Making your own plant feed: nettle, comfrey and horsetail

Behind my grandmother's compost heap stood a blue barrel with a wooden lid, and not one of us children ever went near it willingly. Once a week she stirred whatever was inside, and then half the garden smelled like a wet cowshed. Two days later her tomatoes stood there as though somebody had flipped a switch.

That barrel is the oldest fertiliser factory a garden has ever known. You throw in the greenery that grows along your fence anyway, pour water over it and let the bacteria do the work. What sits in there two weeks later is a fair picture of exactly what the plant pulled out of the soil beforehand.

One thing at a time: what separates a fermented feed from a decoction, which plant carries which nutrient, and how far you dilute before any of it reaches a root.

A fermented feed is not a decoction

A fermented plant feed is left to work. Greenery and water stand somewhere warm for one to three weeks, microorganisms break the plant matter down, it bubbles and it reeks. At the end the nutrients are dissolved and available to plants. That is genuine fertiliser, and it goes to the root.

A cold-water extract stands for only 12 to 24 hours in cold water and is strained before fermentation gets going. It barely smells, it does not feed, and it works as a contact treatment against aphids. It keeps for a day or two.

A decoction you cook. The greenery soaks for 24 hours, then the brew simmers for half an hour. Cooking releases substances that cold water leaves behind, above all the silica in horsetail. A decoction strengthens the plant, it is not a fertiliser.

The base recipe: one kilo of greenery to ten litres of water

The ratio stays the same whichever plant you use. What changes is the moment you harvest and how far you dilute afterwards.

1. Cut before flowering

   Cut while the plants are still succulent and have not set seed, otherwise the seeds survive fermentation and you spread them across the bed later. On nettles that means the top 30 to 40 cm of the shoots. Gloves on.

2. Chop roughly

   Cut the material into hand-length pieces with secateurs. The more cut surfaces there are, the faster the bacteria reach the cell contents.

3. Pick a container, but not metal

   A plastic bin, a wooden tub or stoneware. The finished feed is strongly alkaline and attacks zinc, aluminium and copper; dissolved metal ions are the last thing you want in your watering can. Fill it three quarters full at most, because the brew foams up at the start.

4. Top up with rainwater

   One kilo of fresh greenery to ten litres. Rainwater is softer and lower in lime than tap water, so fermentation gets going faster.

5. Cover it, but not airtight

   A wooden lid or fine mesh keeps leaves and insects out while letting the fermentation gases escape. A sealed lid bulges, and the brew turns putrid. Weight the lid down or place the barrel where small children and pets cannot reach it: an open barrel of liquid is a drowning hazard even at a shallow fill level.

6. Stir every day

   Stirring once a day with a stick brings oxygen in and keeps the smell within reason. The barrel wants warmth, but not a spot next to your terrace or your boundary fence.

7. Strain and store in the dark

   Pour it through an old sieve; the plant residue goes on the compost. Kept cool, dark and tightly sealed, the feed lasts until next spring.

Nettle: the engine for leaf growth

The stinging nettle (Urtica dioica) grows where the soil is rich in nitrogen, and it hands that nitrogen straight back later. Its feed is the nitrogen fertiliser among home remedies, and it brings potassium, magnesium and iron along with it. There is little phosphorus in it; you cover that with compost.

Use it in early summer, when plants are meant to put on bulk: pumpkin, courgette, kohlrabi, potatoes and young tomatoes before fruit set. One watering every two weeks is enough.

What gets none of it are peas and dwarf beans: they fix their own nitrogen through root nodule bacteria, and they repay any extra with plenty of foliage and very few pods. For where plant feeds sit among the other organic fertilisers, see the overview on feeding your garden organically.

Once peppers, cucumbers and tomatoes move into fruiting, potassium matters more than nitrogen. It governs the water balance of the cell and makes sure fruit ripens properly instead of staying watery. Anyone still topping up with nettle feed in August gets foliage instead of a harvest.

Strawberries run to a different clock: they crop in June and are finished before the changeover even starts. Their important feed comes straight after picking in July, when they build new leaves and next year's flower buds. That is the moment for one more dose of nettle, comfrey after that, and nothing containing nitrogen from mid-August onwards.

The switch is easy to remember: as long as the plant is growing, it gets nettle. Once it is bearing, it gets comfrey. On fruiting vegetables that changeover falls somewhere between early July · CW 27 and late July · CW 31, depending on the weather. For how to keep hungry feeders supplied across the season, see feeding fruiting vegetables.

Here the recipe is a different one: a kilo of fresh greenery or around 150 g of dried material steeps for 24 hours in ten litres of water, then the brew simmers for half an hour. Let it cool, then strain. Dilute it 1:5 and spray in the early morning of a dry day. Every 14 days from bud break is the usual rhythm, especially on roses, tomatoes and cucumbers. If mildew is already sitting on the leaf, you need something else: see spotting and treating powdery mildew and the overview of biological plant protection.

Diluting and applying

One litre of feed to ten litres of water is the standard dose for established plants in the bed. For young plants and anything in a pot you halve the amount of feed: half a litre to ten litres of water, so 1:20. Containers hold little substrate and therefore little buffer, and a dose that is too strong does damage there quickly.

How much of the finished solution each plant gets depends on its size: one to two litres for a full-grown tomato or courgette, around five litres per square metre of bed.

Water onto moist soil. If the ground is bone dry, wet it with clear water first, otherwise the salts go straight to the fine roots. Early evening is the best time.

Getting the smell under control

The stink is the reason most people give up after their first attempt. It appears when fermentation slips into the oxygen-free range.

The rhythm across the season

The season for setting up a barrel runs from mid-April · CW 16 to early September · CW 36. Before that there is too little greenery, after it the weather turns too cool for brisk fermentation. The first nettle brew goes into the barrel in spring, because young growth is the most nutrient-rich of the whole season; you apply it after the last frosts from mid-May · CW 20. Up to CW 26 the hungry feeders get a watering every two weeks, and in parallel the first horsetail decoction goes over roses and tomatoes.

From CW 27 you switch your fruiting vegetables to comfrey. If you are reading this in late July · CW 31, now is the moment for your first comfrey brew: in this warmth it will be ready in seven to ten days. Better to run two small bins in rotation than one big one.

From mid-August · CW 33 there is nothing but potassium and silica. Nitrogen has no business in the bed by then, because shoots that keep growing will not harden off and the first frost catches them. Leftovers in the barrel go onto the compost in autumn, where the nitrogen is bound up in organic form and nothing is lost. Do not pour them on cleared beds or freshly sown green manure any more: whatever the plants cannot take up now, the winter rain washes down into the groundwater.

A plant feed is a craft with a memory. It takes until your third year to notice that the brew in partial shade finishes faster than the one in full sun, and that your tomatoes bear better on comfrey from early July · CW 27. Note the start date, the plant you used and the dilution in your garden journal. Then what stands beside that barrel is no longer a mystery but five summers of your own experience.

Common questions about plant feeds

How long does nettle feed need to ferment?

At temperatures around 25 degrees it is through after seven to ten days; in cool weather it takes up to three weeks. The test is the stir: when no more bubbles rise, fermentation is complete. The liquid is dark brown and smells earthy rather than sharp.

How do I dilute plant feed correctly?

For established plants in the bed you take one litre of feed to ten litres of watering water, so 1:10. For young plants and anything in a pot you halve that to half a litre per ten litres, so 1:20. Horsetail decoction is the exception: it goes on at 1:5, because it does not feed the plant but strengthens the leaf.

What helps against the smell of plant feed?

A good handful of primary rock or basalt dust per ten litres binds the odour gases. Add a daily stir, and keep the barrel covered without sealing it airtight.

Can I use nettle feed on every plant?

No. Peas, beans and other legumes fix their own nitrogen and respond with lots of foliage and little yield. Mediterranean herbs like rosemary and thyme lose aroma on a rich diet. Leafy crops like lettuce, spinach and chard get only a very sparing dose, and nothing at all in the last two or three weeks before harvest, otherwise they accumulate nitrate. From mid-August fruiting vegetables and woody plants get no more nitrogen, or their shoots will not harden off and go into winter vulnerable to frost.

What is the difference between a fermented feed and a decoction?

A feed ferments for one to three weeks, and that process unlocks the nutrients. It is diluted, poured at the root, and it fertilises. A decoction is cooked, usually 24 hours of soaking plus 30 minutes of simmering. It barely fertilises, but it releases silica and is sprayed onto the leaf.

Am I allowed to make plant feed on an allotment?

Making and using it in your own garden is permitted, as long as no serious odour nuisance arises and nothing reaches watercourses or the sewer. On allotment sites the site rules cover the details on top of that. Selling plant feed lands you in regulation quickly: as a fertiliser it falls under fertiliser law, as a plant strengthener it needs listing with the German BVL. These legal notes refer to Germany; Austria and Switzerland have their own plant protection and water protection rules, though the basic principle is the same everywhere.