Crop Rotation on Organic Farms

Interplanting Crops with Partially Overlapping Growing Seasons

SARE Outreach
Charles L. Mohler, Sue Ellen Johnson | 2009 | 154 pages

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.

Table 7.1 Cover crop interseeding systems
Cover cropCash cropMethodBenefitsProblems
Red clover, annual or perennial ryegrass1Corn, late sweet cornBroadcast after last cultivation2Good organic matter production; N-fixation by red cloverEstablishment may be poor in a dry summer; annual ryegrass may winter-kill
RyeCorn, late sweet cornBroadcast in early fall at 3 bu/AReasonable establishment most years3Stand may be patchy; stand may be poor if the fall is dry
Annual ryegrass, creeping red fescue, red clover, white clover, alfalfa4Soybean (planted in rows)Broadcast after last cultivation2No interference with harvest; N-fixation by legumesEstablishment may be poor in a dry summer
Rye, winter wheat, speltSoybeanBroadcast at 1.5X usual rate at leaf yellowingAllows establishment in wet years and late harvested soybeanTiming is critical5; stand may be poor if the fall is dry
Crimson cloverSoybeanBroadcast at 20 lb/A at leaf yellowing N-fixation; good production Useful only from zone 6 & south – further north it winter-kills after little growth
Red clover, alsike clover, alfalfa, yellow sweet cloverWinter wheat, speltSow on frozen ground in early springGood organic matter production and N-fixation before next spring cropGround may not freeze sufficiently to support tractor
Bell bean6Fall brassicasPlant two rows of bell bean between crop rows after last cultivationN-fixation for next crop; grows fast and then winter-kills; upright growth does not interfere with cropExpensive seed. Cover crop will be damaged at harvest.
Annual ryegrassTomato, pepperBroadcast after last cultivation2Good dry matter production by next springMay winter-kill
Hairy vetchLate harvested vegetablesPlant 1 or 2 rows between rows of vegetables after last cultivation N-fixation for next crop; no interference with crop; spreads out to give fair winter cover and good spring productionNone apparent
RyeLate harvested vegetablesPlant 1 or 2 rows between rows of vegetables after last cultivationFalls over to give fair winter cover; no interference with cropNone apparent
RyeLate harvested vegetablesBroadcast at 2 to 3 bu/A 3 to 5 weeks before harvestProvides more uniform cover than drilling between rows3Stand may be patchy; stand may be poor if the fall is dry; interferes with harvest of short, leafy crops
Note: Only systems that cause negligible yield reduction have been included. All of these systems provide either a living or winter-killed cover through the winter. All resist damage during harvest of the cash crop unless otherwise noted, but some damage is inevitable.  
1Alfalfa, yellow sweetclover, crimson clover, birdsfoot trefoil, white clover, alsike clover and hairy vetch can also be established by this method, but fall cover and spring dry matter production tend to be less than for red clover, annual ryegrass or perennial ryegrass.
2Seed can be applied at cultivation by attaching a forage seeder box to dribble seed onto the ground behind the cultivator tools. It can also be spun on with an attachment or by hand. Seeding should be completed before the first rain after cultivation.
3Rye is the only cover crop that usually establishes well when surface sown in the fall. Spelt and annual ryegrass also have a reasonable chance of success. Other surface seeded cover crops usually either fail to germinate or are heavily consumed by seed and seedling feeding insects (species that are normally considered beneficials due to their consumption of weed seeds and pest insects).
4Wheat or rye seeded by this method will grow several inches above the height of the lowest pods by harvest.
5If the grain is sown too soon before leaf drop, grain establishment will be poor due to shading by the soybeans. If the grain is sown after leaf drop, many seeds will not make contact with the soil and those that do will not have the benefit of coverage by dead leaves.
6Bell bean is a small seeded variety of fava bean. It is preferred over field pea in this application because it does not fall over or twine into the crop.