MY comfrey patch, planted nearly 20 years ago, is coming to the end of its useful life, so this month I’m going to put in some new offsets.

Russian comfrey (Symphytum x uplandicum) is an enormously valuable plant to fruit and vegetable gardeners because it’s an extremely rich source of potassium.

It has long roots, which bring this essential nutrient up from deep in the soil and store it in large, green leaves, thus making it easily available to the gardener.

It’s a long-lasting herbaceous perennial, which regrows rapidly and vigorously after being cut down to a couple of inches above the ground, allowing it to be harvested three or four times a year starting in April and finishing in October.

That means that an ordinary sized garden or allotment won’t need more than a few plants.

Although at its best in a sunny position in deep soil, comfrey will do well enough almost anywhere, so it can be tucked away in a semi-shady part of the plot where not much else would grow.

Each plant produces masses of foliage, which can simply be added to the compost heap. However, two more popular ways of using it are as a mulch and in liquid feeds.

When the wilted leaves are laid around plants such as tomatoes or peppers they preserve moisture in the soil, as do all mulches, but they also feed the crops as they rot down.

Another advantage which many gardeners notice is that the rough, hairy leaves seem to deter slugs and snails.

For the same reason, it’s best to wear gloves when you’re handling comfrey to avoid uncomfortable skin reactions.

The easiest way to turn the leaves into liquid fertiliser is simply to stuff a load of them into a container, such as a barrel or water butt, fill it with water and wait about a month.

The unpleasantly smelly “tea” which results can be used undiluted, especially in the greenhouse and on plants grown in containers.

Making a liquid concentrate of comfrey, which doesn’t pong and can be stored, is more complicated — but if you don’t mind a bit of DIY, you’ll find instructions readily on the internet.

New comfrey plants can be planted, or old ones divided, at just about any time of year other than in the depths of winter.

I prefer to do it in autumn simply because it’s convenient, but in colder areas of the country you might be better off waiting until spring. Don’t take a harvest during the first year after planting.

Whether buying root cuttings or getting them from a neighbour’s plants, make sure you have the cultivar named “Bocking 14” — all other kinds are liable to spread and become a weed.

Plenty of water and nitrogen are comfrey’s main needs, so keep it well mulched with manure.

Ideally you should cut it regularly enough that it never flowers — but if it does, you’ll find it’s an excellent plant for attracting bees.