How do I make plant feed from comfrey or nettles?

Two free liquid feeds with genuinely different profiles: nettle for leafy growth in spring, comfrey for flowers and fruit from midsummer — plus the concentrate method that avoids the smell.

Difficulty: beginner
Time: 40 min
Read: 4 min
Safety: caution

Short answer

Nettle is high in nitrogen and suits leafy crops in spring. Comfrey is high in potassium and suits flowering and fruiting plants from midsummer. For either: pack leaves into a container, weight them down, cover with rainwater, and leave three to five weeks. Dilute the result to the colour of weak tea — roughly one part to ten — and use it as a soil drench.

Comfrey is worth growing solely for this. Its roots go down well over a metre and mine potassium out of the subsoil that nothing else can reach, and the leaves are cut three or four times a season. Nettles do a similar job for nitrogen from ground nobody wants anyway. Between them they cover most of a season's liquid feeding for the price of a hose to fill the barrel.

Safety

Comfrey and nettle brews smell strongly enough to cause genuine complaints — site the container away from the house and from a boundary. Wear gloves when handling nettles, and be aware that comfrey leaves have irritant hairs. The liquid is not for drinking and should be kept away from children and pets. A large open container of liquid is a drowning risk for small children and a hazard for wildlife — keep it covered and weighted.

What you'll need

A lidded container — a water butt, dustbin or bucket
Comfrey or nettle leaves
A brick or stone as a weight
Rainwater
A watering can and something to strain with

Step by step

Grow the right comfrey.Bocking 14 is the cultivar to plant: it is sterile, so it does not seed itself around, and it is high-yielding. Wild comfrey seeds prolifically and is genuinely difficult to remove once established. Plant it somewhere permanent — the roots regrow from any fragment left in the ground.
Cut at the right time.Comfrey is cut when it is around 600mm tall and before it flowers, three or four times between late spring and early autumn. Nettles are cut in spring before they flower, when the nitrogen content is highest.
Pack the container and weight the leaves down.Fill it loosely with leaves and put a brick or a stone on top to keep everything submerged. Floating leaves rot unevenly and the brew goes mouldy on top.
Cover with rainwater.Enough to submerge the leaves. Rainwater is better than tap water, which is chlorinated and slightly alkaline. Then put the lid on — this genuinely matters for the smell.
Leave it three to five weeks.Warm weather speeds it up. It is ready when the leaves have collapsed into sludge and the liquid is dark. It will smell strongly. That is normal and it is why the lid and the siting matter.
Strain and dilute properly.Strain the liquid off. Dilute to roughly one part concentrate to ten parts water — the colour of weak tea is the practical guide. Undiluted it is far too strong and will scorch.
Use it as a soil drench during the growing season.Around the roots, weekly or fortnightly, onto already-moist soil. Comfrey on tomatoes, peppers, courgettes, potatoes, soft fruit, roses and containers from midsummer. Nettle on brassicas, leafy crops and anything building growth in spring.
Consider the concentrate method instead, if the smell is a problem.Pack leaves into a container with a hole and a tap at the base, weight them down, and add no water at all. Over a few weeks they collapse into a thick black liquid that drips out. It smells far less, and it is used at around one part to fifteen or twenty.
Put the sludge on the compost heap.The spent leaves are excellent compost activator. Nothing is wasted.

Tips

Comfrey leaves can be used directly as a mulch around potatoes, tomatoes and soft fruit, or laid in the bottom of a potato trench. This gets most of the benefit with none of the smell.
Nettle brew works as a compost activator as well as a feed — a can of it poured into a slow heap gets it moving.
Neither brew is a complete fertiliser. Comfrey is roughly a 1-1-5 profile and nettle roughly 2-0-2, so both are usefully unbalanced towards a particular job rather than general feeds.
The smell is caused by anaerobic decomposition. It cannot be avoided in the water method, only contained. The no-water concentrate method genuinely does smell far less.
Three or four comfrey plants supply enough leaf for an average vegetable garden's feeding through a season. It is one of the highest-value things you can plant in a corner.

Common mistakes

Planting ordinary wild comfrey rather than Bocking 14 — It seeds everywhere and the deep roots regenerate from fragments, so it becomes a permanent weed. Bocking 14 is sterile and stays where you put it.
Using the concentrate undiluted — It is strong enough to scorch roots and foliage. Weak tea colour is the target and it is far more dilute than people expect.
Siting the barrel near the house or a boundary — The smell is genuinely powerful and lasts all summer. Put it at the far end, downwind, with a lid.

Questions people ask

How long does the diluted feed keep?

Use it within a few days. The concentrate keeps for months in a sealed container; the diluted liquid deteriorates and starts to smell worse quickly.

Can I use other plants?

Yes — borage, nettles, docks and even weeds generally will produce a usable liquid. Comfrey is used because its deep roots make it unusually rich in potassium, and nettles because they are unusually rich in nitrogen. Random weeds produce a weak general feed.

Is homemade feed as good as a bought one?

For its particular job, yes — comfrey liquid is a genuine high-potash feed and behaves like one. It is less consistent in strength than a bought product, which is why the weak-tea dilution guide matters more than a precise ratio.