Comfrey Bocking 14 Tea Brewed in a Water Butt for a 12-Plant Tomato Feed

Bocking 14 is a sterile cultivar of Russian comfrey (Symphytum x uplandicum) selected at the Henry Doubleday Research Association trial ground in the village of Bocking, Essex. Its seed is non-viable, so a clump planted at the end of a raised bed stays where it is put instead of colonising the whole plot the way wild comfrey does. The leaves accumulate potassium in concentrations that suit fruiting crops, which is why tomato, chilli, and squash growers cut it repeatedly from May onward.

A mature Bocking 14 crown can be cut three or four times a season once it is two years established. Each cut removes the leaf mass down to about 5 cm above the crown, and regrowth is fast enough that a single plant supplies a household growing tomatoes in eight to twelve grow bags. The nutrient profile of the leaf is roughly balanced in nitrogen and phosphorus with a markedly higher potassium fraction, which is the ratio a tomato plant draws on heavily once trusses begin to set.

Packing the water butt and starting the steep

A 200-litre plastic water butt with a tap near the base is the practical container. Cut leaves are pushed in loosely to fill roughly two-thirds of the volume, then topped with water until submerged. The lid matters: comfrey brewing in an open butt produces a smell that carries across a garden and draws flies within a week. A tight lid keeps the anaerobic process contained.

The steep runs for four to six weeks in a British summer, faster in a heatwave, slower if the butt sits in shade. Bacterial breakdown converts the leaf into a dark brown liquid while the fibrous stalks collapse into sludge at the base. When the liquid stops bubbling and the leaf residue has broken down, the concentrate is ready to draw off through the tap.

The raw liquid is far too strong to apply neat. It is diluted at roughly 1 part concentrate to 10 parts water for tomatoes, producing a weak-tea colour in the watering can. A 200-litre butt of concentrate therefore stretches to around 2,000 litres of usable feed, which covers 12 plants watered twice weekly from first flower to the final ripening trusses in September.

Why potassium is the leaf’s payload

Tomatoes shift their nutrient demand as they move from vegetative growth to fruiting. Early on, nitrogen drives leaf and stem. Once flowering starts, potassium governs fruit set, sugar development, and the movement of water through the plant, and a deficiency shows as uneven ripening and blotchy green shoulders. Comfrey concentrate delivers potassium in a form that dissolves readily and reaches the roots within a day of watering.

The deep taproot of an established comfrey plant reaches subsoil layers that annual vegetables never touch, drawing up minerals and depositing them in the leaf. Cutting that leaf and steeping it transfers those minerals into a liquid the tomato roots can take up immediately. This is the mechanism that makes comfrey feed distinct from a general-purpose fertiliser: the potassium is high relative to nitrogen, matching the fruiting phase rather than the leafy one.

A worked feeding schedule for 12 plants

Twelve indeterminate tomato plants in grow bags need roughly 1.5 to 2 litres of water each on a warm day, more in a heatwave. Feeding twice a week from the appearance of the first flower truss means each plant receives around 2 litres of dilute comfrey feed per application. Twelve plants at 2 litres is 24 litres per feed, 48 litres a week.

Across a fruiting season of roughly 16 weeks, that comes to about 768 litres of dilute feed, drawn from around 77 litres of concentrate at the 1:10 dilution. A single 200-litre butt, yielding perhaps 120 litres of usable concentrate after the sludge is discounted, comfortably covers a season with a reserve for chillies and courgettes. Topping the butt with a second batch of leaves in July extends supply into autumn without a break.

Enviromesh over the carrots while the comfrey steeps

Carrot root fly (Psila rosae) locates its host by scent, flying low and homing in on the volatiles released when carrot foliage is bruised or thinned. Enviromesh, a fine woven barrier with a mesh aperture around 1.35 mm, physically excludes the adult fly when laid over the bed and sealed at every edge with soil or timber battens. The barrier goes on at sowing and stays on, because the fly has two main flight periods, one in late spring and a second in mid to late summer.

The mesh only works if it is sealed completely. A single gap at a corner lets a female through, and once she lays at the base of the plants the larvae tunnel into the roots regardless of the cover overhead. Raised beds simplify sealing because the mesh can be pinned down the vertical sides of the bed frame. Sowing carrots thinly to avoid thinning later reduces the scent release that draws the fly in the first place, which lightens the load the mesh has to carry.

Companion planting that earns its space

French marigolds (Tagetes patula) interplanted among tomatoes release limonene and other volatiles that deter whitefly, and the effect is measurable enough that commercial glasshouse growers have trialled the pairing. The marigolds occupy the bare soil at the base of the grow bag where nothing else grows, so they cost no productive space.

Basil sown between tomato plants competes very little for root volume and is harvested continuously through the season. Nasturtiums planted at the edge of a bed act as a draw for blackfly, pulling aphid pressure off broad beans nearby. None of these pairings replace a comfrey feed or a mesh barrier, but they reduce the number of interventions a bed needs across a summer.

Slugs, and the limits of a barrier

Slug control without metaldehyde, now withdrawn from UK sale, relies on methods that each work partially. Ferric phosphate pellets, sold under names such as Sluggo, break down in soil and are the closest direct replacement, killing slugs that ingest them without the wider toxicity of the old blue pellets. Nematode drenches of Phasmarhabditis hermaphrodita, watered into moist soil at temperatures above 5C, infect and kill slugs below ground and give around six weeks of protection per application.

Beer traps sunk to the rim catch a proportion of the population but need refilling every few days and can draw slugs in from neighbouring plots. Copper tape around grow bag rims deters crossing through a mild galvanic reaction with slug mucus, though a determined slug bridges it on a leaf that touches the ground. Removing daytime shelter, the boards and pot rims where slugs cluster, cuts the resident population more than any single barrier. The most effective approach layers two or three of these, because a hungry slug population routes around any one of them within a week.

What none of these methods answers is what happens to the comfrey sludge left in the base of the butt at season’s end, a wet fibrous mass still holding minerals: whether it composts fast enough to return to the bed before spring, or sits sour and airless until it does more harm than good.