The climbing crops benefit from the nitrogen supplied by the trees, from the support
the trees provide and are also protected from wind and direct sun. In addition the land
area is used to produce a crop of food at the same time as the trees are growing. Various
types of climbing beans are suitable for this.
Also known as nurse crops, these are two or more crops that are grown together in the
same field. At least one of these crops, and often more than one, benefits from the
presence of the other. Examples include onions, garlic and marigolds planted together
with vegetable crops to protect the vegetables from some harmful insects.
Carrots for example can benefit if onions or other Allium species are grown close to
them because the Carrot Fly is repelled or confused by the smell of the onions.
Similarly, many insect pests of tomatoes and cabbages are kept away if marigolds are
grown close to them.
Trap crops are similar in that they are planted specifically to attract certain harmful
insects; they are then ploughed under or destroyed as soon as this has happened. Trap
crops can also be used to stimulate Striga weed seed to germinate, the trap crops
themselves not being harmed by this weed (1L, pages 88–90).
Mixed Cropping (1Hb, page 66)—two or more crops which are grown together in
various systems of intercropping or multiple cropping almost always behave as
“companions” towards each other.
For example when maize and beans are grown together the maize benefits from the
nitrogen produced by the beans while the beans often (but not always) benefit from the
shelter which the maize provides, such as from hot dry winds, and vertical support.
When a crop provides protection to either a second crop or to the soil it is known as a
cover crop. For example, wheat or barley “covers” the more delicate grasses or clover
which are often planted under these cereals (the cereals are said to be undersown with
grass/clover), or grass planted under fruit trees “covers” and protects the soil from
leaching and erosion.
In the tropics Kudzu Vine (Pueraria lobata syn. P. thunbergiana, P. montana), a
hairy, leguminous perennial climbing plant, is often used as a cover crop, which is used
later as forage for animals. Once established, the vines can grow 20m in one season,
about 30 cm per day! It only grows well in warm areas & is ploughed in after 2–3 years.
In most climates, but especially in wet, dry or hot conditions, it is usually a good
idea to cover the bare soil to protect it from erosion and loss of nutrients. Either a mulch
or a cover crop can be used, though cover crops have the advantage over mulches of
“holding” or “locking up” soluble plant nutrients in the soil, which reduces the leaching
of these nutrients.
Cover crops are often ploughed under a few weeks before planting the main crop, in
which case they serve the dual purpose of cover crop and green manure (page 69).