How to calculate chill units and grow great fruit

Without a certain amount of cold – calculated as chill units – plants can’t complete their dormancy, leading to poor flowering and fruiting. Find out what fruits are best for your area.

Without a certain amount of cold – calculated as chill units – plants can’t complete their dormancy properly, leading to poor flowering and fruitings. Picture: Jorg Bauer / Unsplash

When choosing the best deciduous fruit tree varieties to grow, a critical factor affecting success is their chilling requirement. This applies to stone fruits, pomes (apples and pears), nuts (pecans, walnuts, almonds), figs, kiwi fruit and grapes. Without a certain amount of cold – calculated as chill units – these plants can’t complete their dormancy properly, leading to poor flowering and fruiting. Chill units are the accumulated hours during winter when temperatures are below 7C. In warmer areas, you also need to subtract the number of hours where temperatures are above 16C. A simple method for calculating your suburb or town’s chill units is explained on fruit tree grower PlantNet’s website (plantnet.com.au/plant-care/chill-hours-guide/). High chill varieties need more than 650 chill units, medium need 450-650, and low chill varieties need 450 or fewer. There are now low-chill types of apples, almonds, nectarines, peaches, pears and plums that can succeed even in the subtropics. Choose self-pollinating varieties but be aware most apples and pears, and some plums and cherries need another – compatible – variety to cross-pollinate them. This information is on the plant labels and in pollination charts accessible when you order.

Dwarf Varieties - Small Wonders

Bare-root fruit trees are sold in winter online and in nurseries. Dwarf varieties allow you to grow full-size fruit even in small gardens and in large pots.

Fireball apricot

Tango flat nectarine

See plantnet.com.au for stockists of these four dwarf stone fruit varieties.

Sir Don cherry

Spring Satin plumcot

Q&A

Having failed to control the rust decimating my frangipani, I hope to replace it with a small crepe myrtle for summer shade and winter sun. Is that a good choice? What variety could I keep at about 4m tall? Will the rust spores stay in the soil or affect the new tree?

Gregory Frazer, Sydney

Frangipani rust is specific to frangipanis and won’t affect other trees, but spores would remain in the soil to infect other frangipanis. Crepe myrtles are an excellent choice, as they’re low maintenance, waterwise and tough in full sun, with four seasons of interest. The Indian Summer series (at flemings.com.au) range from 3m to 8m tall in various colours. You can prune crepe myrtles after flowering or in winter to control their size.

I’m a big fan of making compost but my heap is a breeding ground for curl grubs. My chooks love them but I’m loathe to use compost on the garden for fear of spreading the problem. Any ideas?

Caroline Underwood, Moree, NSW

Curl grubs are the larvae of various beetles; they help in the composting process, so are beneficial in compost heaps. In gardens and lawns, they can also eat roots but are only a problem in large numbers. You can treat heavy infestations with Eco-Neem but predatory wasps control curl grubs, so plant flowers to attract them. Letting your chooks clean the compost of curl grubs will make it fairly safe to put on the garden.

Some of my lemons have a silvery/sharkskin texture. Is this citrus broad mite? How should I treat it? Can I use the fruit?

Craig Gumley, Perth

Citrus broad mite causes these symptoms; it’s worst in warm, humid, summer-autumn weather. The fruit become blemished when small, and young foliage can also become narrowed and distorted. It’s too late to spray now, but next summer use a hand lens to watch for mites; spray early with lime sulfur or a horticultural soap. You can still use the flesh and juice.

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