How to Grow Comfrey in Australia

Comfrey (Symphytum officinale) is a long-lived perennial in the borage family, Boraginaceae, grown for leaves that make mulch, compost and a potassium-rich liquid feed. It is a garden worker, not a kitchen herb, so never eat or drink any part of it. The mistake most gardeners regret is planting it where they dig, because any piece of root left behind grows into a new plant.

Three mistakes that matter

1. Planting it in the vegie patch. Each root piece a spade leaves behind can grow, so comfrey in a dug bed soon takes over. Give it a permanent spot beside the compost bay, under fruit trees or in a large pot.
2. Letting common comfrey set seed. Seed-grown common comfrey self-sows, unlike the sterile 'Bocking 14', and seedlings can come up well away from the clump. Cut the flower stems before the seed ripens.
3. Cutting too hard in the first year. A young plant needs its leaves to build the deep root that feeds later harvests. Take one light cut in the first season, then cut established clumps about every six weeks.

When to plant

Comfrey seed is slow, taking about 25 to 30 days to come up in mix held at 20 to 22°C. Start it in trays under cover, on a seedling heat mat in cool weather. Plants live for years, so one sowing is enough.

Where frosts are regular, as in Canberra or Ballarat, keep autumn-sown seedlings in pots in a sheltered spot and plant them out in spring. In the tropics, plant seedlings out while the soil still holds moisture from the wet season.

Site, soil and setup

A corner that is never dug suits comfrey better than the best bed in the garden, because its roots run deep and it stays put.

Container growing

Pots are the surest way to keep comfrey from spreading. Choose one that holds at least 20 L; a standard 40 cm pot holds about 28 L, and a deep pot gives the taproot room. Fill it with quality potting mix.

• Water: whenever the top 5 cm of mix is dry, deeply, at the base rather than over the leaves.
• Feed: fish emulsion at half strength, fortnightly from spring to late summer.
• Position: sun with afternoon shade, on paving rather than soil, so roots cannot escape through the drainage holes.
• Dividing: lift and divide the clump every few years, as it slowly fills the pot.

Planting Crowns

Start common comfrey from seed and plant the young crowns out; multiply established clumps, and the sterile 'Bocking 14', by crown division or root cuttings.

Crown division

Divide in early spring while the plant is dormant, or in autumn where winters are mild. Drive a spade under the crown about 8 cm down and split it into pieces that each carry roots and a growing point. For root cuttings, cut roots into 3 to 10 cm lengths and lay them 3 to 5 cm deep. Dividing perennials gives the timing for each zone.

Sowing seed

• Sow 5 mm deep in trays or punnets of seed raising mix.
• Keep the mix at the temperature in When to plant, moist but never wet or dry.
• Expect seedlings in 25 to 30 days, and do not give up early: germination is slow and uneven.
• For a quicker start, try cold stratification (a cold, moist spell that breaks dormancy), as seed treatments explains.
• Prick out (lift and move) each seedling into its own small pot once it has true leaves.

Planting out

• Plant out once seedlings are 5 to 10 cm tall and have their first true leaves.
• Harden off (gradually accustom seedlings to sun and wind) over one to two weeks, as hardening off seedlings describes.
• Space plants 60 to 90 cm apart.
• Water in with seaweed solution at half strength, 15 mL per 9 L.
• Keep the soil moist until new leaves appear.

Care by growth stage

Water and regular cutting decide how much leaf a clump gives. Once settled, comfrey is deep-rooted and tough, but young plants need steady moisture.

Cutting Back

Cutting is how you harvest comfrey, and it also stops common comfrey going to seed.

• First year: take one light cut, and remove flower stems as they form so the plant puts its effort into root and leaf.
• Established clumps: cut the whole plant to about 5 cm above the soil, roughly every six weeks in the growing season.
• Frosty zones: stop cutting in early autumn so the plant can recover before it dies down.
• Late autumn or early spring: clear dead leaves back to ground level.

Companion planting

Comfrey belongs with long-lived plants in ground that is never dug.

Grows well with

• Fruit trees, where cut leaves go straight down as mulch under the canopy.
• Asparagus, another permanent bed that is never turned over.
• Rhubarb and berries, long-lived neighbours that stay in one place.
• The compost bay, so leaves go straight into the heap.

Keep apart

• The annual vegie patch, where digging chops the roots into new plants.
• Small herbs and seedlings within about 60 cm, which the big leaves crowd out.
• Lawn edges and paths, where edging and weeding tools cut into the roots.

Harvesting Leaves

Cut comfrey when the clump is full of large leaves and before the flower stems open. Seed-raised plants reach that point in about a year.

• Wear gloves and long sleeves, because the stiff hairs on leaves and stems can irritate skin.
• Cut the whole clump at about 5 cm, or take outer leaves at the base and leave the centre.
• Let leaves wilt for a day before laying them as mulch.
• Lay a 5 cm layer of chopped leaves and grass clippings around fruiting plants and fruit trees.
• Add a few shredded leaves in layers through the compost heap to get it working.

Comfrey leaves do not keep, so use them the day they are cut. For comfrey tea, half fill a bucket with torn leaves, top it up with water and fit a lid. After two to six weeks the brew turns dark and smelly: strain it, and put the sludge on the compost.

Dilute the tea 1 part to 10 parts water and pour it on the soil around flowering and fruiting crops, such as tomato, fortnightly. Keep it off leafy salad crops. Comfrey is for the garden only: every part contains pyrrolizidine alkaloids, which can damage the liver if swallowed. Over time it can also poison grazing livestock, so do not use it as fodder.

Common problems

Trays still bare. Comfrey seed can take a month to show, and cool mix slows it further, so keep the tray warm and wait before resowing.

Seedlings popping up around the garden. Common comfrey self-sows when flowers are left to set seed; see Three mistakes that matter.

Comfrey regrowing where it was dug out. Root pieces left in the soil sprout again. Cut the patch to the ground, cover it with several layers of wet newspaper and a thick mulch, and leave it to rot out.

Scorched or limp leaves in summer. Strong afternoon sun can burn the large leaves, and heavy rain can leave them drooping; Site, soil and setup covers shade.

Pests and diseases

Comfrey has few serious pests, and most damage falls on seedlings and young plants.

Garden snail (Cornu aspersum) and slugs. They chew ragged holes in seedlings and young leaves, mostly at night. Hand-pick them after dark and remove the damp places where they shelter by day. Read more

Aphids (several species). They cluster on soft new shoots in warm spring weather, and a firm jet of water knocks most colonies back. Read more

Damping off (Pythium, Rhizoctonia and other soil organisms). Seedlings collapse at soil level, a real risk during comfrey's long wait in moist mix. Read more

Rust (rust fungi). Orange-yellow to brown spots appear on the leaves, mostly underneath, and plants can look pale and stunted. Cut affected plants back and put the leaves in the bin, not the compost.

Powdery mildew (fungal disease). A white, powdery coating spreads over older leaves late in the season, when plants are tired. Cut the clump back and let clean new leaves replace them. Read more

Prevention first

• Sow into fresh seed raising mix in clean trays, and water seedlings from below.
• Check seedlings and young plants for snails after dark, especially after rain.
• Give plants their full 60 to 90 cm so air moves between them.
• Cut clumps regularly and clear out old, tired leaves as autumn arrives.

Tips

1. Push a stake in beside each clump before winter, so no one digs into it while it is dormant.
2. Cut common comfrey just after its first flush of flowers: the regrowth flowers again, which gives bees a second round.
3. Plant a strip several plants wide along a fence line to stop running grasses creeping into the garden.
4. Stand the tea bucket well away from outdoor seating, because the brew smells strongly.