Any one who has ever listened to a talkback gardening program will have heard the proverbial lemon tree question. It seems that everyone grows a lemon and at some stage, everyone has had a problem with one. So what are the things that challenge our lemon trees, and for that matter citrus in general. Citrus challenges could be broken down into four groups:
Pest and disease problems
Incorrect planting time
Below are further details of the main challenges home gardeners experience.
Citrus trees must have good drainage, however more lemon trees suffer from not enough water rather than too much. Lack of water or inconsistent watering will cause citrus trees to drop leaves, flowers and fruit, fruit can split and even worse, the plant can die. As with all plants, it is worthwhile preparing the soil well prior to planting by incorporating compost to improve the soil's water holding capacity and encourage the earthworms and other soil microbes that make for healthy soil. Applying a good layer of organic mulch will help to keep moisture in the soil.
Citrus leaves often go yellowy in patches or right across the leaf. At this time of year, general yellowing across the leaf can be caused by the cold slowing the plants metabolism so that it cannot process the available nutrients, although it may indicate a hungry plant. A common deficiency is iron which causes a condition called chlorosis. It can be seen where the leaves go yellowy or pale but the veins remain green. It is particularly common in alkaline soils, like those common in areas of the Adelaide Plains. It can be treated by using a product called Iron chelates, applied as a foliar spray or simply mixed with water and applied to the soil. Several doses may be required to correct this nutritional deficiency so that the leaves return to green. Other deficiencies that are common in South Australia are lacks in magnesium, manganese and zinc, often exacerbated by our alkaline soils. It is best to feed citrus in spring, summer and autumn with a balanced, organic based fertiliser such as Sudden Impact for Roses. This contains extra potash to help with the flowering and fruiting processes and extra iron to combat chlorosis.
Pest and disease problems
Citrus leafminer causes wiggly lines on the new growth of citrus trees. It is the larvae of a tiny silvery moth and it can affect all citrus trees. Its larvae tunnel in young leaves leaving wiggly lines and crinkling which causes the leaves to curl up when they pupate. Citrus leafminer can infest any new growth, but moth’s egg laying is most prevalent in late summer and early autumn. A less important pest on mature trees, it is kept in check by various parasitic wasps and predatory lacewing larvae. Trees under five years of age however, often sustain significant damage and growth may be stunted. Insecticides are not very effective and will also kill the valuable natural enemies, so rather than try to kill the leafminer, use an oil spray to deter the moth from laying its eggs. Watch new foliage from January to March to see if significant leaf damage starts to occur, and if it does, spray with garden oil such as Eco-oil. Several applications may be required.
This is a common problem on a number of plants and is more likely to affect plants that are under stress. There are many different species of scale insects attacking different plants, but generally they are small rounded, sap sucking insects which, when attached to a leaf or stem, looks like an inanimate black, brown or white lump, approximately 1-2mm long. Scales sucks the sap of the plant and are found on stems, trunks or leaves depending on the species. They reduce the plants strength to a limited extent, but in the process they exude a sweet honey-dew secretion. This secretion will appear as a sticky and slightly shiny covering on the stem or leaf, and it is in this secretion that an opportunistic fungus called sooty mould grows.
The mould gives the leaf and stem a dirty black appearance, which can actually be rubbed off with a damp finger. Eventually as this mould spreads in the secretion it can actually block the sunlight from reaching the leaf and thus affect the plants ability to photosynthesise. Scale is carried onto plants by ants which farm the scale to milk the honey dew secretion.
Consequently, sooty mould is a symptom of another plant problem and until the other problem is addressed it will not go away. Once these insects have gone and the honeydew supply stops, the mould can no longer grow. Spraying with a fungicide is unnecessary. The unsightly mould will eventually dry and fall off the leaves.
Another sap sucking insect that exudes a honey dew which can result in sooty mould is whitefly which may harbour underneath the citrus leaves, especially in the cooler months when it leaves deciduous trees in search of somewhere to reside over winter. An oil spray will help to smother the breathing apparatus of the whitefly and significantly reduce their numbers.
Incorrect planting time
Finally it is really important to only plant citrus when the ground is warm and in the Adelaide Hills this can often mean from October onwards. It also means that planting should have ceased by late March just to be sure. Plants that are planted into the ground over the cooler weather when the ground is cold may 'sit and sulk', go yellow, lose leaves and even die. More often however they fail to thrive and it can take a number of years to overcome this early set back.
When you do plant a citrus of any type, use a scoop of a seaweed based plant tonic such as Seamungus in the bottom of the hole, or apply regular treatments of Seasol to help over come the transplant shock.
What to do in the Garden
In many areas the risk of frost is over, and now with the ground starting to warm up, it is the time to plant frost tender plants such as citrus and passionfruit. If you are in a frost prone area, prune back frost sensitive plants when the risk of frost has passed.
Tomatoes are the most popular vegetable at this time, with many gardeners in cold districts growing their seedlings on in pots, where they can better protect them from any late cold weather. Basil seedlings and seeds can be planted as the ground warms up.
Other vegies to plant include beetroot, cabbage, capsicum, celery, ucumber, eggplants, silver beet, squash, sweetcorn and zucchini.
October is also melon and pumpkin planting season. Sow watermelon seeds now and, in late summer just when the heat threatens to become unbearable, you'll be able to cool down with your own home-grown watermelons.
Beans can be sown now for summer harvest.
With the warmer weather, our plants burst into new growth, but so do the weeds! Hand weed where possible, especially where the weeds are growing close to existing plants. The use of herbicides is often necessary for larger areas such as driveways or paths.
Now is most definitely the time to feed anything that you have not already fed this spring. October is perhaps the most important growth month in any garden and every plant is bursting forth with new shoots, so make sure that you have nourished and 'rewarded' your plants. Treat the garden with generous amounts of slow release organic fertiliser such as Neutrog's Rapid Raiser. Your lawn also requires nourishing at this time with a boosted organic fertiliser such as Sudden Impact for Lawns or Upsurge.
Snails & Slugs - They are at their most active during moist weather and they particularly love new spring seedlings - so be warned! There are many products available to combat these pests, including a number that are not harmful to pets or birds and other forms of wildlife. In my garden I use Multiguard Snail and Slug Killer. This product kills snails and slugs with minimal danger to pets and native animals. It contains no dangerous poisons and is certified as an allowable input on organic farms by NASAA. Multiguard Snail and Slug Killer is iron based so that when the pellets break down they add a trace of iron to the soil and are taken up as a beneficial nutrient.
Rose Aphids - These tiny soft-bodied, harmless-looking insects are sap-suckers, which are particularly fond of developing shoots and flower buds. A few aphids can be tolerated, however severe infestations can result in deformation of new buds, loss of flowers and even defoliation of the plant. They have a very short generation time and can build up in numbers very quickly as the weather warms up.
Traditionally, many gardeners have reached for insecticidal sprays when they see their new rose shoots smothered in these aphids. The problem with such sprays, even those such as low toxic, pyrethrum-based sprays is that they do not distinguish between beneficial insects such as ladybirds, lacewings and wasps as well as other harmless insects, and the harmful Rose Aphid.
The one draw back of natural biological control is that it takes longer to work. Often it can take up to two weeks for the natural predators to build up in numbers and wipe out the aphids. It is in this period that most gardeners panic, and reach for an insecticidal spray. The best thing to do is simply to rub or hose them off the buds. Do this for several days in a row. This will not get rid of them all together, but it gives the natural predators of the rose aphid, such as ladybirds and their larvae and the Rose Aphid Predatory wasp, a chance to build up in numbers.
For a more proactive approach, many home gardeners are now purchasing and releasing beneficial insects such as Lacewings, a practise known as 'integrated pest management', common in commercial horticulture. This allows you to treat insect infestations earlier that waiting for natural biological control to happen. For more information on purchasing lacewings, visit http://www.bugcentral.com.au
Feed – All the public talk and panic over water restrictions has caused some gardeners to overlook a very important spring task – one that can help their plants and gardens cope with the ravages of heat and possibly drought in the months ahead. Gardens must be fed with an organic based fertiliser in spring and again in early summer. This helps to replenish energy expended during the plant’s flush of activity in spring and to ensure the plant is in optimal health before the stresses of summer arrive. Try to time the application so that rain can help to wash it in. Want to know the definition of a keen gardener? It is the one scattering fertiliser pellets in the dark or in the drizzle because it is about to rain!
Mulch – top up the mulch on any areas of the garden where it has become thin or blown away.