For many decades in certain parts of the old world it has been the practice to grow African or French marigolds of a certain type to repel white fly on tomatoes in greenhouses or outdoors. It has also been found that the tomatoes grow better and bear more fruit with marigolds around them.
Sow seeds late winter to mid spring 3mm deep at 21-24C in a good seed compost. Germination usually takes 5-10 days. When seedlings are large enough to handle, transplant into 8cm pots and grow on in cooler conditions. For use as a Nematode and Whitefly repellent plant between the tomatoes in the greenhouse border or outside after hardening off, two or three tagetes for each tomato should prove sufficient.