Some years ago, when planting roses, gardeners began using Rootgrow, a product containing mycorrhizal fungi which help new roses establish healthy roots and fight off rose replant sickness.

The more that has been learned about soil and how plants grow in it, the more we know about the role these microscopic mycorrhiza play.

Mycorrhiza, literally "root fungus", have been around for 400 million years and have developed a symbiotic relationship with 85 per cent of plants (some exceptions are the brassica family, including wallflowers and such plants as spinach, rhododendrons and dianthus). The fungus plugs in to a plant's root system, then develops a network of microscopic threads called mycelia. These collect dissolved nutrients and exchange them for sugars through the plant's roots, benefiting both plant and fungus. As part of this exchange, the plant receives phosphorus, which plays a key role in developing a healthy root system.

Recent research has shown that this symbiotic relationship between plants and mycorrhizal fungi affects crop health and yields. Essentially, though, it is a long-term partnership and provides no quick fix.

Although mycorrhizal fungi don't generally function well in peaty soil, they can improve growing conditions in peat. When trying to find ways of reducing peat in commercial compost, scientists in Germany made an interesting discovery. In 2007, Henrike Perner used pelargoniums in a trial, and added organic compost to a peat-based growing medium. She mixed 20 per cent and 40 per cent organic compost to peat and added mycorrhizal fungi to half of these combinations. She found the compost increased the dry weight of the plants and the mycorrhiza on their own increased the number of buds and flowers. But she got more buds and flowers more quickly in mixes with both compost and mycorrhizal fungi. So grow your pelargoniums in homemade compost and add a sprinkling of Rootgrow.

Three years ago, Italian scientist Andrea Copetta also found that a mix of mycorrhizal fungi and green waste produced better, tastier tomatoes. I'll try using mycorrhizal fungi with homemade compost next year.

Mycorrhizal fungi develop large underground networks that can affect plant health as well as producing more flowers and tastier tomatoes. Several plants are often linked together by these mycelial threads and, as a result, share information and some nutrients. One plant can warn another one if it is being attacked by a pest or disease, which lets its neighbour prepare its chemical defences. This may sound far-fetched, but we're only just learning how sophisticated communication between plants can be.

But, like countless soil organisms, mycorrhizal fungi only thrive in organically managed soil. They can't tolerate fungicides and herbicides - and that includes weedkiller run-off from treated paths and lawns.

Digging the soil also damages mycorrhizal fungi, breaking up the mycelial networks. In 2002, Canadian researchers found soil treated with mycorrhizal fungi produced two good maize crops and then a better crop of soya beans than crops grown in disturbed soil. Like peas and other beans, the plants produce root nodules that fix nitrogen from the atmosphere. In undisturbed soil, the mycorrhiza stimulated strong root growth and root nodules four times larger than plants in disturbed ground. The beans absorbed more nitrogen and, at harvest time, the plants were nearly half as heavy again as those in disturbed ground. So "no dig" gardening sometimes pays off.

If you give them the right conditions, mycorrhizal fungi are good news and can even be used in containers and baskets.