Cercis siliquastrum, the Judas tree, is one of the most spectacular small trees in a UK garden. Its cauliflorous flowers, appearing directly on the bare branches and trunk in vivid rose-purple every April and May, stop visitors in their tracks. The large, rounded, kidney-shaped leaves that follow are a quiet glaucous blue-green all summer. When those leaves begin to curl, it is worth investigating promptly because, while most causes are manageable, one (Verticillium wilt) can be fatal.
This guide covers the principal causes of leaf curl on Cercis siliquastrum, from the two most common problems to less frequent but serious threats, with practical UK garden advice for each.
About Cercis siliquastrum
The Judas tree is a small deciduous tree or large shrub in the subfamily Cercidoideae of the Fabaceae family. In ideal conditions it can reach 6 to 10 metres, but most specimens in UK gardens stay considerably smaller, often 3 to 5 metres, and are slow to establish. The leaves are orbicular-cordate: almost circular, notched at the base, and quite large relative to the size of the tree. They are a distinctive glaucous blue-green and have a smooth surface. The cauliflorous habit, producing flowers directly from old wood and the trunk rather than from new shoot tips, is extraordinary and unusual in temperate trees.
Cercis siliquastrum is native to southern Europe and the Middle East and is well suited to warm, sunny, sheltered gardens in central and southern England. It tolerates alkaline soil and, once established, is considerably more drought-tolerant than many other ornamental trees. It can take five to ten years from seed to first flower, but the wait is widely considered worthwhile.
Cause 1: Botryosphaeria leaf spot and leaf curl
The most common fungal cause of leaf curl on Cercis siliquastrum is infection by Botryosphaeria or related fungal species that produce a leaf spot disease. The symptoms begin as small brown spots on the orbicular leaves, which enlarge and coalesce. Affected leaf tissue dies and the edges of the large round leaves begin to curl inward as the dead tissue contracts. In a wet summer this can progress quickly, leading to premature leaf drop and a sparse canopy from August onward.
The fungus thrives in humid conditions and spreads via spores on water droplets. Overcrowded planting and restricted airflow around the canopy encourage infection. The pathogen can also attack weakened or damaged shoots, causing dieback beyond the leaves alone.
To manage Botryosphaeria leaf spot, improve airflow around the tree by removing competing plants or lower branches if the canopy is dense. When watering, apply water to the soil rather than over the foliage, particularly in warm weather when wet leaves dry slowly. Remove and destroy infected leaves as soon as you notice them, collecting any that have fallen around the base of the tree. Do not compost them. In a severe case a copper-based fungicide applied at the first sign of infection may slow spread, though it will not reverse existing damage. The following season, if the tree has retained its general health, it should produce a full canopy of fresh leaves.
Cause 2: Drought stress
The orbicular leaves of Cercis siliquastrum are large in proportion to the tree's modest root system, particularly in young specimens. In a hot, dry summer, or on free-draining sandy or gravelly soil, the tree can lose water through the leaf surface faster than the roots can replace it. The result is a characteristic wilting and curling at the leaf edges, often most pronounced on the south-facing or most exposed parts of the canopy.
Unlike Botryosphaeria, drought-related leaf curl is usually temporary. Once rain arrives, or after a good watering, the leaves recover and uncurl within a day or two. There are no spots or lesions on the leaf surface: the leaves simply look slightly wilted and curl at the margin in the afternoon heat.
Established Cercis siliquastrum are considerably more drought-tolerant than young trees and rarely need supplementary watering once they have been in the ground for three or four years. Young trees, particularly those planted in the last one to two seasons, can struggle in a prolonged hot dry spell and benefit from a deep, thorough watering once a week rather than frequent shallow watering. A mulch of bark or well-rotted compost around the base, kept clear of the trunk, helps the soil retain moisture.
Other causes of leaf curl
Aphid infestation
Small colonies of pea-type aphids sometimes colonise the soft new shoot tips of Cercis in spring and early summer. Heavily infested shoots carry clusters of soft-bodied green or black aphids and the leaves at the tips curl around the colony, partly as a natural plant response and partly because the aphids are sheltering there. On mature, well-established trees, natural predators including ladybirds, hoverfly larvae, and blue tits usually bring aphid numbers under control without intervention. On younger trees or where infestations are heavy, a strong jet of water from a hose directed at the shoot tips dislodges most of the colony. Avoid using systemic insecticides on a tree you value for pollinators: the Cercis flowers are popular with bees in spring, and a systemic applied to the roots or foliage can persist into flowering the following year.
Late frost damage
Cercis siliquastrum flowers on bare branches in April and May, before the leaves emerge. The new leaves are thin and tender and can be caught by a late ground frost in April or a cold snap in early May. Frost-damaged leaves show scorching, browning at the edges, and inward curling, sometimes giving the impression of a disease. The damage is usually cosmetic: the tree pushes out a second flush of leaves within a few weeks and recovers fully. In colder areas of the UK, or where the tree is in a frost pocket, position Cercis siliquastrum against a warm south-facing or west-facing wall or fence, which provides both heat and shelter from late frosts at the vulnerable moment of leaf emergence.
Verticillium wilt
Verticillium wilt is a serious soilborne fungal disease that Cercis siliquastrum is susceptible to. Unlike the other causes described here, Verticillium wilt is potentially fatal to the tree. It infects the vascular tissue through the roots, blocking the upward movement of water. The first symptom is usually the sudden wilting and die-back of one or more branches, followed by leaf curl and leaf drop on those branches. If you cut into an affected branch at the base, the sapwood will show characteristic brown or olive-green staining in a ring pattern.
There is no cure for Verticillium wilt. Remove affected branches promptly, cutting well below the point of visible discolouration into clean, white sapwood, and sterilise your tools with dilute bleach or surgical spirit between each cut to avoid spreading the pathogen. Some trees survive if the infection is caught early and confined to a few branches; others decline over several seasons. If the tree dies, do not replant another Cercis or other susceptible species (which includes dahlias, strawberries, and many Acer) in the same site for several years, as the fungus persists in the soil.
Canker on branches
Cercis siliquastrum can develop cankers on branches, typically at wound sites or where branches have been damaged by frost. Affected bark is sunken and discoloured and the branch above the canker may show poor growth, leaf curl, and die-back. Remove cankered branches cleanly with a sharp saw, cutting back to healthy wood, and dispose of the material away from the garden.
Prevention and long-term care
Most problems with Cercis siliquastrum stem from siting and soil. Plant in the warmest, most sheltered position available, ideally against a south-facing or west-facing wall in colder parts of the UK or in a sheltered courtyard garden. The tree needs excellent drainage: waterlogged soil in winter weakens the root system and opens the way for fungal infection and Verticillium wilt. Neutral to alkaline soil suits it well.
Water young trees regularly in prolonged dry spells but do not overwater established trees. Apply water at the base rather than over the foliage to reduce the risk of leaf spot. Maintain good airflow around the canopy. Watch for the progressive branch die-back and sapwood staining that signal Verticillium wilt and act quickly if you see it.
For gardeners in colder, wetter parts of the UK, or those who want a more reliably flowering specimen, Cercis canadensis 'Forest Pansy' (the North American redbud) is a hardier and more widely available alternative. It offers dramatic burgundy-purple foliage all summer. Named UK cultivars of Cercis siliquastrum worth seeking out include 'Bodnant' (selected for flower quality) and 'Alba' (white flowers, slightly less showy but quietly beautiful).
Frequently asked questions
Why are my Judas tree leaves curling at the edges?
The two most common causes are Botryosphaeria leaf spot (a fungal disease that causes brown spots and leaf-edge curling in wet summers) and drought stress (the large orbicular leaves wilt and curl quickly in hot dry spells). Check whether you can see brown spots on the leaf surface: if yes, fungal infection is likely; if the leaves look otherwise healthy but droop and curl in afternoon heat, drought stress is the more probable cause.
What is the Judas tree legend and where does the name come from?
The name Judas tree comes from a tradition, recorded across southern Europe and the Middle East, that Judas Iscariot hanged himself from this tree after betraying Jesus. The tree's small, dense, pendulous flower clusters were said to have turned from white to the deep rose-purple seen today out of shame. An alternative folk etymology suggests the name is a corruption of "Judaean tree", referring to the tree's native range in the eastern Mediterranean, including Judaea. Both explanations are widely cited and the true origin is debated.
Is Cercis canadensis 'Forest Pansy' a good alternative to Cercis siliquastrum in the UK?
Yes, Cercis canadensis 'Forest Pansy' (the North American redbud) is a popular and hardier alternative that is more widely available in UK nurseries. It offers striking burgundy-purple foliage throughout the summer rather than the blue-green of the true Judas tree, and it tends to flower more freely at a younger age. It is generally considered more reliable in colder or wetter parts of the UK. The trade-off is that the foliage colour makes it a very different-looking plant: 'Forest Pansy' reads as a foliage tree first and a flowering tree second, whereas Cercis siliquastrum is grown primarily for its extraordinary cauliflorous spring display.
Can I save a Judas tree with Verticillium wilt?
Verticillium wilt is the most serious disease Cercis siliquastrum faces and there is no chemical cure. The fungus lives in the soil and infects the vascular system, blocking water movement. Affected branches die back progressively and the sapwood shows characteristic brown staining when cut. Remove all affected branches, cutting well below the discoloured zone into clean white wood, and sterilise your tools between cuts. Some trees recover if the infection is caught early and limited to a few branches; others decline over several seasons. Do not replant another Cercis or susceptible plant in the same soil.
My Judas tree flowered but now the leaves look scorched and curled. What happened?
The most likely cause is a late frost. Cercis siliquastrum produces its extraordinary rose-purple flowers on bare branches in April and the new leaves emerge in late April and May. Those young leaves are thin and tender and a ground frost after emergence can scorch them quickly, causing curling, browning at the edges, and a scorched appearance. The tree will almost always recover and push out a second flush of leaves. In colder parts of the UK, grow Cercis against a warm south-facing wall or in a sheltered courtyard to reduce frost exposure at this vulnerable moment.