Many companion planting ideas are questionable, but some are sound tried and established practices.
Leguminous plants (eg. peas and lupins) are known to have the ability to fix nitrogen (ie. take nitrogen gas from the air and convert it into a nutrient form which the plant can use). It is common in horticulture and agriculture to use legumes to 'feed' other plants.
Ideas on companion planting, a lot of which is folk-lore, are commonly criticised as having no solid scientific basis. Few companion planting techniques have been researched sufficiently for us to draw solid conclusions that the practice is a substantially effective cultural technique. Your own experience is your best source of knowledge.
Read all you can about the inter relationships between plants both in texts you have received with this course, and any other sources you can find. However do experiment yourself, as you will probably learn more about companion planting by trying it than you ever will by reading about it.
Although there is no scientific explanation for the effects of companion planting however companion plants are believed to work in several ways:
  • May act as a barrier to the crop
  • May camouflage the crop
  • May confuse insect pests
  • May attract insects away from the main crop
  • Produce exudes from the roots that appear to deter pest attack
  • Produce chemicals that repels pests or masks

Repellent Plants

Certain plants will repel insects or other pests from an area. This usually works by the aroma being released from the plant (as such a repellent plant might not work unless it is brushed or broken frequently and the smell is released).
Plants said to work in this way are:
  • Fennel for fleas
  • Peppermint for mice and rats
  • Wormwood for snakes
  • Pennyroyal for ants
  • Tansy for flies

Attractant Plant
  • These are plants which keep pests away from where you want them by attracting pests to the herb (eg. a nasturtium, grown at one end of the garden may attract aphis, keeping them away from plants at the other end of the garden).
  • Moths are attracted to some types of lavender
  • Hyssop attracts cabbage white butterfly
  • Marshmallow plants (ie. Malva sp.) attract harlequin bugs

Plants Which Affect the Soil
  • Plants can affect the soil in many different ways to create desirable or undesirable affects for other plants. For example:
  • Legumes such as peas, beans or lupins have colonies of bacteria on their roots which have the ability to take nitrogen from the air and convert it into a form of nitrogen which the plant can absorb.
  • French marigolds exude a chemical from their roots which deters the development of nematodes in the soil.
  • Garlic and other onion type plants will increase the level of sulphur in the soil in desirable forms, leading to some control over fungal diseases.
  • Mustard seed will give some control against powdery mildew when dusted on affected leaves.

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