Marigolds planted with tomatoes dose keep nematodes at bay.
African marigolds in particular. Tagetes and other marigolds will help stop eelworm and other pests.
You could also plant nasturtiums as these will deter whitefly.
It must be said that there is no scientific evidence that these plants, and others, do what is claimed, but people seem to think they do help.
It is probably due to chemical diffusion given off by the plants.
I believe just plant in and around the tomatoes and consider the size of the plant that you are planting as to allow space for the tomatoes to develop and not get crowded out. Regards.

Hi all, I forgot to list the type of Marigolds I planted and where I got them. The Marigolds in the picture were from:

French Dwarf (Tagetes Patula) Bonzana Blend from Botanical Interests.

They are suppose to grow to 8" - 10" tall, I think mine are over 12" and still growing but this may have to do with the Tomato plant food I used with 12-10-5. The 12% nitrogen I think is causing more leaf growth then would be the case with less nitrogen. In any case I really like the way the Dwarf Marigolds look and hope they help with the nematodes.

Take care, DC

Here is a link that might be useful: Bonzana Blend

Hi all, I ran across a web page that goes into more detail on the different Marigold species and nematode control then any I have seen before. Check it out at the link below.
---cut from the web page---
"Research has shown that marigolds have nematicidal
properties, and can help to manage root-knot
nematodes when planted as a cover crop.
The nematicidal compound released by marigold
roots, a-terthienyl, is one of the most toxic
naturally-occurring compounds found to date
(Gommers and Bakker, 1988). The active
ingredient of an effective synthetic nematicide,
metam sodium (Vapam), is related to this chemical."

Hope this helps, DC

Here is a link that might be useful: Lot's of Marigold info

I believe when they talk about a "cover crop", they are not talking about growing them along side your tomatoes.

You would grow a cover crop when you are not growing tomatoes, then incorporate that crop into the soil before you plant out your tomatoes.

Corrie

I believe the best way to rid the nematodes is over time (sorry- not what most people want to hear)- plant your marigolds heavily -close to the tomatoes, and then at the end of the season turn them into the soil. Repeat yearly.

I had a root magot/nematode problem awhile ago in my garden. I bought some marigold seeds that said in their description that they were the best marigolds to plant for protection.

They did the job! I planted a ring of marigolds around everything. The only problem was that I was expecting the marigolds to grow about 8 inch tall marigolds and these special marigolds grew the entire season and actually got as big and bushy as my tomatoes. It made picking tomatoes a pain and my garden looked like an overgrown wildflower meadow, but the plants were completely protected.

I also heard that a lot of the chemical that works against the nematodes are in the flower buds, so I hand picked bags of the flowers off and threw them into the garden and tilled them in at the end of the year. I haven't had a problem since and the marigolds didn't hurt anything. I had lots of beneficial insects and worms.