Steeping comfrey leaves in water for around 30 days produces a liquid with a similar nutrient content to a commercial tomato feed, according to Garden Organic’s member trial on homemade liquid feeds. Steeping for longer than 30 days did not increase nutrient content further.
Potassium is the nutrient fruiting plants need most once they begin to flower and set fruit, and comfrey leaves are exceptionally rich in it. The plant’s unusually deep roots draw up minerals from well below the surface, concentrating them in the leaves. Steep those leaves in water and the nutrients leach into a liquid that plants can absorb quickly: faster than compost and without the burn risk associated with animal manures. Comfrey feed can be applied during the growing season without risk of scorching roots or disrupting soil biology.
The right variety: Bocking 14
The comfrey to use is Symphytum × uplandicum ‘Bocking 14’, a sterile Russian comfrey hybrid developed in the 1950s at Bocking in Essex by Lawrence D. Hills at the Henry Doubleday Research Association (HDRA), the organisation that later became Garden Organic. Hills spent decades trialling comfrey varieties and identified Bocking 14 as the highest-yielding and highest-potassium selection available.
Its potassium content can reach around 5% of dry leaf weight, roughly double that of common comfrey (Symphytum officinale) and higher than composted animal manure. Because it is sterile it cannot set viable seed, so it stays where you plant it. Common comfrey self-seeds freely and can become difficult to manage. Bocking 14 is available as root cuttings or crown sections from specialist suppliers and many organic growing organisations.
Two methods
There are two ways to make comfrey liquid feed. Both work, but they produce feeds of different concentration and are applied at different dilution ratios. Garden Organic’s trial found that the steep method produces a more consistent result; the concentrate method can be highly variable in nutrient content, which makes reliable dilution advice difficult.
Method 1: steep in water (recommended)
Pack a large lidded container (a bucket, barrel, or old water butt) with fresh comfrey leaves. Place a brick or heavy stone on top to keep the leaves submerged, then fill with water until the leaves are fully covered. Fit the lid and leave for three to four weeks in summer. In a cool spring below around 12°C allow six to eight weeks; cold temperatures slow fermentation significantly. The liquid will darken to a deep brown colour and smell strongly, which is a normal sign of fermentation.
Strain off the liquid and dilute at 1 part feed to 10 parts water before applying. The diluted feed should look like weak tea. Use the spent leaves as a mulch or add them to the compost heap.
Method 2: concentrate (no water)
Drill or punch drainage holes in the base of a bucket and suspend it over a second container to catch liquid. Pack the upper bucket tightly with fresh comfrey leaves, place a heavy weight on top, and leave without adding any water. Over several weeks the leaves decompose under their own weight into a black concentrate that drips into the lower bucket. This concentrate is much stronger than the steep method and must be diluted at 1 part concentrate to 15 parts water before use.
The concentrate method produces a very strong smell. Garden Organic’s trial also found it produces highly variable results between batches, as the concentration depends on the wetness of the leaves and the weight applied. For most growers the steep method is more practical and more reliable.
Applying the feed
Water the diluted feed directly onto moist soil at the base of plants, not over the leaves. Applying to dry soil reduces uptake; always water the plant first if the soil is very dry. Begin feeding once the first flowers or small fruits appear and repeat every 7–10 days throughout the fruiting period.
Comfrey feed is particularly valuable for high-potassium crops:
- Tomatoes (especially in pots and grow-bags, where nutrients are quickly depleted)
- Courgettes, marrows, and squash
- Runner beans and French beans
- Peppers and chillies
- Cucumbers
- Strawberries
- Fruit trees and soft fruit bushes during the fruiting season
Avoid using comfrey feed on leafy crops such as lettuce, chard, or brassicas during their main growing phase. These crops need nitrogen for leafy growth more than potassium, and a high-potassium feed at that stage is of little value. Nettle tea (see below) is better suited to those plants.
Using comfrey as a mulch
Comfrey leaves can also be used directly as a mulch rather than brewed into a liquid. Cut leaves and lay them in a 5 cm layer around vegetable plants or fruit bushes. As they decompose they release potassium, calcium, and other minerals into the soil and act as a moisture-retaining mulch at the same time. This is sometimes called chop-and-drop. The leaves break down quickly, within a few weeks in warm weather, and can be topped up with each new cut throughout the season.
Comfrey mulch is particularly useful as a lining at the base of potato trenches before planting, or in the bottom of runner bean trenches. As the season progresses and the roots reach the comfrey layer, the nutrients become available exactly when the plant needs them.
Growing and harvesting Bocking 14
Bocking 14 is propagated from root cuttings or crown sections, not seed. Plant root cuttings horizontally about 5 cm deep in spring; they typically take two to six weeks to emerge depending on soil temperature. Plants grown from root cuttings take up to two years to reach full productive size.
Once established, Bocking 14 can be cut three to four times per season between May and September. Cut the leaves to within a few centimetres of the ground; the plant will regrow vigorously within a few weeks. Do not cut after September; leave the plant to die back naturally and build root reserves for the following year. No feeding or watering is needed once established. A single plant in a sheltered spot with reasonable soil will supply enough leaves for a small garden.
Comfrey leaves are covered in rough hairs that can irritate skin during handling. Wear gloves when cutting and processing leaves, especially in quantity. The plant is perfectly safe in the garden and poses no risk to wildlife or soil organisms.
Nettle tea: a nitrogen-rich alternative
Stinging nettles (Urtica dioica) can be made into a liquid feed using exactly the same steep method as comfrey. Nettle feed has an approximate NPK of 2.0–0.5–1.5 compared to comfrey’s roughly 1.5–0.5–5.0. Garden Organic’s trial found that nettle feed contains around 50% more available nitrogen than comfrey feed. This makes it better suited to different plants and different growth stages.
Use nettle feed on leafy crops (lettuce, chard, kale, spinach, brassicas) and on any plant in early vegetative growth before flowering begins. Switch to comfrey once flowers and fruit appear. For a balanced all-purpose feed, steep comfrey and nettles together in the same container; the resulting liquid will have both elevated nitrogen and potassium.
Steep nettles for the same duration as comfrey and dilute at the same 1:10 ratio. Nettles are available freely across most gardens and hedgerows; there is no need to buy them.
Further reading
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Garden Organic – All about comfrey (gardenorganic.org.uk/expert-advice/all-about-comfrey/comfrey)
Garden Organic is the direct successor to the HDRA where Bocking 14 was developed, and holds the most comprehensive UK documentation on comfrey cultivation and use. Their site also links to the full member trial report comparing homemade comfrey and nettle feeds against commercial tomato fertilisers. -
RHS – Three ways to use comfrey to feed your plants (rhs.org.uk/advice/grow-your-own/features/making-comfrey-feed)
The Royal Horticultural Society covers the steep method, the concentrate method, and direct leaf mulching with practical notes on timing and application rates. Their comfrey plant profile at rhs.org.uk includes cultivation notes for Symphytum × uplandicum and guidance on propagation from root cuttings. -
Comfrey UK (comfrey.co.uk)
A specialist UK supplier of Bocking 14 root and crown cuttings with detailed growing guidance. Useful if you cannot source Bocking 14 locally; they also publish a step-by-step liquid feed guide with notes on fermentation times at different temperatures.