For all crops, after tillage is completed and the soil is prepared for planting, the next crucial step is getting a good stand. In vegetable production that is accomplished by direct seeding or by transplanting. In California’s Salinas Valley, there are a number of crops that are now nearly 100% transplanted: annual artichokes, cauliflower, celery, leeks, and peppers. Other crops such as spinach, onions, and baby lettuce, planted at high seed densities, are 100% direct seeded.
Full-size lettuce is in between these extremes. It is not planted at such high densities that necessitates direct seeding, but in the Salinas Valley probably more than 95% is direct seeded. The reason for the extensive use of direct seeding of lettuce rather than transplants is due to various factors: lower cost, as well as perceived improved quality and shelf life. However, establishing a stand with a small-seeded crop such as lettuce is challenging and great care must be taken to avoid problems and pitfalls.
All direct seeded full-size lettuce is planted with air or belt planters using coated seed. Seeds are placed approximately every 2 inches. Depending on the variety, seed costs about $150 per acre, which is cheaper than the cost of growing and planting transplants. Growers use anticrustants sprayed over the seedline immediately after planting to help flocculate the soil and keep it softer and allow the delicate young seedlings to emerge and grow vigorously. Some of the commonly used anticrustants also supply a small quantity of nitrogen and phosphorus which act as a starter fertilizer for the crop.
Once the stand is established and the plants have two to three true leaves (about two to three weeks after emergence), the crop is ready to be thinned (photo 1). For many years, thinning was done by hand; however, in 2013 we saw the first widespread use of automated thinners that use a chemical spray (fertilizer or herbicide) to remove the unwanted lettuce plants and any weeds that may be there as well (photo 2). These thinners use a camera to view the lettuce stand and a computer to process the images and decide which plants to remove and which to leave (photo 3). There are now four companies that produce automated thinners; this technology came at a good time due to a significant labor shortage that also occurred in 2013.
A disadvantage of the automated thinners is that they require excellent seed placement in order to minimize doubles. Currently, doubles are difficult for automated thinners to detect and thin properly. However, the developers of the automated thinners and growers are working to resolve this issue and have made good progress.
A potentially significant advantage of the automated thinner is that the bed is not disturbed in the thinning operation. In hand thinning, when the workers hoe out unwanted plants, they create a divot that can undermine the unthinned plants. This soil is pushed back up to reshape the bed in a subsequent cultivation, but before the soil is returned, the undermined plants can be subject to wind whip which can reduce the stand or cause stunted plants.
Once thinning is completed, the stand of lettuce is basically at the same point that it would be if it had been transplanted. Given the length of the growing season of the Salinas Valley, direct seeding of lettuce allows for double cropping of vegetables. The use of transplants could help reduce the time to establish and grow lettuce, but at present this is not a primary concern of the growers.
However, in the future, factors such as the need to improve water and fertilizer use efficiency or the need to intensify the use of the land could tip the balance toward the use of transplants. At present, the emergence of the new automated thinner technology has strengthened the role of direct seeding lettuce. It is encouraging to see the emergence of a new technology that provides the growers with a way of coping with production challenges and helps them to grow healthy plants.