Integrated weed management (IWM) is the holistic approach to weed growth and suppression in the garden. IWM recognizes that the complete eradication of all weeds from the garden now and for all time is not only impossible but probably not desirable.
What Weeds Are
Weeds are plants growing any place in the garden where you don’t want them to be; weeds are pest plants.
Most weed species are fast-growing and abundant re-seeders. Weeds—like flowers and vegetables—may be annuals, biennials, or perennials. Annual weeds germinate, grow, and re-seed in one season; biennials germinate and grow the first year and re-seed the second year. Perennial weeds live more than two years and commonly reproduce not only by seed but by roots, stems, and stolons. Annual and biennial weeds are generally shallow-rooted and can be slowed if not allowed to flower and set seed. Perennial weeds are deep or long-rooted and can be especially difficult to be rid of.
How to Manage Weeds
Keeping the weed population to a minimum is important not only for overall garden appearance but also to ensure that the vegetables and ornamental plants thrive. Here are some suggestions for dealing with weeds:
• Weed tolerance. You will never rid your garden of all weeds. Decide how many weeds you can tolerate and how you can turn their presence to your advantage. Know the problem: What weed species are growing in your garden? (Identify them and make a list. Know their life cycles and habits.) How aggressive are they growing and spreading? Where are they growing? How much damage are they likely to cause other plants? After you answer these questions, you can begin to manage weeds, not let them manage you.
• Habitat modification. Weeds can be managed with the modification of the growing conditions which allow them to thrive—habitat modification. Where weeds thrive you can limit the availability of water, nutrients, and sunlight. You don’t have to nurture weeds; you can manage them.
• Irrigation. Drip irrigation or directed watering of vegetables will eliminate the widespread availability of moisture to weeds. Avoid overhead watering which spreads moisture across the garden—not just where it is needed. Use a drip irrigation system that places water in the root zones of edible plants. There will be little moisture left over for weeds.
• Grow on mounds. If you don’t use drip irrigation, plant vegetables on raised soil mounds, and irrigate via furrows between mounds. You can easily cultivate furrows and the edges of mounds to be rid of weeds growing close to crops.
• Fertilizers. The application of fertilizers or plant nutrients around crops rather than across a garden bed will limit the nutrients available to weeds. Sidedress crops with compost or fertilizers—place fertilizer close to crop roots and stems. Avoid using more nitrogen than necessary; many grassy weed species thrive on nitrogen.
• Sunlight. Limit sunlight available to weeds by closely planting crops and covering unplanted soil with mulch. Intensive planting allows weeds to be plucked or cultivated out of the garden early on. As intensively planted crops mature they will touch leaves and shade out new weeds.
• Mulch. Weed seeds find it difficult to germinate if mulch keeps sunlight from reaching the soil. Apply light mulch such as straw 6 inches deep; apply heavy mulch such as leafmold 3 inches deep.
• Focus. Focus your weeding efforts on weeds that exceed your tolerance level. Suppress weeds that threaten vegetables and other cultivated plants rather than all of the weeds in the garden.
Benefits of Some Weeds
Weeds can serve a purpose; here are benefits weeds offer:
• Soil fertility and water holding capacity. Perennial weeds such as thistles, nightshades, and pigweeds are deep-rooted and penetrate the subsoil. Root penetration breaks up the soil and allows water and nutrients to move up and down through the soil. Moisture seeps down and phosphorous and potassium are drawn to the surface for use by other plants.
• Habitat for beneficial insects. Weeds in the sunflower (Asteraceae), parsley (Apiaceae), and mustard (Cruciferae) families are nectar sources for beneficial insects. Most weed flowers are shallow throated making their pollen and nectar easily available to beneficial insects.
• Trap crops. A border or row of weeds along a garden will attract pest insects that otherwise might concentrate in the garden and harm crops. You can encourage weeds that attract pest insects. Keep weeds lush and irrigated to attract insects, but cut back the top of weeds before they flower and set seed. Monitor weedy trap crops for insect eggs and pest insect larvae; handpick or spray pest insects in trap crops before they move into the garden.
• Soil protection. Low growing weeds that compete poorly with vegetables can protect the soil from drying out. As long as weeds are not competing with crops for moisture and nutrients, they can hold the soil in place and keep the sun from baking the soil.