For those of us who use soil as a foundation to produce a product, weeds are our nemeses. No matter how hard you knock them down, those seemingly unbeatable rivals keep coming back for more. They are our competitors and the bottom line is we must control them if we want to have satisfactory crop yields.

There is no doubt that weeds can have a negative effect on yield. Left to their own devices, they will consume a cultivated patch of land and gorge on its nutrients, water and sunlight, leaving your tender, weakened crop plants behind in a shroud of shade. Weeds are simply stronger competitors than plant cultivars, but this doesn’t mean you have to be out there tackling weeds every other day from time of transplant to harvest. Every crop plant has a specific stage in its growth during which competition from weeds has the greatest impact on yield. If a plant does not have to compete with weeds during what’s termed as the critical weed-free period, yield will generally not be affected. For example, the critical period for carrots is the first three to six weeks postemergence. A carrot that competes with weeds during this period will exhibit a significantly reduced growth rate, but weeds should not significantly affect a carrot’s growth rate before and after this period. Which is not to say weeds should be ignored before and after the critical period. Weeds with a strong foothold are more difficult to pull, hoe or till. They could also harbor pests and disease, hamper harvesting efforts and contribute to the soil’s weed seed bank if allowed to go to seed.

Table 1. Examples of Critical Weed-Free Periods for Various Horticultural Crops

Crop Critical Weed-Free Period Location
Cabbage 2 to 4 weeks after planting Wisconsin
Corn 5 weeks after emergence Turkey
Chili Pepper .7 to 3.2 weeks after transplant Mexico
Organic Sweet Potato 2 to 6 weeks after transplant North Carolina
Peanut 3 to 8 weeks after planting North Carolina
Pickling Cucumber 0 to 4 weeks after planting Ontario
Radish first 3 weeks after planting Iowa
Snap Bean 2 to 3 weeks after emergence Jordan
Tomato (direct seeded) 0 to 9 weeks after planting Ontario
Watermelon 0 to 6 weeks after transplant North Carolina

This is a pretty interesting and important discovery that has been extensively studied with corn, soybeans and cotton, and increasingly so with vegetable crops, including cucurbits, brassicas and members of the nightshade family (see Table 1). It is even being explored in apple orchards. What has been learned, and what comes as no surprise given the dynamic nature of producing fruits and vegetable, is a crop plant’s critical weed-free period is not set in stone. It varies according to a number of factors, including planting date, weather, soil type, dominant weed species and even tillage systems. It also varies according to your level of acceptable yield loss, or what percentage of yield you’re willing to sacrifice to the weeds. There is an economic threshold below which it is not cost-effective to control weeds based on the estimated loss of produce and the cost of weed control. The three to six-week postemergent period I mentioned for carrots was based on a 5 percent yield loss (i.e., a yield loss of more than 5 percent should be expected if your carrot crop is not kept free of weeds during the critical weed-free period). Five percent is the most common threshold quoted in the literature, although anywhere from 1 percent to 10 percent is used.

Knowing the critical weed-free period for the crop(s) you grow can help you optimize your time end energy when it comes to weed control. Although investigations into this concept were done so as to focus the timing of herbicide applications and consequently reduce expenses associated with herbicides, it is a concept that every organic grower should be reading about and exploring on his or her farm. Without synthetic herbicides, organic growers are dependent on hand-pulling weeds, hoeing, tilling and other cultivation methods for weed management. Tilling too frequently can stimulate microorganisms, leading to the rapid breakdown of organic matter. It can affect the soil’s weed seed bank with respect to its dynamics, and it can influence the depth at which these seeds can be found. It can also harm the soil’s tilth, break down soil aggregates and make the soil more prone to crusting. This in addition to the costs of labor and fuel associated with tilling. For many decades we have known how over-tilling can have negative effects on soil quality. Knowledge of when a crop’s critical weed-free period occurs will aid in reducing the amount of time and money spent on tilling and optimize weed control for the betterment of your soil and crops.

The early growth stage of weeds, the white thread stage, is when weeds should be targeted. At this point, the weed seeds have germinated and begun growing, but they have not broken through the soil surface and consequently are not visible from your vantage point. When weeds are in this early stage of growth, they are much easier to control with respect to time and energy. Cultivating very young weeds will also reduce the chance of harming your crops. This stage occurs roughly five days after planting, and shallow cultivation will uproot the young seedlings. By cultivating the soil in a shallow manner, you will also avoid bringing seeds buried deep in the soil to the top.

Shallow cultivation or tilling should not be the only tool up your weed-control sleeve. Rotating your crops annually will prevent weeds from adapting to a specific crop’s growth cycle and its management. Cover crops are strong weed competitors that will also enhance soil tilth when incorporated into the soil. Mulch, whether plastic or straw, is also a well-known option for smothering weeds. What you don’t want to do is till your soil to death. An organic system is characterized by diversity, not only with respect to what crops you’re growing, but also with how you manage those crops and the soil they are rooted in. The more tools you have up your sleeve, the healthier and more productive your growing system will be.

The author is a biologist who lives and farms in Vermont’s Northeast Kingdom.