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Crop competitiveness

Except for a few very weakly competitive crops like carrot and onion, most weed control comes from competition with the crop rather than from weeding by the gardener. You can easily demonstrate this for yourself by leaving two patches of ground unweeded for a few weeks in the middle of the season - one in the crop and one that is left unplanted. Because crop competition is an important element of weed management, competition from the crop should be encouraged.

Dense planting

High crop density provides early canopy closure and more effective competition against weeds than a sparse planting. Any crop that makes multiple units of produce on a single plant (e.g., squash, beans, tomatoes) and most leafy greens (e.g., chard, collards, kale) can be planted at higher than recommended rates without yield loss. Each plant will yield less, but the yield of the overall planting will change little provided the high density does not encourage the spread of disease (evaluate where your problems lie!). Root crops will tend to make small roots if planted too closely, although surprisingly dense plantings are possible if soil tilth and fertility are high.

Solid seeding

For many vegetables with small plants like carrots, parsnips, leaf lettuce etc. seeds can be sown in continuous beds of a convenient width for your arm rather than in rows. This fills the space up with crop plants allowing less room for weeds. Carrots, for example, are usually considered to be a relatively noncompetitive crop, but once the foliage is well established on a continuously sown bed of properly spaced carrots, emergence and growth of new weeds becomes difficult.

Solid seeding generally precludes hoeing. Vegetable crops that make small plants usually have to be hand weeded within the row, however, even if they are hoed between the rows, and depending on the spacing between the plants, solid seeding may not require extra hand labor. Solid seedings should always be reserved for relatively weed free parts of the garden.

Use competitive varieties

Many factors enter into variety selection. Most gardeners consider produce quality, yield, and disease resistance more important than ability to suppress weeds. Nevertheless, as you evaluate varieties note whether the variety has features likely to suppress weeds. Among these are vigorous early growth, speed of canopy closure, height, and foliage density. All crop species vary in capacity for weed suppression among varieties.

Use transplants

Most annual weeds have very small seeds and consequently they establish relatively slowly. If the crop is also relatively small seeded (e.g., cole crops, lettuce, tomato, leek etc.) then growing the crop in weed free soil and transplanting after the plants are well established gives the crop a substantial head start over the weeds. Few professional organic growers direct seed small seeded crops that tolerate transplanting. If the bed was prepared before the transplants are ready, the surface should be stirred vigorously to a depth of 1-2 inches to kill any weed seedlings before transplanting. This should be done even if seedlings have not yet emerged so as to give the transplants the maximum head start over the weeds.

Planting date

Many of our vegetable crops have their origins in the tropics or subtropics (e.g., corn, beans, squash, tomato, peppers etc.). Consequently, they grow most vigorously when the soil and air are warm. In contrast, every season has weeds that are well adapted to the prevailing weather conditions at that time of year. Consequently, pushing warm season crop species to get an exceptionally early or late harvest puts the crop at a disadvantage relative to the weeds. You may find harvesting warm weather crops outside of their usual season worth the additional effort, but expect to do extra weeding.

Intercropping

Another way to increase the competitiveness of crops is to plant them in mixtures. Some mixtures may be less competitive against weeds if the crops compete more with each other than they do with the weeds, so the mixtures must be chosen carefully.

-- Radishes grow rapidly whereas carrots establish slowly. Consequently, the early crop of radishes in a mixed sowing helps suppress weeds but leaves space for subsequent carrot growth once the radishes are pulled.

-- Lettuce is harvested much sooner than tomatoes. Consequently, a row of lettuce next to a row of tomato plants competes with the weeds but is harvested before it can compete with the tomatoes. Note that many other crop combinations could be substituted here (e.g., spinach for lettuce and Brussels sprouts for tomatoes).

-- Sweet corn, although it is tall, lets a lot of light reach the ground and this allows weed growth. A strategy for improving late season weed control is to allow winter squash or pumpkins to run in under the sweet corn to compete with the weeds. Squash or pumpkin yield will be greatly reduced (e.g., by 1/2 to 3/4) under the corn so the total planting should be increased accordingly.

-- Finally, where skips occur in a row due to poor emergence or early death of the crop, some other crop should be planted to fill in the gap. If you do not have a crop that fits appropriately into the space, sow a rapidly growing cover crop like oats or buckwheat in the gap (Summer cover crops).