- different nutrient needs and therefore don't compete for nutrients
- grow in different spaces (above ground/ below ground) and therefore don't compete for space
- have different root systems (shallow/deep rooted) and therefore don't compete for root area
- take different times to germinate, grow and mature and therefore are not in the same area at the same time or for the same time.
Companion planting however takes intercropping a step further by also looking at positive synergies between different plants. Crops from high Nitrogen need can benefit from Legumes (beans, peas, etc.), which are Nitrogen fixing. Onions and carrots grown together both have a strong smell that can deter each others pests (onion fly and carrot fly), other plants have just through observation and experience shown that they benefit from each other in one way or another.
The table above gives a comprehensive overview of positive as well as negative effects of plants on each other. To read the table choose a plant from the rows down and check it against plants in the columns across. If the crossing square is marked with a + these the plant in the column has a positive effect on the plant in the row. If it has a - it has a negative effect on the plant in the row and should not be planted close to it.
- Avoid monocropping: Monocrops are an easy target for pests and diseases. They also all need the same nutrients from the soil and therefore compete with rather than support each other.
- Beneficial Habitats: By planting flowers and herbs among vegetables and tolerating some weeds and wildflowers in corners of the garden, beneficial insects and birds can be attracted, which are predatory or parasitic to pests and are the natural way of pest control.
- Deter Pests by intercropping: Strong smelling flowers, herbs or neighbouring crops produce odour, which confuse pests and deter them from finding your main crop by hiding or masking a crop from pests.
- Trap Cropping: Sometimes, a neighbouring crop is selected for its use as a trap crop. These trap crops have proven to be more attractive to pests than the main crop distract them off them. Nasturtium is a trap crop for aphids for example and Radish attracts leaf miners away from spinach.
- Symbiotic Nitrogen Fixation: Legume, such as peas, beans, clover, alfalfa and sun hemp, have the ability to fix atmospheric nitrogen and also benefit of neighbouring plants through symbiotic relationship with Rhizobium bacteria. Some plants like chamomile and comfrey are also able to make nutrients available from the soil, which would otherwise be locked up and not available to the other plants.
- Biochemical Factors like pest or growth suppression: Some plants exude chemicals from roots or aerial parts that suppress or repel pests and protect neighbouring plants. French marigold (Tagetes) e.g. releases thiopene, a nematode repellent, making it a good companion for many garden crops. The manufacture and release of certain biochemicals is also a factor in plant antagonism. Some allelochemicals can suppress the growth of some other plants and their proximity should therefore be avoided. An example for this is potatoes, which stunt the growth of many other crops.
- Physical Spatial Interactions: Tall-growing, sun-loving plants may share space with lower-growing, shade-tolerant species; plants, which take up mainly root space can be intercropped with plants, which take up space mainly above the ground. Their proximity might produce a beneficial microclimate for both crops.