Caring for young citrus trees

Date: December 2004

Watering

After the initial watering at planting, the soil surrounding the young trees  should not be allowed to dry out. The frequency of watering will vary with soil type and seasonal conditions, but it is likely to be at least weekly while the trees are being established.

The tree roots occupy only a very small soil volume, so soil moisture should be monitored.

Frequent irrigations of very short duration are required. Mulching will dramatically reduce surface evaporation from around the young trees, allowing longer intervals between irrigations. Mulching will also reduce peak daily soil temperatures, which can assist early establishment.

Soil management

Every effort should be made to maintain or improve the stability and fertility of the soil. This means keeping cultivation to a minimum and maximising the addition of stable organic matter to the soil.

Nutrition

The only added nutrients which young trees are likely to need in the first, non-cropping years are nitrogen and phosphorus. Because phosphorus does not move readily in the soil, it is best applied early in the preparation of the site, preferably before ripping, so that it will be well mixed in the root zone.

Nitrogen fertilisers are readily soluble and move easily through the soil with rain or irrigation, so they should be applied when and where they are needed. Young trees should be fertilised with nitrogen frequently during periods of active growth.

During the first growing season the total amount of actual nitrogen should not exceed 50 g per tree. Spread it carefully around the drip ring (an imaginary circle on the ground below the edge of the canopy), as root burn and tree death can be caused by careless application or over-application of nitrogen to young trees.

Tree vigour and leaf colour are one indicator of nutrient levels, especially in young trees. Large, deep-green leaves indicate an adequate supply of nitrogen. Leaves that are pale green to yellow can indicate a shortage of nitrogen. However, yellowing may also be due to root damage, overwatering or poor aeration, so do not apply fertiliser without first being satisfied that lack of nitrogen is the problem. For more information, see Citrus nutrition.

Wind protection

Young trees need to be well protected from strong and especially hot and dry winds. Short-term protection can be achieved by growing tall annual crops between the rows before planting. However, permanent windbreaks should be established prior to planting.

Species suitable for short-term protection include tall winter-growing cereals such as rye corn or barley between the rows. Summer-growing hybrids of sorghum, such as ‘Jumbograze’, can grow to 3 m and provide excellent short to medium term wind protection. Hybrids have the advantage of not self-seeding and so do not spread into the orchard.

Many tree species are used for windbreaks in Australia, and species selection varies with location.

• More permanent wind protection can be achieved with Barna grass windbreaks every five to six rows. Barna grass (Pennisetum purpureum) is a perennial species that will persist for many years. This practice is widespread in Sunraysia (Victoria) and the Riverland (South Australia).
• Casuarina species are commonly used as windbreak species in Australian orchards.

Trunk wraps

Trunk wraps of insulation foil, or more permanent PVC, protect the young trees from vertebrate pests such as mice, rabbits and hares. They are also useful for reducing suckering on the tree trunk, protecting the tender tree trunk from sunburn in summer, and providing some frost protection in winter.

Trunk wraps are only required for the first year or two and should be regularly checked to ensure they are not tight, cutting into the trunk and restricting tree growth.

Ants will often colonise the space between the tree trunk and the tree wrap. They build columns of dirt up around the trunk, which retain moisture and  can introduce soil-borne pathogens, such as Phytophthora collar rot, above the union into the scion. If ants have colonised this space, remove the wraps, disturb the colonies and apply looser wraps if the trees still require protection.

Pests and diseases

There are a number of pests which have insignificant effects on mature trees but can have severe effects on young trees. These pests include:

• aphids
• soft scales and armoured scales
• snails
• crusader bugs
• bronze orange bugs
• citrus leafminer.

Young trees and newly established blocks do not provide good microclimates for predators. As a result, pests which are often well controlled naturally in mature citrus groves can increase in number significantly in young plantings. The use of insecticides, which is increasingly unnecessary and discouraged for mature citrus trees, may be necessary in young plantings. Consult your local advisor on specific pests for details on diagnosis and control recommendations.

In coastal areas where there is a high rainfall, diseases are also a problem, and applications of protective fungicides may be required.

Weed control

Citrus trees have a shallow fibrous root system compared with some other tree crops. Consequently citrus is susceptible to weed competition. Young trees will respond well to a weed-free growing environment. Mechanical weed control increases the risk of root damage or trunk damage.

Chemical weed control is feasible, although very few herbicides are registered and recommended for use around young citrus trees. They include:

• pre-emergents, which ideally should be applied to bare soil surfaces;
• non-systemic knockdowns, which are very effective on young tender weeds.

These herbicides will not eliminate perennial weed species such as nutgrass, couch or mature paspalum, hence the need to adequately control these weeds prior to planting. For further information, consult your local advisor.

Non-chemical weed control

Weed matting

Weed matting is a woven polypropylene material that is used to cover the soil surface to prevent weed growth. Weed matting allows passage of water and air but excludes light from the soil surface, preventing weed germination. It is widely used in the nursery industry and has been successfully used to a limited extent in citrus groves to eliminate weed competition without the use of herbicides.

Mulching

Mulching around trees with organic materials such as straw, rice hulls or compost will assist in suppressing weeds by excluding light from the soil surface. As mentioned previously, mulch will also help retain moisture in the topsoil by reducing surface evaporation, as well as moderating soil surface temperatures. The mulch should not be in contact with the trunks of the trees, as this can provide a site for pests and diseases to attack the tree — the bark becomes soft and vulnerable to cold and to attack by pest and disease  if it is covered by moist organic matter.

Cultivation

Alternatively the tree lines could be cultivated. However, when the soil is moist, cultivation can lead to smearing at the bottom of the ploughed layer, resulting in a barrier to root growth. There is also the risk of bringing up weed seeds when the soil is disturbed.