When two or more crops are grown together, each must have adequate space to maximize cooperation and minimize competition between the crops. To accomplish this, four things must be considered: 1) spatial arrangement, 2) plant density,
3) maturity dates of the crops, and 4) plant architecture.
At least four basic spatial arrangements are used in intercropping. Most practical systems are variations of these.
· Row intercropinpg - growing two or more crops at the same time with at least one crop planted in rows.
· Strip intercropping - growing two or more crops together in strips wide enough to permit separate crop production using machines but close enough for the crops to interact.
· Mixed intercropping - growing two or more crops together in no distinct row arrangement.
· Relay intercropping - planting a second crop into a standing crop at a time when the standing crop is at its reproductive stage but before harvesting.
To optimize plant density, the seeding rate of each crop in the mixture is adjusted below its full rate. If full rates of each crop were planted, neither would yield well because of intense overcrowding. By reducing the seeding rates of each, the crops have a chance to yield well within the mixture. The challenge comes in knowing how much to reduce the seeding rates. For example, if you are planning to grow corn and cowpeas and you want mostly peas and only a little corn, it would be easy to achieve this. The corn-seeding rate would be drastically cut (by 80% or more) and the pea rate would be near normal. The field should produce near top yields of peas even from the lower planting rate and offer the advantage of corn plants for the pea vines to run on. If you wanted equal yields from both peas and corn, then the seeding rates would be adjusted to produce those equal yields.
Planting intercrops that feature staggered maturity dates or development periods takes advantage of variations in peak resource demand for nutrients, water, and sunlight. Having one crop mature before its companion crop lessens the competition between the two crops. An aggressive climbing bean may pull down corn or sorghum growing with it and lower the grain yield. Timing the planting of the aggressive bean may fix the problem if the corn can be harvested before the bean begins to climb. A common practice in the old southern U.S. cotton culture was to plant velvet beans or cowpeas into standing corn at last corn cultivation. The corn was planted on wide 40-inch rows at a low plant population allowing enough sunlight to reach the peas or beans. The corn was close enough to maturity that the young legumes did not compete. When the corn was mature, the beans or peas had corn stalks to climb on. The end result was corn and beans that would be hand harvested together in the fall. Following corn and pea harvest, cattle and hogs would be turned into the field to consume the crop fodder.
Selecting crops or varieties with different maturity dates can also assist staggered harvesting and separation of grain commodities. In the traditional sorghum/pigeonpea intercrop, common in India, the sorghum dominates the early stages of growth and matures in about 4 months. Following harvest of the sorghum, the pigeonpea flowers and ripens. The slow-growing pigeonpea has virtually no effect on the sorghum yield.
Plant architecture is commonly used strategically to allow one member of the mix to capture sunlight that would not otherwise be available to the others. Widely spaced corn plants growing above an understory of beans and pumpkins would be a classic example.
Traditional Corn-Bean-Squash Mixed Intercrops
Throughout Mesoamerica, a common intercrop of corn, beans, and squash is traditionally grown. Grown together, these three crops optimize available resources. The corn towers high over the other two crops while the beans climb up the corn stalks. The squash plants sprawl along the ground, capturing light that filters down through the canopy and shading the ground. The shading discourages weeds from growing.
This mixture was compared to the individual crops grown separately in a study near Tabasco, Mexico. In the study, corn yields were considerably higher in the mixture than in a pure stand planted at optimum densities. Bean and squash yields suffered considerable yield reductions when grown in mixture. In this example if corn was the most important crop, it was beneficial to grow it in mixture with squash and beans. The beans and squash were just a bonus.
Intercropping is sometimes called interplanting, and a special form of interplanting is called companion planting.
Interplanting involves growing two different vegetables in an area at the same time. This method of planting is another way to save space. Some gardeners believe that certain plants perform better when grown together. This type of interplanting is called companion planting.
Interplanting flowers and herbs among vegetables can add interest to a backyard vegetable garden that often looks too plain. Furthermore, the planting of flowers and herbs in the vegetable garden may attract beneficial insects that will help control pests.
To successfully interplant, consider plants with similar nutrient and moisture requirements. A common example is to plant radish and carrot seeds at the same time. Radishes germinate and grow quickly. They mark the rows of the carrots that take much longer to germinate and mature. Radishes are harvested as the carrots are just getting established. Leaf lettuce is a good crop to interplant among larger vegetables since it tolerates shade and has few pest problems.
Leeks may be planted among sweet potatoes. The sweet potato leaves help shade the developing stem. Surplus onion sets may be placed around other plants as a source of scallions. Young tomato plants or pole bean seeds may be planted among declining pea vines to replace them on the trellis. The spent pea vines may be used to mulch the new crop.