Answering your green thumb questions and home gardening queries. Many thanks to Helen Yung, Horticulturist, for her expertise.
Q – Is ‘Timor Black’ bamboo good for screening? How should we plant them to minimise invasiveness? (Mountain River, Tasmania)
Bambusa lako is from Timor and prefers warm climates. It reaches 20m in the tropics and 8-10m in the sub-tropics, but much less in your climate. It’s said to tolerate down to -3°C with light frosts. The large leaves usually begin about a third of the way up the glossy black stems and it is more open than some bamboos, so it forms a light screen. It is a clumping, not a running bamboo, so is not invasive but you can install a root barrier for extra security.
Q – I’ve sprouted an avocado seed in a glass of water. How big a pot should I transplant it to? I’m in an apartment. Also, what fertiliser should I use? (Melbourne)
So much of home gardening is about expectations. Avocados make attractive pot plants but you shouldn’t expect fruit. Start your seedling in a 20cm pot, using top quality potting mix. In a year or so, go up to a 30cm pot and so on. Gradual increments are better than putting it into a too-large pot. Use liquid fertiliser such as Charlie Carp, PowerFeed, Harvest or Thrivebou every month or two. A sunny spot with wind protection is best, but it will take part shade.
Q – Can we put chipped prunings of bay tree and oleander in the compost heap?
Studies have shown the glycosides and other toxins in oleander are destroyed by composting and are not taken up by plants grown in that compost. Similarly, the pungent oils in bay leaves or other aromatic plants will break down when composted. The smaller the pieces, the faster they decompose.
Q – What care do I give my new air plants, positioned on an inside wreath?
Tillandsia species are small bromeliads that capture water and nutrients through their leaves, so can grow without soil. They need bright, indirect light. Mist with water – frequency depends on their microclimate but let them dry out between waterings. Add fertiliser to the misting water each month. Soaking in a bowl of water for two hours occasionally is recommended, so add it to a list of your home gardening chores.
Q – What flowers could I grow to attract butterflies and for my grandchildren to pick? (Adelaide Hills)
Butterflies love nectar-rich, bright flowers, particularly clusters of small flowers or ones with open flat petals. Butterfly bush (Buddleia) is the standout: look for sterile, dwarf cultivars that won’t become weedy. Others include daisies, heliotrope, sunflowers, pentas, verbena, cosmos and lavender. More at Butterfly Conservation South Australia
Q – What small plants would add colour and interest to our sunny and windy triangular bed in the dry tropics (North Queensland) ?
Try dwarf ixoras, Barleria ‘Purple Dazzler’, Cuphea ‘Mad Hatter’ in mauve or white, blue Evolvulus pilosus and society garlic (Tulbaghia). Natives include yellow buttons (Chrysocephalum apiculatum), strawflower (Xerochrysum bracteatum), native gardenia (Gardenia psidioides) and white fan-flower (Scaevola albida).
Q – Is it a gardening myth that free ferns (Dicksonia antarctica) benefit from some sugar or golden syrup on the crown and should be watered from the top in a trickle?
Plants manufacture their own sugars (glucose) through photosynthesis. No definitive studies have proven refined sugar solutions will benefit them; in fact they can be detrimental. Sugar water can, however, help cut flowers last longer. Tree fern trunks are essentially a root mass, so watering all over and from the top is beneficial.
Q – What sculptural plant would suit a large pot in part shade to feature in my 6m x 4m courtyard? I live in the Southern Highlands of NSW
For a contemporary edge, try tree aloe (Aloe barberae), Agave ‘El Mirador’ or dragons blood tree (Dracaena draco). For lush foliage, fiddle-leaf fig (Ficus lyrata) or lady finger palm (Rhapis excelsa) are good. You could also try cycads such as Cycas revoluta and native burrawang (Macrozamia communis).
Q – Is applying liquid fertiliser and seaweed to leaves harmful? Do they block the leaves’ stomata and disrupt evapotranspiration?
A – Foliar feeding is not harmful but is not very efficient. Much of the benefit comes from fertiliser running off leaves into the soil anyway. However, it is a beneficial way to rapidly supply major nutrients, or to apply iron, and manganese to plants growing in alkaline soils, or to feed plants where soil nutrients are stolen by aggressive plants nearby. Nutrients enter through the leaf cuticle and stomatal pores without detriment.
Q – My foxgloves are huge. I thought they finished flowering but they keep coming up. What should I do after flowering?
Foxgloves traditionally are biennials that grow a rosette of foliage in their first year and produce 1-1.5m-tall flower spires in spring and summer the second year, before dying. Newer strains can flower within one year. After the first flush, cut the main flower stems just above the basal leaves to encourage several shorter flower stems to follow. Foxgloves readily self-seed so you can have an ongoing supply. The plants are highly poisonous.
Q – What shrubs or climbers would hide a sunny 1.5m-tall brick wall but not make a mess in our pool just 2m away? (Ashmore, Queensland)
As living things, most plants shed something. Clumping palms such as cascade palm (Chamaedorea cataractarum), gingers, elephant’s ears and broad-leafed cordylines are some of the tidiest plants but may burn in hot sun. Consider bird of paradise, cannas, Euphorbia ‘Firesticks’, Nandina ‘Gulf Stream’, Dracaena reflexa, dwarf oleanders, and maybe gardenias. Climbers include bower vine (Pandorea jasminoides), glory bower (Clerodendrum splendens) and stephanotis; these will drop more flowers.
Q – My columnar, spiky cactus has produced several offshoots at the base. Can I propagate them?
Yes. Cut them off with a clean, sharp, sterilised knife, and bevel the cut end towards the central core; allow cuttings to dry until they’re callused over, then plant into coarse sand. Spring is the ideal time. Water sparingly until roots form.
Q – What Australian plants can I grow indoors? Would banksias work?
Only plants adapted to low light can survive indoors. Banksias need full sun. Try ferns such as bird’s nest, maidenhair and Blechnum species. Larger plants include umbrella tree (Schefflera), black bean (Castanospermum) and any of the native palms and Ficus species. Hoya and kangaroo vine (Cissus antarctica) are good climbing or hanging plants.
Q – For my suburban citrus orchard, which would be best: seedless Valencia or Washington Navel oranges, or Imperial mandarin? (Melbourne)
Valencia is more climatically adaptable, better for juicing, and the fruit holds on the tree for months from late spring through summer, becoming sweeter with age. Navels are winter cropping and deteriorate faster; easily peeled and segmented, they’re most popular for eating but are more prone to mites and alternate bearing (good crops every second year). Growing both provides oranges for much of the year.
Imperial mandarins crop from early winter over many weeks and can be alternate bearing. The seeded fruit are easy to peel. Adding other varieties that crop later will extend your harvest. Choice of rootstock for your region is important.
Q – How can I get rid of root-knot nematodes in my raised vegie bed?
This serious pest affects most vegetables and also strawberries. Try growing a green manure crop of marigolds, mustard or Japanese oats. Increase organic matter, including chicken manure, to feed predatory micro-organisms. Practise good hygiene and crop rotation; some vegetables that are not affected include corn, leeks and brassicas.
Another option is to give up on soil-based growing altogether and start looking into a basic hydroponic set up. If this appeals to you, you can read more here.
Q – Our well-established lillypilly hedges are infested with soft, white, pea-size pests. Removing by hand seems tedious and ineffective. Can you help? (Sydney)
White wax scale is common inside mature (and possibly stressed) lillypilly hedges where it’s shady and protected. The thick waxy coating around adult scales protects them from sprays; hand removal with disposable gloves is effective, although tedious. Spraying PestOil or Eco-Oil treats crawlers and nymphs; repeat after two weeks. Pruning to open up the interior of the hedges to light helps. Control ants, and keep the hedges well watered and fertilised.
Q – Colonies of small brown ants invade my pots, laying eggs and affecting my plants’ health. Are there any safe remedies? (Menangle, NSW)
Ants like dry and sandy soils; they won’t nest in moist potting mix. Use a soil wetting agent to correct water-repellent potting mix. Try submerging pots in a bucket of water with some insecticidal soap for a few hours. Diatomaceus earth on the soil and around the pots kills ants but is less effective when wet and must not be inhaled. Control scale and aphid pests that ants feed upon.
Q – Why do the buds and flowers of my double pink hibiscus drop in quantity, especially after rain? (Mission Beach, Qld)
Double-flowering hibiscus are prone to bud drop. Excess soil moisture and fluctuating temperatures are triggers. Hibiscus flower beetles burrow into buds, causing them to drop. Collect and destroy fallen flowers. Place white plastic containers with water and some detergent to attract and drown beetles. Hibiscus mealybug could be the culprit – use Eco-neem.