with clear plastic. The plastic is sealed at the edges with dirt and left in place for three to six weeks – longer if the weather is marginal. Some weeds will survive this ordeal, but the vast majority of seeds in the top two to three inches will be killed.
Cultivation is one of the oldest methods of weed control. Discing will certainly reduce weed competition, but there’s a serious downside to discing: Bare ground is vulnerable to erosion and will cause dusty conditions that can aggravate pests like mites. Also, shallow roots may be damaged. The unfortunate tendency of discing to spread perennial weeds that grow from little bits of root or stem is another problem. Anyone who has inadvertently sown their property with field bindweed knows the true meaning of despair.
For multiple fruit trees, since the area directly under a tree is the most critical for weed competition, a management approach that preserves cover on the floor while reducing competition is to keep the tree rows clear of weeds and allow vegetation to grow between rows. These aisles can be kept mowed and the plant cover will protect the soil. If nitrogen-fixing plants such as clover or vetch are planted, there will be an additional benefit in increased fertility. Insectary planting is also an option that provides habitat for beneficial insects while providing erosion control.
Mulching is an excellent way to suppress weeds; when weed seeds have no light, the seeds don’t germinate. Mulches also conserve moisture and prevent erosion and soil compaction. The downside is that purchasing mulch can get very pricey. Mowers that throw the clippings into the tree row provide a self-mulching service and sometimes you can find a free source of wood chips or other organic material. The caution here is to be wary of introducing diseases or pests; be sure you know what’s in your mulch.
Another sort of barrier to weeds is weed cloth. A heavy-duty, woven weed cloth placed in the tree rows provides excellent weed protection (shop for the commercial grade used by nurseries). The material is expensive initially, but since the material lasts for 10 years, the cost is pretty reasonable when you amortize it over a decade. Secure weed cloth well at the edges with turf staples; weed cloth doesn’t last very long at all if you chew it up with the mower.
Herbicides are often used to keep the tree’s soil free of weeds. In recent years, a number of less toxic herbicides that use active ingredients like acetic acid (vinegar), soaps, orange oil and clove oil have been developed. There’s generally more regrowth of weeds with these products than with conventional herbicides.