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Q COLOUR CHANGE The red poinsettia I bought last Christmas gradually turned green when I repotted and fertilised it. What’s the trick to get red flowers again?

MARIA SLADE, AUCKLAND

The brilliant red bracts A of poinsettia (Euphorbia pulcherrima) are coloured leaves which surround a cluster of nubby, small, yellow true flowers.

Red is the classic Christmas colour but there are white, pink, cream and marbled poinsettias too.

Photoperiodism is the term used to describe the reaction of plants (and animals) to relative lengths of light and dark each day.

Poinsettias need darkness for 12-14 hours a day for at least six weeks in order for the bracts (specialised leaves) to change colour. Poinsettias planted outside in New Zealand turn red in winter while potted plants here are tricked into turning red for a summer Christmas by being kept in artificial conditions with controlled exposure to light.

Either wait for the long winter nights to arrive, or cover your plant with a light-proof box or keep in a dark room for 12-14 hours a day to induce the colour change.

If planted outside, the colourful bracts will last all winter. Plant in a sheltered, sunny spot. Protect from frost. They can grow into leggy shrubs 3m or more in height; prune after flowering to keep plants compact. Like other euphorbias, poinsettias ooze a milky sap Wear long sleeves, gloves and goggles to avoid skin and eye irritation. Barbara Smith

Q BUTTERFLY FOOD

Weevils released to control pest plant Buddleja davidii also decimate non-weedy buddleias loved by butterflies and bees. How can the weevils be controlled? LORRAINE BARNETT, MARTON

A Buddleja davidii is classified as a weed throughout New Zealand and the buddleia leaf weevil (Cleopus japonicus) was imported to play an important role in protecting both native and plantation forests. Buddleia is highly invasive and establishes on disturbed sites before native colonisers can take hold. Buddleia also colonise stream and river beds and cause flooding. The weevil gets into remote and hard-to-reach areas, and is doing a great job controlling buddleia without herbicides.

Jacqui Knight from the Moths and Butterflies of New Zealand Trust says members first noticed the weevil on their buddleias 11 years ago. They applied Yates Mavrik Insect and Mite Spray, following the manufacturers’ instructions to avoid interfering with other biodiversity in the garden.

Jacqui has buddleias in her own garden (not the pest one!). Although she did not use pesticides, the plants seem to have developed a resistance to the weevil so she has beautiful flowers, much to the delight of her admirals and monarch butterflies. She suggests feeding your plants well and mulching too.

Gardeners can also switch to other plants favoured by butterflies. For plant lists, seeds, events and information visit the MBNZT website: nzbutterflies.org.nz. Barbara Smith

Q TOPKNOT CHOP

I A cabbage tree near my compost heap is growing quickly. Can cabbage trees be pruned? Will cutting off the top harm the tree in any way. ETHNE ANDERSON, LOWER HUTT

A Cabbage trees can be pruned but, as occurs when other trees are pruned, doing so alters the growth pattern and ultimately the shape of mature trees. Nipping out the centre of the tuft of leaves of a young tree stimulates a clump of new shoots. You can then thin out and nip off any shoots growing in the wrong direction (encroaching on other plants or blocking gutters, for example) and eventually it will grow into a multi-headed tree.

For larger trees, cut across the trunk leaving a stump at the height you want the tree to regrow from. New shoots sprout randomly from the trunk. Again you can leave the shoots that are growing in directions you want them to go and nip off the ones you don’t want. The stump can look rather ugly for a year or so until the canopy fills out. There’s no way of telling where the side shoots will appear so you might end up with lopsided regrowth.

Mature trees with a large root system will often regrow if the trunk is cut down to ground level. Shoots can grow from the cut stump and also from the roots. You can sometimes see a ring of cabbage trees in a paddock. There would once have been one tree in the centre and the others would have grown from its roots after it had been cut down or burnt when the land was cleared.

Barbara Smith

Q SICK CITRUS

My cirus trees are dying one by one. The leaves go yellow and drop off. But a grapefruit and an orange tree just 3m away are fine. I mulch, spray with copper, water and feed organically. VIVIEN DEAN, BOMBAY

Citrus trees aren’t equally A hardy. ‘Meyer’ lemons, for example cope with colder temperatures than other citrus. It’s possible that the healthy trees are hardier, so more able to cope with whatever the problem is.

Yellow leaves usually indicate lack of nutrients. Drying, curling and dropping leaves (if no insect pests are present) indicate the trees are stressed – too cold, too much or too little fertiliser, frost, too wet, too dry, wind, collar rot or root damage.

You are doing the right thing with the copper spray and watering, but what do you mean by organic feeding? If it is just compost, it might not have the full range of nutrients citrus need. Citrus are heavy feeders and need a fertiliser with extra trace elements including iron and magnesium. Apply a citrus specific fertiliser in spring, summer and autumn, but don’t feed over winter when the trees aren’t actively growing.

If the trees are in a windy spot, they might be rocking just enough to damage the roots and prevent uptake of nutrients and water.

The photos don’t show the tree trunks. If mulch is piled up touching the trunk, the constant dampness and lack of air movement can cause collar rot – again affecting the tree’s ability to take up nutrients and water. Leave a gap of at least 10cm between the mulch and the trunk.

Barbara Smith