Planting marigolds with tomatoes in your greenhouse can help repel whitefly from your tomatoes.  They produce a chemical called limonene, which helps repel whitefly naturally. This was verified in a controlled greenhouse study published in 2019 that can be found here.   Keeping your tomato plants free of disease is key, so the more you can do preventatively the better. Companion planting with marigolds is a grea...