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YOUR GARDENING QUESTIONS THIS MONTH

Q

MIX & REFRESH I I don’t want to waste the money spent on last season’s purchases of potting mix and seed-raising mix. Can I reuse old mix? How much fertiliser needs to be added? IRENE BLYTHE, COROMANDEL

A

Spread used potting mix in a thin layer over the surface of garden beds as mulch.

If the bed needs it, you could sprinkle a handful of general fertiliser, lime, blood and bone or sheep pellets underneath (or perhaps a specific fertiliser like citrus food if the old mix is going around a lemon tree, for example). The amount of fertiliser to use per square metre should be in the instructions on the container.

Used potting mix has poor structure and lacks nutrients. There is also a risk that diseases and soil pests could be transferred. But if previous plants weren’t diseased, the old mix can be refreshed by adding water retention crystals, a rewetting agent, sheep pellets, slow-release fertiliser pellets, and if it’s for succulents or striking cuttings, add some pumice for increased drainage.

Refreshed potting mix is handy as a bottom layer in large containers or raised beds. Plant into a layer of fresh mix on top.

Don’t skimp on seed-raising mix. Use a fresh batch to avoid the fungal disease damping off. Barbara Smith

Q

F1 HYBRIDS I wanted to save my tomato seeds but they are F1 hybrids that won’t produce stable seed. How do seed companies produce F1 hydrids if they can’t save the seeds? NIK RAKELS, CHRISTCHURCH

A

F1 hybrid seed is the result of a cross between two plants that are genetically different but still similar in order to gain the benefits that either of the parent plants might offer. This could be characteristics like uniformity, plant vigour, earliness to fruit, disease resistance, shelf life, taste, colour and so on.

The seed stock of the parent plants are open-pollinated types which are kept separate for crossing again when the hybrid offspring runs out.

Tomato seed has quite a long shelf life of up to 10 years if it’s harvested at its optimum stage and stored correctly.

As long as it is a stable cross, the F1 hybrids will always be the same, although there is always a chance that a rogue offspring will occur that’s quite different, such as a reversion to a parent.

A hybrid can take many years to breed and stabilise before it is introduced to the market. Quite often a breeding company will be growing these plants in a controlled-environment greenhouse so they can grow several generations in a year rather than doing one grow out a year if they were outside. Usually breeders repeat grow outs one after the other to check the stability of the hybrid cross. Gerard Martin, Kings Seeds, kingsseeds.co.nz

Q

NIGHT BLOOMER Can you please identify this plant. I was told the flowers open at night possibly to be fertilised by moths. The flower is on the end of a long succulent-like stem. JUDY WALLBANK, AUCKLAND

A

There are several cacti with similar blooms, including Epiphyllum oxypetalum and Selenicereus grandiflorus, known as night blooming cereus or queen of the night (not to be confused with the pest plant Cestrum nocturnum which is also commonly called queen of the night).

The large, heavily scented flowers are fleeting and rare – appearing for one night only. Buds open slowly and often look their best around midnight.

Plants have many lanky, drooping, flattened stems which can be grown over a trellis for support or in hanging baskets. Place plants where the blooms can be seen at night. They can be grown indoors or outside where they get morning sun but are sheltered from the hottest afternoon sun.

Don’t overwater. Water more often in summer but keep plants slightly dry over winter and protect them from frost. Feed with dilute liquid fertiliser in spring to encourage the growth of more stems, each of which has the potential to bloom at the tip.

Propagate more plants by taking cuttings. Cut a broad stem into 10cm lengths. Leave cuttings to dry out for a few days until the raw ends callous over. Place in moist, freedraining potting mix in bright, indirect light. The cuttings should root in three to six weeks. Barbara Smith

Q

SICK HEBES What’s wrong with my hebes? They looked so good last year but now the leaves are tatty and the new growth is deformed and yellow, and some leaves have holes. SACHA MINCHIN, NEW PLYMOUTH

A

Auckland Botanic Gardens Manager and hebe breeder Jack Hobbs identified the brown spots on the leaves as septoria leaf spot. The leaves with browned tips are showing signs of downy mildew infection.

Septoria leaf spot is a common affliction of hebes, notably the numerous hybrids that have Hebe speciosa in their parentage. It is most prevalent in humid conditions so hebes are best planted in sunny, windy places well spaced away from neighbouring plants so air can circulate freely.

Jack does not advocate spraying. He says they have not sprayed with any insecticides or fungicides at Auckland Botanic Gardens for 20 years and their plants have never been healthier.

Jack’s advice is that if a hebe is badly diseased, it’s an unsuitable variety for that region, or it’s simply in the wrong place and might be happier in a sunnier, windier spot.

New Plymouth has a relatively high rainfall climate. In places with a drier climate, the problem would be less prevalent.

Some cultivars such as Hebe diosmifolia and ‘Wiri Mist’ are seldom prone to septoria leaf spot and downy mildew even when grown in humid conditions that are less than ideal for most hebes. Barbara Smith