Thyme - Attracts bees and deters cabbage worm so plant near members of cabbage family. Beneficial effect on plants near it. Valerian - Excellent addition to compost heap as stimulates phosphorous activity in soil. Earthworms love it. Wormwood - Grow by itself edge of garden as inhibits growth of other plants.
Spray discourages slugs and snails.
Yarrow - Increases essential oil content enhancing aromatic quality and making more resistant to insect attack.