It draws in parasitic insects that feast on pests. Like most benificials, they need next to no care and do their job with no fuss. Companion planting books cover the subject well and you can apply tomato suggestions to Mary.

There's a long list of plants that help keep insects in balance and add nutrition to the soil. Each helps in it's own way.

Here's a link with some plants used for organic ferts.
http://boards.cannabis.com/organic-g...nutrients.html

Using cover crops like alfalfa, winter rye, buckwheat, and clover in outdoor beds helps conserve water, keeps soil healthy, and prevents soil erosion. Turning them into the soil before planting adds nutrients and other goodness.