Helichrysum is a varied genus that covers two quite different plants popular in UK gardens. Xerochrysum bracteatum, still widely sold as Helichrysum bracteatum, is the strawflower: a half-hardy annual grown for its papery, everlasting blooms in shades from white through yellow and orange to deep crimson, traditionally started under glass in February or March and planted out after the last frost. Helichrysum petiolare, the licorice plant, is a tender perennial grown for its silver-felted or lime-green trailing foliage and used extensively in hanging baskets and mixed containers. Both plants share similar vulnerabilities. When the leaves begin to curl, the cause is usually either aphids on the soft young growth or downy mildew developing in cool, humid conditions, though overwatering, red spider mite under glass, botrytis on spent flowers, and nutrient deficiency in containers can all play a part.
Aphids
The peach-potato aphid, Myzus persicae, is the aphid most commonly found on helichrysum and is one of the most widespread and destructive soft-bodied pests in UK horticulture. The adults and nymphs are soft, oval, and pale to mid-green, roughly one to two millimetres long, and they cluster densely on the shoot tips, around developing flower buds, and on the undersides of the youngest, most tender leaves. Their feeding withdraws sap from the plant's vascular system, and the feeding site also injects a toxic saliva that disrupts normal cell growth in the surrounding tissue. The result, visible before the colony itself may be noticed, is a pronounced downward curling of the leaves on and immediately below the infested shoot tip. In moderate infestations the shoot becomes distorted and stunted; in heavy ones the shoot tip may be entirely hidden beneath a dense mass of insects and completely fail to develop. Both strawflowers and licorice plant are susceptible, and the problem is consistently worse during the warmer months of late spring through summer when aphid populations reproduce most rapidly. Under glass, where predator populations are low and temperatures are higher, aphids can build up from a handful of individuals to damaging numbers within a week.
The first sign is usually the curling leaf before you find the colony, so check the shoot tips of any helichrysum that shows distorted new growth. Squash small colonies by hand immediately. For larger infestations, a strong jet of water directed at the shoot tips knocks most aphids off and interrupts the colony rapidly. Follow up with an insecticidal soap spray or a neem oil solution applied thoroughly to all shoot tips and to the undersides of the newest leaves, repeating after seven days. Encourage natural predators including ladybirds, lacewings, and parasitic wasps by reducing pesticide use in the garden generally. Keep plants under glass well ventilated and inspect new arrivals before placing them near established plants, as aphid colonies move readily on new plant material brought in from outside.
On strawflowers in particular, honeydew excreted by the colony coats the leaves below the feeding site and rapidly develops a growth of black sooty mould. The sooty mould does not attack the plant directly but it reduces the leaf's ability to photosynthesise and makes the plant look poor. Once the aphid colony is eliminated and the plant resumes normal growth, rain and time wash the mould away, though you can accelerate this by wiping affected leaves gently with a damp cloth.
Downy mildew
Downy mildew is a serious disease of helichrysum that is especially damaging to seedlings raised under glass in the UK spring, when the combination of cool nights, high humidity inside the propagation house, and the dense spacing of young seedlings in trays creates near-ideal conditions for the pathogen. The causal organisms are Peronospora or Bremia species, water moulds that are distinct from the fungi causing powdery mildew and that behave very differently. Downy mildew spreads in cool, moist, still air, the opposite of powdery mildew which favours warm, drier conditions.
The first symptoms are angular, pale green to yellow patches on the upper surface of the leaves. The patch edges are often bounded by leaf veins, giving them a geometric rather than rounded shape. On the underside of the leaf, directly beneath each patch, there is a corresponding growth of soft, grey-white or off-white sporulation that has a faintly fuzzy or downy texture. This is the key diagnostic detail that separates downy mildew from all other diseases that produce yellow patches on helichrysum leaves. As the infection progresses, the affected leaves curl, collapse, and turn brown. In severe infections, or in seedlings that are already stressed by cold, poor drainage, or overwatering, the disease can kill the plant within days.
Under-glass seedlings are particularly vulnerable during UK spring, when the days are lengthening but nights remain cold and the temptation to keep the greenhouse closed overnight for warmth creates the still, humid air that allows spores to germinate and spread. The practical response is to balance temperature and ventilation carefully rather than choosing one over the other: provide ventilation even on cold nights if the humidity is high, water seedlings from below rather than overhead, and avoid splashing water on the foliage when watering. Remove and bin any affected seedlings immediately, as the sporulation on the leaf underside is producing millions of spores that will infect neighbouring plants if left in place. Apply a copper-based fungicide as a preventive spray to seedlings during periods of high disease risk. Outdoors, space plants generously to allow air to circulate freely between stems. If downy mildew appears on established outdoor plants, remove affected leaves promptly, switch to base watering only, and treat with copper fungicide.
Other causes of leaf curl
Botrytis grey mould (Botrytis cinerea) is a constant risk on helichrysum, particularly on the papery spent flowers of strawflowers in wet weather. The pathogen colonises dying petal tissue very readily and from there spreads to adjacent leaves and stems. The first sign is typically a grey, fuzzy mould on an old flowerhead followed by the stem below it turning brown and collapsing. Leaves near the affected area may curl and die back. The solution is consistent deadheading throughout the season, removing spent flowers before they begin to die back on the plant, and improving airflow around the stems by not overcrowding in the border or container.
Overwatering and root rot affect both species when grown in containers. Helichrysum petiolare in hanging baskets is frequently overwatered, and strawflowers in pots are susceptible if drainage is poor. Symptoms are yellowing leaves that later turn brown, wilting that does not recover after watering, and a generally declining plant. Check that containers have adequate drainage holes and that the compost is not remaining wet for extended periods between waterings. Remove the plant from the pot and inspect the root ball if decline persists. Healthy roots are white to pale tan; rotten roots are brown, soft, and may smell unpleasant. Trim away rotten roots, treat with a fungicide drench, and repot into fresh, free-draining compost.
Red spider mite is a significant problem on helichrysum grown under glass. The mites are tiny, yellowish or greenish-red, and visible to the naked eye with difficulty, but their feeding produces a distinctive fine mottling or bronzing of the leaf surface and fine webbing on the undersides. Leaves may curl as the population grows. Maintain high humidity under glass to discourage mites, which thrive in hot, dry conditions. Introduce the predatory mite Phytoseiulus persimilis as a biological control from late spring onwards in greenhouses.
Nutrient deficiency is worth checking in container-grown plants that show persistent yellowing alongside leaf curl. Helichrysum in hanging baskets and pots depletes the nutrient reserves of potting compost rapidly, particularly nitrogen. Feed container helichrysum with a balanced liquid fertiliser at fortnightly intervals through the growing season. A slow-release granular fertiliser incorporated at planting reduces the feeding workload considerably.
Prevention
- Water at the base of the plant rather than overhead. Wet foliage promotes both downy mildew and botrytis.
- Deadhead strawflowers regularly throughout the season to remove dying flowers before botrytis can colonise them.
- Provide maximum ventilation to seedlings under glass, particularly at night in cool, humid spring conditions when downy mildew risk is highest.
- Sow into fresh, sterile compost to reduce damping off and early downy mildew infection at the seedling stage.
- Harden off seedlings properly over ten to fourteen days before planting out, exposing them gradually to outdoor temperatures and wind to strengthen the foliage and reduce transplant stress.
- Space plants generously in the border and in containers to allow air to circulate between stems and leaves.
- Inspect new plant material brought in from outside for aphid colonies on the shoot tips before placing near established plants.
- Ensure containers and hanging baskets have adequate drainage and allow the compost surface to dry slightly between waterings to avoid root rot.
- Feed container plants fortnightly with a balanced liquid fertiliser to maintain vigour and resistance to both pest and disease pressure.
Frequently asked questions
Why are my helichrysum leaves curling?
The most common causes of helichrysum leaves curling are aphids and downy mildew. Aphids colonise the shoot tips and soft young growth, causing leaves to curl downward around dense pale green colonies. Downy mildew produces angular yellow patches on the upper leaf surface and a grey-white furry mould beneath, with the affected leaves curling and collapsing in cool, humid conditions. Overwatering and container root rot can also cause wilting and curl, as can red spider mite under glass.
What are the small pale green insects on my strawflower shoots?
Pale, soft-bodied green insects clustered on the shoot tips and around flower buds of helichrysum are almost certainly the peach-potato aphid, Myzus persicae. This aphid feeds on a very wide range of plants and is one of the most common pests on helichrysum in UK gardens and under glass. The colonies feed on sap, cause the surrounding leaves to curl downward, and excrete sticky honeydew that coats the leaves below and supports the growth of black sooty mould. Treat by squashing colonies by hand, applying a strong jet of water, or spraying with an insecticidal soap or neem oil solution.
Can I save a helichrysum with downy mildew?
Yes, in most cases, if you act early. Remove all affected leaves, improve airflow around the plant, stop overhead watering, and apply a copper-based fungicide to the remaining foliage. Downy mildew spreads in cool, humid air, so thinning crowded seedlings under glass and spacing plants generously outdoors significantly slows the disease. Badly infected plants or those where the disease has reached the crown are unlikely to recover fully and are best replaced to prevent spread to neighbouring plants.
Why are my helichrysum seedlings collapsing under glass?
Collapsing helichrysum seedlings in late winter or spring are most often victims of downy mildew or damping off. Downy mildew causes yellowing patches and grey sporulation on the undersides of leaves before the seedling collapses. Damping off, caused by Pythium or Rhizoctonia species, rots the stem at soil level and causes the seedling to topple. Both spread in cold, wet, poorly ventilated conditions. Sow into fresh, sterile compost, water from below rather than overhead, provide maximum ventilation, and maintain a minimum temperature of around 15 degrees Celsius to reduce the risk of both problems.
Do strawflowers need deadheading?
Deadheading strawflowers (Xerochrysum bracteatum) prolongs the flowering display and, importantly, removes the spent flowerheads before they can become infected with botrytis grey mould in wet weather. Botrytis colonises the dying petals of old flowers and can spread from there onto adjacent leaves and stems, causing a grey furry mould, collapse, and dieback. Regular deadheading, carried out every week or two during the season, keeps the plant tidy, encourages fresh flowers, and removes the primary infection sites for grey mould before the pathogen can establish.