Why Are My Rhubarb Leaves Curling?

Aphids

The dock aphid (Aphis rumicis) and several other species colonise rhubarb in spring and early summer, feeding on the underside of young, newly emerging leaves. The leaves curl inward and pucker around dense aphid colonies, and the growing tips of the stalks may be distorted where feeding is concentrated. Aphid colonies on rhubarb are often attended by ants, which farm the aphids for their honeydew secretions. Natural predators, particularly ladybirds and parasitic wasps, typically bring aphid populations under control on rhubarb without intervention by late spring or early summer.

What to do

• Inspect the underside of emerging rhubarb leaves in April and May for aphid colonies. Small to moderate colonies can be left for natural predators to manage: established rhubarb crowns tolerate aphid feeding without significant long-term harm.
• Apply insecticidal soap spray to large colonies that are causing severe curling of the young leaves, covering the underside of affected leaves thoroughly. A single application in spring is usually sufficient.
• Control ant access to the crown: ants protect aphids from predators by farming them, so disrupting ant access with a sticky barrier on the stalks (if practical) can help natural predator populations control the aphids more quickly.

Crown rot

Crown rot on rhubarb is caused by several fungal and oomycete pathogens, most commonly Phytophthora and Botrytis species. The infection attacks the central growing crown, rotting the tissue at the base of the emerging stalks and causing the stalks to collapse, turn brown, and smell unpleasant. The leaves on rotting stalks twist, distort, and yellow as the stalk fails. Crown rot is most common in poorly drained soils, after prolonged wet winters, or where rhubarb has been planted too deeply with the crown bud buried below the soil surface.

What to do

• Remove badly rotted stalks promptly, pulling them cleanly away from the crown. Inspect the crown: if the rot is confined to individual stalks, the crown may recover with improved drainage.
• Improve drainage around the crown by incorporating coarse grit into the surrounding soil and ensuring water does not pool around the base of the plant.
• A severely rotted crown that has lost most of its buds is unlikely to recover and is best dug out and replaced with a new division from a healthy plant on a fresh site with well-drained soil.

Drought

Rhubarb is more drought-tolerant than many garden vegetables, but the large leaves transpire significant quantities of water in hot weather and the stalks become thinner and tougher in very dry conditions. Drought-stressed plants produce leaves that droop and curl at the margins, and the stalks may become stringy. Prolonged drought in summer can cause the plant to die back prematurely.

What to do

• Water rhubarb during extended dry spells, giving the crown a deep soaking every 10 to 14 days rather than frequent shallow watering.
• Apply a generous mulch of compost or well-rotted manure around the crown in spring (keeping the mulch away from the crown bud itself) to conserve soil moisture through summer.

Honey fungus

Honey fungus (Armillaria species) occasionally attacks rhubarb crowns, causing the plant to fail and die despite apparently healthy growing conditions. Symptoms include progressive dieback, collapsing stalks, and a white, fan-shaped fungal growth under the bark of the crown or on the roots. Black, boot-lace-like fungal strands (rhizomorphs) are sometimes visible in the soil around the crown. Honey fungus is much less common on rhubarb than crown rot but is incurable once established.

What to do

• Dig out and destroy the entire crown and as many roots as possible if honey fungus is suspected. Do not compost the material.
• Do not replant rhubarb in the same position. There is no chemical control available to UK home gardeners for honey fungus.

Rhubarb virus

Rhubarb is susceptible to several viruses (including Rhubarb virus 1 and Strawberry latent ringspot virus) that cause mottled, mosaic, or distorted leaves and reduced plant vigour. Affected plants may produce fewer, thinner stalks and the leaves may curl or crinkle in unusual patterns. Viruses are spread by aphids and by dividing infected crowns. There is no cure for viral infection in rhubarb.

What to do

• Control aphid populations on rhubarb to reduce the spread of virus between plants.
• Replace persistently unhealthy crowns with certified virus-free divisions from reputable UK suppliers, and plant on a fresh site.

Frequently asked questions

Why are my rhubarb leaves curling?

Rhubarb leaves curl most often from aphid infestations, crown rot, or very dry conditions. Aphids colonise the underside of young rhubarb leaves as they unfurl in spring, causing them to curl inward and pucker around the feeding sites. Crown rot (Phytophthora and Botrytis species) attacks the central growing crown, causing the emerging leaves to twist, distort, and collapse. Drought stress causes the large leaves to droop and curl at the margins. Rhubarb leaves also curl naturally when first emerging in spring and as they unfurl: this is normal and does not indicate a problem.

Can you eat rhubarb leaves?

No. Rhubarb leaves are toxic and must never be eaten. They contain very high concentrations of oxalic acid and other compounds that are poisonous to humans and animals. Only the red or pink stalks (petioles) are edible. Always remove and discard the leaves before using rhubarb stalks in cooking. Do not compost rhubarb leaves in a hot compost heap that might produce compost used on edible crops; they are better discarded in general waste or left to rot in an isolated corner of the garden where the toxins are diluted safely into the soil over time.

Why is my rhubarb not growing?

Rhubarb that fails to grow vigorously is most often suffering from one of these causes: planting too deep (the crown bud should sit at or just below soil surface), a waterlogged or poorly drained position (rhubarb rots in wet soils), an exhausted crown that has never been divided (crowns should be lifted and divided every 5 to 10 years), or harvesting too heavily in the first 2 years (the crown needs time to establish before heavy picking begins). Apply a generous mulch of compost in autumn and spring and do not harvest at all in year one, and only lightly in year two.

When can I start picking rhubarb?

Do not harvest rhubarb stalks in the first year after planting, so the crown can establish a strong root system. In the second year, harvest only 2 or 3 stalks per plant in May or June to avoid weakening the crown. From the third year onward, harvest freely from April to June, but stop harvesting by midsummer (June or early July) to allow the plant to build energy reserves in its roots for the following season. Remove any flower stems (rhubarb seed heads) as soon as they appear to keep the plant's energy directed into stalk production.