Brains Trust

Questions answered by Brent Whiter, gardening expert, nursery owner and ABC South East NSW radio talkback presenter, plantsplustura@gmail.com

FIRE-AFFECTED GARDENS

How do I repair burnt soil?

After fire, there remains copious amounts of ash on the ground and this contains potassium carbonates and elements such as phosphorus and magnesium. This is beneficial to all gardens at the rate of one shovelful per square metre. Dig in with an eco-hydrating soil-wetter to overcome hydrophobicity.

How do I make the right decisions about caring for fruit trees after damage from radiant heat and fire?

Don’t cut back the burnt trees immediately after the firefront has passed through. Always wait at least six weeks and check to see if there is life in the branches. Scratch the bark through to the cambium layer (the tissue layer just under the bark where plant growth occurs), focusing from the extremities towards the trunk. When dealing with grafted fruit trees, the normal occurrence is new growth movement below the graft. If there is no leaf growth above the graft, ie, the scion, you have probably lost the tree. Citrus trees seem to be more badly affected than other fruit trees because of the nature of the sap. Those that survive will need to be carefully pruned back to live growth. The application of eco-seaweed, a solution containing 16% potassium, helps relieve plant stress after fire events and will assist recovery. Apply the mixed solution over the foliage and around the dripline, monthly, during the growing season.

GENERAL GARDENING QUESTIONS

I have dieback and lichens on my old fruit trees – will it harm them?

Lichens form crusty patches which normally favour humid conditions where it is cooler and very shady. Generally lichens don’t harm trees, but they can penetrate bark. On heavily affected trees, they often indicate a loss of vigour and poor air circulation. To improve conditions for the trees, thin overcrowded branches at the appropriate time straight after harvest and aerate the soil if needed. Add compost and fertiliser during the growing season to improve the plant’s general health.

Two years ago I removed an unwell fig tree. It was growing particularly well but didn’t bear any fruit. It was 1.5 metres tall and was in a morning sun position.

Often when trees are struggling and they are given good growing conditions, they concentrate all their energy to grow a good structure and canopy. Fruit production should begin three-to-four years after transplanting. Figs thrive in lots of sun and heat, so fruit production is compromised in a cool position. You may need to move your tree in mid-winter to a sunnier position.

I can’t control the weeds in my vegetable garden even though I am hand weeding. What can I do?

For wind-borne weed seeds, limit germination by laying down lucerne, pea straw or sugar cane mulch to a depth of 30 mm. Any weeds that sprout in the mulch will pull out easily. Suppress weeds that have persistent bulbs, such as oxalis and onion weed, with up to six layers of paper under the mulch. Always remove weeds before they set seeds.

We compost our vegie garden regularly but get lots and lots of weeds. Help!

Generally, compost needs to reach 60 degrees Celsius to kill pathogens and weed seeds. Nearly all home compost bins and heaps don’t reach this temperature. You require a compost heap of one cubic metre, the right blend of ingredients and aeration from turning the material regularly. The easiest solution is not to put weeds and bulbs into your compost. Never put diseased material into your compost. Use mulch on your vegie beds to suppress weed growth.

I have lots of casuarina needles nearby and want to use them as mulch in my garden.

Nothing tends to grow under casuarina or sheoak trees because of the leaf litter and root mass and, in part, due to allelopathy – where plants exude chemicals that suppress growth of other plants. Species that exude these chemicals include bottlebrush, eucalypts and conifers. Phytotoxins break down as the fallen needles age and will compost; this makes them usable, but only sparingly, as mulch.

Should I remove all the flowers and fruit from my lemon tree for the first two years after planting?

Yes. Removing fruit before they develop allows a young tree to put all its energy into growing a strong root system and a good branch framework. Strong growth benefits more fruit in the long-term.

Will planting more native trees and shrubs attract more native bees to my garden?

There are about 1800 native bee species in Australia, including ground-dwelling and hive-based bees. Hive-based native bees perform better in urban areas than natural forests, due to a diverse and regular food and water supply. These bees tend to forage about 500 m from the hive, finding exotic and native plants containing pollen and nectar resin. A combination of plant species is beneficial in your garden.

Is using old dishwashing water harmful to my plants?

Using this water alternately with clean water is safer for your plants. Use it on more robust plants instead of young plants and edibles, or find a more eco-friendly form of dishwashing liquid.

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