The following plants have reputed or known companion uses. Try them and see!
BASIL - Plant at the base of tomato plants to deter aphids.
BORAGE - Reduces the susceptibility of strawberries to fungal disease if planted amongst them. If planted amongst tomatoes, their flavour will improve. As Borage attracts blackfly, if you plant it where you have infestations then the blackfly will go to the Borage and leave your other plants alone.
CHAMOMILE - Good companion to cabbages and onions. Planting it next to ailing plants is alleged to cure them. Cut flowers will revive and last longer if Chamomile tea is added to the water.
CHERVIL - Radishes have a hotter flavour if Chervil is planted by them. Lettuces may be protected from ants and aphids if Chervil is planted by them. Chervil also repels slugs.
CHIVES - Plant amongst other plants to deter aphids, mildew and apple scab. Will deter carrot fly if planted amongst carrots.
CORIANDER - Repels aphids and carrot fly.
DILL - Beneficial to cabbages if planted near them. Its flowers attract hoverflies which prey on aphids.
FENNEL - Encourages hoverflies who eat aphids.
FEVERFEW - Grown near roses, it will deter aphids.
FOXGLOVE - Cut flowers last longer in water if Foxgloves are added.
LAVENDER - Plant with roses to repel aphids.
LUPIN - Planted near to Lupins, plants thrive because Lupins fix nitrogen in the soil.
ROSEMARY - Good deterrent to cabbage white butterflies and slugs.
RUE - Good companion to roses and raspberries.
SAGE - Good deterrent to slugs and vabbage white butterflies. Will also deter carrot fly if planted amongst carrots. Sage will stimulate the growth of Rosemary and Rue.
THYME - Good deterrent for cabbage white butterflies, slugs and flea beetles.
YARROW - If planted amongst herbs it will make them stronger and more disease-resistant.