Question about natural control of aphids

We have a small organic community garden plot in Berkeley, California. Lately we have been experiencing ever-increasing numbers of aphids which are doing considerable damage to our winter greens. We've tried spraying them off with a high-pressure nozzle, hand-picking them, and just recently, applying a mild soap solution. Can anyone recommend any other successful strategies for dealing with these critters, or ways of maintaining our plot that will make it less desirable for them to inhabit?
 
Thank you!
Paul  Revier
 
Paul,
Aphids seem to be attracted to stress of one kind or another.  In community gardens where space is limited, stress is often from over-crowding. If spacing is the problem, it can often be overcome by removing/thinning every other plant or even more. Over-watering, under-watering, and soil deficiency are other stress inducers, but overcrowding is the most common in a community garden.  Remove the stress and the aphids will usually disappear.  When   planting winter greens put yourself in the seed's shoes.  How large do you want to be when you grow up?  Give the seed enough space to become all that it can be and it usually will.  When the plant is growing whenever the leaves touch the neighbor plant's leaves, try removing the neighbor plant.  Roots don't like to intrude on another plants root zone...very stressful for a brassica. 
 
Add a good amount of compost and/or a brown/green mulch around all the plants. The mulch will retain moisture and will feed plants when decomposed.  Add mulch layers regularly.
Wishing you bountiful cabbages,
Jon Rowley
Interbay P-Patch
Seattle
 
 
 
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