For gardeners on the east coast – don’t forget that it is a legal requirement in some areas to remove galls from citrus trees by the end of August. Gall wasps lay eggs in main stems or fruit stalks. This forms a swelling and gall wasp larvae weaken trees. As trees may be forming blossoms, delay in pruning may reduce next year’s crop. If you haven’t done so, check citrus trees (especially grapefruit and lemons) this week and prune off any galls before adult wasps emerge to lay eggs in new shoots. Burn affected prunings or dispose of them in a sealed plastic bag. See: Citrus gall wasp
A reader has found that the soil around his fruit trees has become quite hard and has asked should he dig around the trees to loosen the soil.
It is not a good idea to dig around fruit trees as citrus, for example, have very shallow roots. Digging around these and stone fruit trees will damage roots and quite often cause suckers to grow from the root stock. Avocado trees deeply resent any root disturbance.
Hard soil can be a problem in extreme weather conditions, particularly if the soil has not been covered with mulch. If the area is weedy, cut off the weeds at ground level rather than pulling, or digging, them out. Plants obtain most of their energy for growth from light. As you are going to deprive them of light with this method, you can leave the roots to break down and add organic matter to the topsoil.
If grass has covered the area, cut it short and, with a sharp edged spade, cut through the runners 20 cm outside the ‘drip line’ of the tree, which is the area of soil directly below the outside edge of the foliage canopy of the tree – so called because it is where rain drips off the foliage canopy and the feeder roots of trees lie in this area.
Apply a light application of organic complete fertiliser to the soil surface in the drip line area and give the tree a thorough watering, but not close to the trunk. Immediately after watering, cover the soil surface beneath the tree out to the drip line with a 3 cm layer of compost, keeping the compost at least a hands width from the trunk. Compost contains microorganisms (and often earthworms) that will help break down the weed roots and make the soil more friable.
Then cover the compost with a 5 cm layer of organic mulch to keep compost damp and deter weed growth, keeping the mulch well clear of the trunk and extending it to 20 cm beyond the drip line of the tree. Don’t use compost as mulch, as it will dry out and you will lose most of its benefits.
I love warm, still spring days when the air is heavy with the sweet fragrance of citrus and Jasmine flowers, and the lethargic buzzing of bees that seem intoxicated by the perfume.
Citrus trees produce many more flowers than the tree could support if they developed to full maturity as fruit, and a reader has asked if she should remove some of the flowers.
This is not necessary, as most species of citrus will shed the excess flowers at the end of the flowering period when the developing fruit is no larger than a pea. A further shedding of tiny fruit often occurs when they are about 2 cm across. However, there are other causes of fruit drop beyond what the tree intends. Water stress and sap sucking by Bronze-orange or Spined-citrus bugs are common causes. A deficiency of copper (which is only needed in tiny amounts) will cause citrus trees to drop small fruit, and flowering stage is a good time to give your trees a drink of seaweed extract tea, especially if your soil is low in organic matter.
Make a note of when your citrus trees flower because 3 months after flowering the final amount of fruit the tree will produce that season is established. At this time you can remove excess fruit, particularly on young trees.
On some species of citrus, flowers are well spaced as in the photo above. On other varieties flowers form in clusters and, even if only a third of the flowers in this photo developed into fruit, the weight of the fruit will cause new wood to bend downwards, resulting in a poorly shaped tree in years to come. The weight of excess fruit can also split young trees. It is worthwhile being patient with young citrus trees, and foregoing fruit until they have established a sturdy framework.
On older trees, simply pinch out any young fruit that will form a compacted cluster when mature to avoid putting too much weight on new wood at the end of branches.
Autumn is a good time to check your citrus trees for magnesium deficiency. Citrus have a high magnesium requirement and magnesium is essential for the formation of chlorophyll (green colour) in leaves. Without enough magnesium plants will not be able to make sugars and starches, and growth will be poor.
Magnesium deficiency often shows up in citrus in autumn because magnesium is also required for developing fruit and many citrus species produce fruit over the cooler months.
Because magnesium is very mobile in plants, a shortage of this essential element results in magnesium being drawn from the older leaves to new growth. Deficiency shows as pale leaves, beginning with inter-vein yellowing of the outer edges of the oldest leaves, so that a green V remains with the point of the V at the leaf tip, and widest part of the V closest to the stem. In extreme cases, entire leaves may yellow.
Magnesium deficiency can occur in several ways. If soil is too dry roots can’t absorb magnesium, so regular watering of citrus is necessary. It is more common where soil is quite acidic and this can be remedied by watering in some dolomite, which will supply magnesium, plus calcium to raise the pH. If soil pH is in a suitable range for citrus, magnesium deficiency can also occur where heavy rain has leached it from soil, or where excess potassium has been added to soil – this includes use of wood ash, or overuse of seaweed fertilisers, which can be very high in potassium.
In these situations, a quick remedy to save this year’s crop is to dissolve some Epsom salts in a small amount of warm water, then dilute it in a full watering can of cold water, and water it into the soil under the outer part of the foliage canopy. You will need about 250 g of Epsom salts for a very young tree, and up to 2 kg for a fully-grown tree.
Magnesium is also important for sweetness of fruit. If your citrus fruits are not as sweet as you would like, it could be due to magnesium deficiency.
However, a general yellowing or paleness of all leaves (chlorosis), while only the veins remain green, could be the result of iron deficiency. Iron deficiency begins in the youngest leaves. This can occur where soil is too alkaline for the tree to absorb iron. If the alkalinity occurred through an accidental overdose of lime or dolomite, the pH can be lowered by adding elemental sulphur to the soil around the tree. If soils are generally alkaline, including some well-rotted cow or horse manure (under mulch, but not dug in) as part of your fertiliser will help reduce the pH by replacing some of the calcium ions in soil with hydrogen ions as it decomposes. To prevent crop losses, it is worthwhile checking soil pH around citrus trees each spring, and correcting it, if necessary.
See also: Feeding citrus
I have revised this post to provide more detail because there have been a few enquiries recently about fertiliser requirements for citrus, particularly trace elements.
Time to fertilise
There is no set time of the year for fertilising citrus as different species produce fruit in different seasons. Many citrus trees are producing crops at this time of year (through winter) and it is not a good time to give them a good dose of fertiliser. As citrus go through a growth cycle after fruit has matured, a good general rule is to apply fertiliser to the soil surface, under the outer part of the canopy, after fruit has been harvested, then cover the fertiliser with about 5-8 cm of organic mulch. Avoid scratching fertiliser into the soil surface as citrus roots lie close to the soil surface, and never apply fertiliser to dry soil, as it will burn tree roots. Repeat the fertiliser application in approximately six months, but avoid applying fertiliser in very hot weather.
Suitable citrus fertilisers
Citrus trees require a good supply of fertiliser that contains a full range of both major and trace elements for good growth, and pest and disease resistance. Trees to five years of age can use up to 500 g of complete organic fertiliser per year, as the inclusion of organic matter in soil will make nutrients more readily available. This should be divided into 2 applications. Older trees may require a little more. Very young trees should receive a proportional amount, as they will also benefit from the occasional application of manure tea. Too much high nitrogen fertiliser will attract aphids, scale, the citrus butterflies and citrus leaf miner. Over fertilising can also kill citrus trees.
Compost is the best fertiliser for citrus, but worm castings, poultry-based fertilisers, and well-rotted manures will also keep trees healthy.
Major nutrient elements are: nitrogen, phosphorus, potassium, calcium, magnesium and sulphur. Part of the role of phosphorus in plants is to promote root formation and early growth. Heavy applications of high nitrogen fertilisers can make phosphorus unavailable to plants. Phosphorus is also only available within a narrow pH range. Keeping the soil pH around citrus trees close to neutral will improve their growth, and phosphorus from organic sources is more readily available to plants. Citrus have a fairly high requirement for the major element magnesium. Signs of magnesium deficiency appear first in older leaves where yellowing begins at the outer edges of the leaves and moves inwards, resulting a green V shape at the stalk end. It can also cause cupping of leaves and lack of sweetness in ripe fruit. This problem is common in citrus in autumn when fruit it maturing. This deficiency can be corrected quickly by watering in some Epsom salts: about 250 g for a young tree up to 2 kg for a fully-grown tree. If pale leaves have occurred on your citrus trees in the past, in future, apply one handful of dolomite per square metre of tree canopy when fertilising after harvest. In soils of SE Queensland that contain high quantities of magnesium, this problem will only occur where far too much potassium has been applied, or where soil has become quite acidic.
Yellow leaves in late winter, or early spring, are often caused by cold soils, if the tree has been adequately watered and fertilised. This problem will correct itself as the soil warms, and the tree begins to extract nitrogen from soil.
As mentioned above, citrus trees require a full range of trace elements. These are: iron, copper, zinc, molybdenum, manganese and boron. Their availability to plants is dependent on soil pH and the presence of organic matter in soil.
Copper deficiency will cause fruit drop and can, in more serious cases, cause gum to form inside and outside fruit, and on shoots. Iron deficiency reduces citrus crop size and causes leaves to gradually become pale green, and then fade to pale yellow, preventing the tree from manufacturing carbohydrates. Zinc deficiency reduces fruit bud formation, and manganese deficiency prevents vitamins forming in fruit. Zinc and manganese deficiency both produce yellow mottling between veins on young leaves, but in zinc deficiency leaves are smaller than normal and bunch together. Boron is essential for flower production and fruit quality, but boron toxicity can be a problem where laundry grey water containing borax has been used for irrigation.
Trace element deficiencies can occur if your soil, or the fertiliser you have been using, does not contain a particular micronutrient. However, they are most commonly caused by your soil being too acid or alkaline where nutrients are locked into compounds that plants can’t absorb. Humus in soil has a pH of around 6.5 where all nutrients are available to plants. Humus is also able to hold trace elements in a form that is easily absorbed and prevent nutrients leaching away through soil.
It is unwise to apply trace elements individually because, as the name implies, they are only required in tiny amounts, and excess applications can be toxic to plants, causing another range of problems.
To avoid trace element deficiencies, add a moderate amount of compost to the soil surface around your citrus trees. Organic mulch will also produce humus after friendly microorganisms break it down, and earthworms distribute it through soil. Also apply a liquid seaweed extract, at weak black tea strength, in autumn and spring to ensure your trees have access to a full range of trace elements. Seaweed also contains a good quantity of potassium to improve fruit quality and build plants’ resistance to pests, disease, frost and drought by strengthening cell walls. If you are experiencing serious problems with the health of your citrus trees, I suggest you test and correct your soil pH, or problems will continue.
Reasonably priced Soil pH test kits are available from most nurseries.
Unlike many other wasps that assist pollination or are pest predators, the citrus gall wasp is a true pest in coastal gardens of New South Wales and Queensland. It is an extremely tiny black wasp that lays its eggs in young stems of citrus trees, particularly lemon and grapefruit varieties, and the native finger lime. The larvae remain within the stem, stimulating the growth of cells, and causing a gall or swelling to form on the infested stem by early summer. Because these wasps are poor fliers, they tend to reproduce on the same tree unless blown by wind to a new host. Trees that are repeatedly attacked will become weaker and produce less fruit.
During winter, after the Full Moon, prune off all galls and burn them, or dispose of them in a sealed plastic bag. Don’t add galls to the compost heap. The wasps emerge from the galls to lay a new batch of eggs in September or October, so it is necessary to remove all galls before the end of August. Failure to do so can result in fines being imposed in some areas.
It is very likely that the gall in the photo missed last year’s pruning because it is unusual for galls to reach that size in one season. As you can see, the tree in the photo is also affected by scale, and it is more common for citrus gall wasp to attack stressed trees. After pruning, water the tree thoroughly, and feed it with a complete organic fertiliser and as much compost as you can spare. A drink of seaweed extract tea will help it to resist further pest and disease attack.
Jane e-mailed: I have a question about my lemon tree. I bought a new one and planted it in a few weeks ago, and this week we have had a lot of rain. It has a fair bit of new growth, but the new leaves are red in some places. Is there something wrong with my tree?
You have no reason to worry Jane, as you certainly have a healthy-looking tree.
Red colouring in new leaves is a common occurrence in plants that have adult leaves with a leathery texture, including avocado, citrus, eucalypts, oaks and roses. The colour is caused by the tree producing red anthocyanins (a type of antioxidant). These are believed to protect the young, tender leaves from ultra-violet light. The leaves will turn green as they toughen.
When citrus produce a lot of new growth as the weather cools, an application to the soil around the tree of seaweed extract at weak black tea strength will build the tree’s resistance to the effects of cold weather. Seaweed contains compounds that strengthen cell walls.