Growing avocados

If you are serious about producing avocados, buy your trees from a nursery. Poor quality trees will grow slowly in the orchard, regardless of how carefully you tend them. Look out for the following:

• The size of the containers in which the plants are grown. Generally, the larger the container, the healthier the root system and the chances of successful establishment.
• Growth medium. A clay soil mix tends to clog the drainage holes in a plastic container. Lighter soil improves drainage and stimulates the development of the root system.
• Foliage. A healthy tree has a glossy, dark-green colour with no deformed or discoloured leaves

• Graft union. Grafting combines tissues from two plants to create a new plant. One plant – the ‘rootstock’ – is chosen for its roots. The other – the ‘scion’ – is chosen for its stems, leaves, flowers or fruit. With avocados, excessive thickening at the union (join) or large differences between rootstock and scion thickness are signs of incompatibility, a poor rootstock or a diseased scion. You should see a smooth union, with few or no irregularities on the stem.

 SOIL OF AVOCADO

A healthy avocado tree has a root system that can penetrate the soil to a depth of 1m. It is essential to determine in advance whether your soil is suitable for avocado production.
To do this, it is necessary to take soil samples. Dig profile pits at least 1,5m deep in different places on your land. Even if the soil on the surface seems to be much the same everywhere, dig at least one profile pit per hectare.
In hilly areas, dig the holes in different positions on the slope to get an indication of the drainage properties. (In a lower-lying area, you might have to dig furrows to prevent water from accumulating).
Note the following in each profile pit:

• Colour: Only reddish-brown, red and dark-brown soils are suitable.
• Texture: Avocados do best in soils with a clay content of 20% to 40%. If the clay content is lower, the soil has a limited water-retention capacity. Too much clay, on the other hand, makes irrigation difficult because over-irrigation and high rainfall lead to over-saturation of the soil.
• Soils that can be broken into hard clods when dry will restrict root development. Ideal avocado soils display only small, fine cracks when a dry profile wall is examined.
• Patches: If there is a light-coloured layer with many patches 1,8m or so below the soil surface, root problems might be experienced. This type of soil is not ideal for growing avocados.

Soil analysis
You will also need a soil analysis. Take a representative sample, using a soil auger or a spade, from the proposed orchard at least nine months, but preferably 12 to 24 months, before planting. This will give you enough time to prepare the soil thoroughly.
Each soil sample should be homogeneous – that is, have no visible differences. If there are variations in colour and texture, provide a separate sample of each part.
Take samples up to 0,3m deep for topsoil and 0,3m to 0,5m for subsoil. A sample should be made up of at least 10 sub- samples, preferably more.
The area represented by the complete sample should not exceed 3ha.
Mix the sub-samples from a particular land in a clean container (not a fertiliser bag). Take a 2kg sample from this, place it in a suitable container and submit it for analysis.
The results will determine which fertilisers should be applied before planting.
If required, lime or phosphate should be thoroughly worked into the soil before planting.
Soil preparation
Loosen the soil as deeply as possible before planting. If the soil is very acidic, heavy lime applications may be necessary.
Distribute about two-thirds of the recommended agricultural lime over the entire area 12 months before planting, mixing it into the topsoil by disking and ploughing it in as deeply as possible.
A cover crop can then be planted and ploughed in six months later to increase the organic-matter content of the soil.
Apply and lightly work in the remaining lime and the required phosphate simultaneously. The trees can be planted three months later. Do not fertilise recently planted trees too soon.
The trees must first become well established. In most cases, it is advisable to wait a year.
These applications should be very light, and the fertiliser must be applied evenly and not come into contact with the stem of the tree.
Irrigate immediately after applying the fertiliserChoosing the cultivar
The above are general guidelines. To choose the specific cultivar, bear in mind climatic conditions. Of the three best-known avocado races, West Indian cultivars are best adapted to continuous hot, humid conditions with a high summer rainfall.
They do not tolerate frost well. The best temperature for growth is 25°C to 28°C (humidity above 60%).
The Mexican races originated in the country’s cool, subtropical highland forests, and mature trees can withstand temperatures of -4°C to -5°C.
They should not be planted in areas prone to frost in August and September, as flowers can easily be damaged by frost. The optimal temperature for growth is 20°C to 24°C at a humidity of between 45% and 60%.
Guatemalan cultivars require a cool, tropical climate without extremes of temperature or humidity.
The flowers are particularly sensitive to frost. High temperatures of about 38°C, especially if combined with low humidity, could cause flower and fruit drop. A humidity level of 65% or higher is required.
The Fuerte, the most commonly grown cultivar in South Africa, is thought to be a natural hybrid between the Mexican and Guatemalan races and has a wider climatic tolerance (especially to cold) than the pure Guatemalan types.
Optimal growth temperature is 20°C to 24°C. High temperatures, especially during flowering, are not tolerated well.
Rainfall/wind

All avocado cultivars produced commercially in South Africa are sensitive to water stress. Annual rainfall in the growing area should be higher than 1 000mm and well distributed, with the only dry period in June and July.
However, most of the suitable areas in South Africa experience dry weather during flowering, necessitating supplementary irrigation.
From a climatological point of view, the best areas for commercial avocado production are the cool, subtropical parts of Mpumalanga, Limpopo and KwaZulu-Natal, where the rainfall is fairly high and mist occurs frequently.
A high humidity (in excess of 50% by 2pm) is desirable as it decreases stress conditions, particularly high temperature. This plays an important role during flowering and fruit set. South Africa’s mist-belt areas are especially suitable.
Source: DAFF in co-operation with the ARC-Institute for Tropical and Subtropical Crops