Lesson 3 - Temperate fruits
As discussed earlier, knowing your climate is critical to successful fruit growing, particularly temperature and rainfall. Temperate fruits are found in in regions with four distinct seasons. Some grow best in ‘cool temperate’ climates, where the average temperature is above 10°C and cold winters with minimums often below 3°C. as discussed, this period of chilling may be needed to break dormancy for many fruiting species. By contrast, some plants have needs for higher temperatures for longer periods (i.e. growing degree days) and are better suited to ‘warm temperate’ climates, where the average temperature across the year is greater than 10°C and never less than 3°C. These climates tend to have long, warm summers and mild winters and not surprisingly, it’s the fruiting plants without a chilling requirement, plus those that need extended warmth for fruit to ripen, that do best in these climates. Sometimes these warm temperate regions are also considered to be a ‘subtropical’ climate.
The easiest fruits to grow in temperate climates are those that will consistently grow well and don’t require significant inputs over the year to produce fruit. Let’s start with citrus, because there is nowhere in Australia where you can’t grow a citrus of some kind – from Key Limes and Pomelos in the tropics, to Meyer Lemons and Mandarins in cooler climates. But it’s the lemon (Citrus × lemon) that will look at in detail as its one of the most popular and widely planted garden trees in Australia, but it can also be one of the more challenging! Lemons are evergreen trees that have been in cultivation for more than two thousand years, resulting in perhaps 30 different cultivars, three of which are commonly grown in Australia – ‘Eureka’, ‘Lisbon’ and ‘Meyer’. These cultivars vary slightly in terms of their climate suitability as well as their dimensions, foliage traits (particularly thorniness) and fruit attributes – so do some research to work out what will be best for you. There are also dwarf lemons that are great for container growing. Cultivars such as ‘Lots A Lemon’ and ‘Dwarf Eureka’ only grow 1.5-2 m in height but still produce full-sized fruit.
Despite the many calls to gardening talkback across Australia, lemons are generally easy to grow in the home garden providing you look after the basics. Lemons will let you know when they are suffering from pests or a nutrient deficiency (observable by yellowing in their leaves). Like many fruit trees, they do best in sunny locations with good wind protection and if you experience frost then provide some protection while the trees are small or consider selecting a more cold-tolerant cultivar such as ‘Meyer’. Lemons are always grafted to rootstocks which provide better tolerance to different soil conditions, disease resistance and increased vigour – resulting in healthier and more productive trees. And while they grow best in well-drained soils, lemons can also tolerate heavier soils, particularly trees that have a Trifoliate Orange (Poncirus trifoliata) rootstock. If lemons are subject to extended periods of drought, they can shed their leaves over a few weeks. However, provided the underlying conditions are OK and the trees are well-watered they will regain their leaves, but it’s not something you want to happen repeatedly.
Lemons do best in fertile soils and benefit from regular fertilising, particularly in the first few years as the tree is establishing, and then during spring and summer when the trees are fruiting, using a well-balanced and complete organic fertiliser. While soils with a neutral pH (6-7) are recommended, they will tolerate soils slightly outside this range. If you grow lemons in acidic soils (pH <6) it’s common to see symptoms of magnesium deficiency, observable as yellowing around the edges of leaves with a very green centre. This can be treated through regular applications of a source of magnesium, such as Dolomite or Epsom salts, although some citrus fertilisers will have this added in as well. Aside from managing plant nutrition, there is little else to do with lemons. Some light pruning every 2-3 years is useful to keep the tree open and improve air movement, aiding disease prevention, Irrigation in dry areas and mulching of trees is recommended.
There are several pests and diseases with growing lemons, although healthy and vigorous trees are often able to overcome these issues. Common pests are Cirtus Leaf Miner, Aphis, Scale, Sooty Mould, Fruit Fly, Bronze Orange Bugs and Lemon Scab, although some of these are only problematic in trees that are stressed or have underlying nutritional problems. One of the more serious pests is Citrus Gall Wasp, a native pest that causes bulging stems and distorted growth (galls) that retards tree vigour and flowering. Unfortunately, the only practical solution is to prune affected branches and allow the tree to regrow. Because of their thin bark, lemons are also suspect to fungi, such as Collar Rot, and in poorly drained sites, Phytophthora Root Rot. Improving drainage and ensuring the lower trunk is free from organic mulch will help to min9imise these fungal problems. Melanose is a fungal disease that can affect lemons, particularly in humid or wet conditions. It appears as small, raised spots on the leaves and fruit, although severe infestations can lead to damaged fruit and stem dieback. Control requires the removal of affected wood and sometimes regular applications of copper-based fungicides.
Another important temperate fruit are apples (Malus domestica), part of the group of fruits known as pomes, which also includes pears, quinces and loquats. Originating in central Asia with cultivation extending back 10,000 years, apples are now one of the most widely distributed fruits grown across the globe. Due to this history there are literally thousands of different cultivars of apples. When considering which apple to grow, firstly consider your climate. Most apples need cool winters with 500 to 1000 chill hours (cumulative hours below 7°C) to break dormancy and produce fruit. However, there are also some ‘low chill’ apple cultivars such as ‘Anna’ and ‘Tropic Sweet’ that require as little as 300 chill hours, making them suitable for growing in warm temperate areas where winters are mild. Apples are also classified on their use (dessert or eating, cooking, cider), their harvest season (early, mid, late), their texture (crisp, soft or firm) and their flavour (sweet, tart, balanced). To fruit successfully, apples need to be cross-pollinated from different cultivars that overlap in blooming and are also the correct ‘pollination partner’. Most nurseries will have this information available to ensure you have the right trees before purchase, but if not, do your research. You can also buy multi-grafted trees (2-3 cultivars on a single tree) that are already the right match for pollination. Neighbouring apple trees can also actively pollinate your trees – crab apples for example are excellent pollen ‘donors’ for most apple cultivars! There is consideration of the tree size. Apples vary significantly in height, partly based on the cultivar, but more from the attributes of the rootstock to which they have been grafted. Most commercially grown apples are semi-dwarf trees that grow from 3-6 m in height, while genuine dwarf apples range from 1-3 metres. It’s the dwarf apples that tend to best in the home garden, especially if you trying to grow them in a small space or even a large container. Again, seek advice before purchase.
Pruning and training fruit trees is sometimes seen as complex but can be managed successfully once you understand a little about the tree flowers and develops fruit. Most apples flowers from fat, plump buds located in short, branched shoots known as ‘spurs’. These can easily be seen during winter when the trees are leafless. Spurs increase when there is greater light into the tree, and when branches are growing horizontal, rather than vertical. So, methods of pruning and training apples that provide these outcomes – such as espaliers and cordons – still have good fruit production but make the tree easier to manage the tree in terms of harvest and protection (netting in particular). Pruning of apples is completed a couple of times during the year – during winter, when the trees are dormant, to develop the overall shape and form of the tree, and again during summer, to encourage greater flowering and fruiting wood for the subsequent year. Pruning should be directed towards increasing light penetration and air movement, as well as removing diseased, damaged or dead wood.
Like most other temperate fruit trees, apples are best in sunny, protected locations with well-drained soils. Apples should be fertilised in both spring and early autumn and benefit from summer irrigation in dry locations to ensure fruit develops. Apples can be subject to several pests and diseases and controlling these is important to maintaining a healthy tree and ensuring a harvest. This includes pests such as codling moth, light brown apple moth, apple dimpling bug and woolly aphids, and diseases including apple scab, powdery mildew and armillaria root rot. These are worse in some climates and locations than others, plus there are apple cultivars and rootstock with good resistance to specific pests and diseases. Which is why choosing the right cultivar for your garden is important. In some sites you may also need to protect your crop from marauding air-borne pests, particularly birds (parrots, cockatoos) and bats. If you do net your trees make sure it is wildlife-friendly, which is a legal requirement in many states of Australia. Preventative methods for pests and diseases are always best and form a part of Integrated Pest and Disease Management (IPDM) strategies, which combine different control methods to minimize the use of pesticides. We will learn more about this in the Module 7 - Plant Health.
Finally, we need to talk about growing a stone fruit. Peaches and nectarines (Prunus persica) originated in China and have been cultivated for centuries as single trees. During the 19th century peach orchards developed, particularly in the USA, due to the development of rootstocks that improved tree heath and vigour. Like apples, peaches also require winter chilling to fruit successfully, although the requirements vary significantly across cultivars. Some low-chill cultivars, grown in warmer climates, require as little as 150 hours, while high-chill cultivars need 600+ hours. They are also one of the earliest deciduous trees to flower, some beginning in late winter in warmer climates. Fruiting attributes, such as taste, flesh colour (yellow vs. white) and ‘freestone’ vs. ‘clingstone’ vary across cultivars and are a consideration when selecting trees to grow. Peach cultivars also range in height including dwarf trees (1.5 – 3 m), semi-dwarf trees (3-4 m) and standard trees (4-6 m), although tree size will also be influenced by the site conditions and how the tree is pruned and trained. Most dwarf and semi-dwarf trees produce full-sized fruit and are best for smaller gardens, although they may not produce the same quantity of fruit as a standard tree. Many of the dwarf cultivars are also excellent for container growing.
Like other deciduous fruit trees, pruning and training is part of sustaining a productive peach tree. Peaches produce fruit on one year old wood (‘laterals’) from what are called ‘triple buds – two flowers flanking a single leaf or vegetative bud. Getting plenty of light into the tree increases production of laterals, so training and pruning peach trees into an ‘open’ or ‘vase-shape’ aids fruiting and makes trees easier to manage. The best time to prune trees is over winter, although some light pruning over summer, mainly to thin out fruiting branches, is also recommended. Most peaches are self-fertile and don’t require cross-pollination to fruit successfully. Although they do flower in early spring, so ensuring there are plenty of pollinators about, particularly bees, can be a challenge in some locations.
There are a few pest and disease challenges when growing peaches, the most notable being Peach Leaf Curl, a fungus causing distorted and discoloured new growth in spring. The only method of control is the use of protective copper-based fungicides during late winter, before buds open. As fruit is ripening fruit fly can also be a problem in affected areas, along with brown rot – both of which may need chemical and cultural controls.