Andrew, this is all I was able to find about herb/herb companions:

Basil should not be planted near rue as they inhibit each other's growth.

Chervil grows well with dill and coriander.

Chives and parsley grow better when planted together.

Dill and fennel like each other, but have a tendency to cross-fertilize.

Rosemary and sage grow well together.

Rue, as well as not being planted near basil, should also not be planted near sage.

Fennel should not be planted near caraway or coriander.

Try this link - its not a bad list and does included herb to herb companions.
http://www.gardenguides.com/TipsandTechniques/herbc.htm

Linda

What's the science behind the companion planting theory? Do the plants enjoy similar soil, climate, water etc? Are there pest repelling benefits?

I'm no expert on companion planting, and there are whole volumes written on the subject. But in brief (and I don't pretend to be scientific about it!) there is a belief that certain plants do better when grown together. Sometimes one plant will keep harmful insects away from the other, sometimes one will bring useful insects to help pollinate the flowers of the other. Basil, for instance, attracts bees, which are needed to pollinate tomatoes, and so they make good companions. Sometimes you will have one plant some distance from another to act as a decoy. Garlic chives will attract aphids away from roses. Often you'll find that companion plants are members of the same family, and like the same conditions. Other times you'll find plants growing together that balance each other in their uses of nutrients - one may use a lot of nitrogen, the other might provide it.

Some act in other ways to be good companions. For instance, Anise deters pests from Brassicas by camouflaging their odour and improves the vigour of plants grown nearby. It deters aphids, fleas, reduces cabbage worms.Coriander and anise seeds sown together will germinate more quickly. Borage accumulates silica and potassium, of great use when growing flowering and fruiting plants such as Broccoli, Brussels sprouts, Cabbage, Cauliflower, Cucumbers, Pumpkin, Tansy, Tomatoes, Rose, Squash, Strawberries. Chillies help plants which are affected by root rot and other Fusarium diseases. French marigold kills harmful nematodes. Horehound improves the fruiting of tomatoes. Yarrow is a good companion plant for most plants because it provides a lot of nutrients for them, especially those will need to produce their essential oils (the highly perfumed plants like lavender and basil, for instance).

Yarrow is one of the so-called Physician Plants. Grown near a sick plant, it helps it to recover. Thyme is also a Physician Plant. So is marjoram/oregano.

Some plants will help prevent grass and weeds from growing nearby. One such is Arrowroot, another is comfrey. Comfrey also is an excellent companion palnt because it keeps soil rich and moist, accumulates calcium, phosphorus, silica, nitrogen, magnesium, potassium, and iron, acts as a trap for slugs and because the rotting leaves make good fertiliser.

I need to check this, but I think it's coriander which helps prevent fennel from seeding too much and becoming a weed, which it's otherwise likely to do. Fennel has the reputation for being the overall worst companion plant of all - it should be grown well away from other plants. It seems to inhibit the growth of many plants. Coriander (if such it be) is about the only plant which helps to control fennel.

There are definitely some scientific reasons for all these things - some answers are already provided. I have heard that elder is a bad companion plant because the roots produce some substance or other which kills off competition plants - and I have found it true that nothing much...

Fennel will cause Coriander to bolt and generally isn't good for much as a companion.

Ther'es an old saying 'plant fennel near your kennel.' It is good for keeping fleas away and the dried leaves can be put in the kennel itself. Medicinally its supposed to help with eyesight and particularly cataracts by making an eye bath with fennel seeds boiled in water and kept sterile. I use it in the kitchen when cooking fish - just put a sprig on top of the fish and it will absorb the smell and generally deororise cooking smells in general. It has so many uses its worth giving it some space, but I've actually grown it near my roses with no ill effects. Avoid putting it anywhere near beans or tomatoes though. Beans just do not flower near fennel.

Borage is fantastic for strawberries. I used to farm strawberries commercially ( 350,000 plants) and always found the better fruits were near to borage. I allowed the borage to seed and it gave a lot of value. The borage flowers are lovely and make an interesting addition to salad and the leaves are OK when young, steamed like other green veggies.

There is no really provable 'scientific' understanding of companion planting, it is mainly lore and word of mouth experience. I study and use the astrological 'doctrine of signatures' which makes loose sense in planting plants that are sympathetic or antipathetic but much of that is based on Medieval apothecaries reasoning when the doctors were also astrologers and pharmacies grew out of the ground.

An interesting book is the late Louise Riotta's " Roses Love Garlic" and "Planetary Planting" even for someone not versed in astrology her books are full of good gardening sense and general folk lore on companions.

Daisy, thankyou for that insight. What you've said makes sense. If there are good reasons to do things that make sense why not take advantage of that wisdon. As for astrology, it's fraud in my opinion and I have no time for it.

That's absolutely fine Jammus, no one implied you needed to 'believe',in astrology in order to be a creative gardener, particularly as you obviously have studied it and are qualified to make sweeping statements. The information was provided for interest's sake, to illustrate how companion planting has 2000 years of cosmic tradition behind it. Nothing more. Have a happy Solstice.
Cheers
Linda
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If you and I were the same one of us would be irrelevant.

I liked Jackie French's Guide to Companion Planting she makes it a fun read and admits to a bit of trial and error experimenting. Some plants release certain chemicals in the soil which may help others resist certain pests and diseases. I was always told never to plant fennel next to anything. I have basil growing between the pumpkin vines this year (not planned at all) and the pumpkins are really healthy and green without a sign of fungus. This could be entirely coincidental of course but I do believe that some combinations work well together.

nettlerose, do a control group along side without the basil and see what happens.

Also try planting some on a full moon, and plant another set 2 weeks later and compare their growth. I wish I had room in my backyard to do experiments like that. One happy pumpkin plant would take up my entire veggie growing area!

I will have to try that next year Jamus as I am a bit like you at the moment. While I have a biggish garden, the vegetable area is already brimming with plants. I am definitely going to try the moon plantings. Maybe you could grow a pumpkin up a fence or wall on a lattice, I did this with two small varities which grew up a camellia with great success.