Interplanting crops that share the field for only part of the season can increase the capture of sunlight over the course of the whole year. Expert panel farmer Drew Norman provides an example of this sort of intercropping. He transplants lettuce next to his tomato plants. The lettuce uses the space that is not yet occupied by the tomatoes and is harvested about the time the tomatoes are branching out to cover the full width of the bed. Usually, when two cash crops are intercropped, they are either planted at the same time or harvested at the same time. Mechanical planting may be difficult if the crops are not planted simultaneously. Without careful planning, harvesting the early crop may damage the late crop.
Nurse crops are another variation on staggered-season intercropping. Forage legumes and grasses establish so slowly that weeds tend to take over the field if a fast-growing, competitive nurse crop like oats, barley, or wheat is not there to use the available sunlight. After the grain is harvested, the forages continue to grow through the remainder of the season and are ready for a first cutting in the autumn.
Interseeding cover crops into established cash crops can increase cover crop productivity and the range of cover crop species that can be used in a region. For example, clover can be interseeded into corn during the last inter-row cultivation (96), whereas clovers sown after corn harvest establish poorly and have little time for growth before winter. Similarly, in New York and New England, hairy vetch sown after September 15 often winter-kills, which potentially restricts it to use following early-harvested crops. It can, however, be interplanted successfully into many full-season crops. For example, NEON collaborators Eric and Anne Nordell plant single rows of hairy vetch between rows of many types of late-season vegetables. Despite trampling during harvest, the vetch sprawls out and provides good winter ground cover and nitrogen for crops the following year.
Most cover crops will compete heavily with the cash crop unless seeding is delayed until the cash crop is well established. Consequently, cover crops planted into established cash crops usually produce negligible organic matter or nitrogen by the time of crop harvest. Attempts to use interseeded cover crops to smother weeds have generally shown that if the cover crop is sufficiently dense and vigorous to suppress the weeds, it also competes with the cash crop. Successful exceptions require subtleties of timing, sowing densities, and relative growth rates of the cash and cover crops that are difficult to repeat consistently. Thus, the primary use of interseeded cover crops is the early establishment of cover. Note that if the cover crop is left after the cash crop is harvested, cleanup of weeds after harvest is restricted to mowing or hand weeding. Consequently, interseeded cover crops are most useful in late-harvested crops or fields with low weed pressure. Table 7.1 shows opportunities and problems with interseeding cover crops into established cash crops.