What are these flies?
Q What are the small black flies on my houseplants? Julie Baker, by email
A The tiny flies are found on the surface of pots and their larvae feed on organic matter in the compost. It’s quite likely you introduced them when repotting with old compost from last year, especially if it had become wet over the winter.
Seeds and plants are expensive compared with the very modest cost of the small amount of growing medium they sit in, so always buy fresh each year, spreading the old on the garden.
Repotting your houseplants in a well-drained houseplant compost with added sharp sand will largely solve the problem. Adult flies can be reduced by placing yellow sticky traps near plants. Many insecticides will control them, but don’t spray when plants are in flower. Q I saw cabbage caterpillars in December. Are they getting hardier? Cyril Jones, Long Sutton, Lincolnshire
A It’s unusual, but not unknown, for cabbage caterpillars to be active in December – bad news for your kalette plants. Generally, winter brassicas escape the predation of caterpillars, while summer brassicas definitely need the protection of butterfly netting.
Typically, the large white has two broods a year. The first hatches from late April with the butterflies from that emerging in July. The second brood is from late July. The caterpillars from that overwinter as pupa or chrysalis to emerge the following spring.
Occasionally, there’s a third brood in autumn that can continue feeding into winter if the weather isn’t too cold. As you’ve found, handpicking is particularly effective in the cooler months as there’s no chance of further egg laying. Q What’s my grandma’s ‘shamrock’ plant? Ruth Simpson, Pontefract, West Yorkshire
A It’s great to hear a houseplant is reputed to have been in the family for over a century. Yours is Oxalis articulata, which comes from south-east Brazil, Uruguay and Argentina.
Oxalis all have the characteristic ‘shamrock-like’ leaf. The family includes some easily-grown houseplants, wonderful alpines, but also some very difficult weedy species.
The plant grows from stem rhizomes and is easily divided in spring by pulling the clumps apart. Keep the divisions large enough to ensure a good flowering display.
Pot up into a well-drained potting compost with plenty of added sharp sand or grit. Water and liquid feed regularly when in growth. Flowering can occur in flushes from April right through to October. Q When will my seed-grown loquat in my polytunnel ever flower and fruit? Gerry Ramsden, by email
A Eriobotrya japonicaa is generally grown in this country for its wonderfully corrugated, leathery evergreen foliage. Despite the name, it originates from central China but has been grown in Japan for centuries. Trees easily reach 6m (20ft) or more.
Grown from seed, it may take many years to reach flowering. The flowers are rather like large hawthorn blossom, but only form on mature trees after warm summers. Some selections of loquat are self-pollinating, but others need cross-pollination.
Fruit develops over the winter so trees rarely fruit in the cool UK climate outside the south west and a few warm, sheltered London gardens. Even with your polytunnel, it would be difficult to fruit as hard pruning will just encourage juvenile growth. Should it ever fruit, you need to thin the clusters, otherwise trees become biennial bearing, only fruiting every two years.