'Lots of green' is key to companion plants

Companion plants need to have lots of green - be it foliage or stems - in order to effectively deter insects from crops, Warwick HRI's Dr Rosemary Collier has revealed.

Collier - speaking at the HDC's Vegetable Technology Day held at Warwick HRI's research station in Kirton, Lincolnshire, last week - told growers that this is one of the key findings of an ongoing LINK project investigating companion planting for pest control.

The project was launched after a number of studies found that growing crop plants in close proximity to non-host - or companion - plants often reduces pest colonisation.

The research - which is sponsored by Defra and whose project partners include Elsoms Seeds, R Fountain & Son, HDC, Marshall Brothers (Butterwick) and Warwick HRI - has so far focused on finding ways to control infestations of cabbage root fly (Delia radicum) on cauliflower crops.

Collier told growers that, in order to effectively reduce the probability that insects will settle on the crop plant to feed or lay eggs, the companion plant must: be green; have a height relative to the height of the crop plants; have quantity - lots of "surface" for the insects to land on; have depth; and be planted close to the crop plant.

Collier added that "leaf size and shape are not important - it's the amount of green that's important".

She also revealed that the companion plants do not work as repellents - the insects are simply "arrested" by them and spend a considerable time "sitting on the plant" doing nothing.

In order to come to these conclusions Collier said researchers have carried out various experiments to determine the characteristics of an ideal companion plant.

These have included cutting out different shapes and sizes of green card to see whether the insects are attracted to any particular forms.

Other experiments included planting one, two and four seeds in modules with the cauliflower, before transplanting them into the field, to see how well the brassica stands up to the companions.

Researchers have also worked with Elsoms Seeds to identify which species make the best companion plants - experimenting with plants such as chicory, tarragon and caraway.

Collier said: "We looked at 16 species in the first year, narrowed it down last year, and this year we have narrowed it down even further.

"Last year showed us that you do need a good amount of alternative green surface compared with your cauliflower - that's the key factor. This year that's what we are focusing on, with lettuce and carrot as our two preferred companions."

The next phase of the project is set to determine the impact of companion plants on natural enemies and other pests, such as diamondback moths.

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