A home orchard needs citrus trees
By Gareth Carter
Spring is a great time to be adding to or planting a home orchard. There are plenty of summer-fruiting berries and frost-sensitive varieties available in the garden centre now. Varieties such as banana, passionfruit, tamarillo and cape gooseberry all grow well in sheltered pockets of home gardens around Whanganui.
It is the ideal time to plant these as they will be established and be of some size by the time the cooler temperatures arrive in June next year.
Citrus are more cold-hardy than the above and perform well in Whanganui. Citrus includes the well known lemons, mandarins, oranges, tangelos, grapefruit, limes, lemonade and some lesser-known fruits including limequat and tangor (a cross between a mandarin and an orange). These popular trees are productive and ornamental. They can grow well in the garden or as container specimens, with sweet-smelling white flowers during spring and summer and the fruit's decorative appearance during the autumn and winter months.
Here in Whanganui we have a climate that generally grows good citrus. Most citrus trees are subtropical or tropical and will tolerate temperatures to around -2°C. Trees are fairly slow-growing, with a mature specimen taking 15-20 years to reach 4-5 metres. Where the location is sheltered from wind and warmer the trees will grow and mature faster.
Citrus trees will generally try to produce fruit from the first year, but at this stage the tree's ability to bring fruit to maturity is often questionable. A good practice is to remove flowers and fruit for 2-3 years to allow a strong branch framework to establish. In subsequent years if the tree is still producing larger crops than it can sustain, the removal of about one-third of the crop will ensure the tree does not get into a pattern of biennial bearing. This is where the tree switches between a year of heavy fruit production and a year of minimal cropping.
To be grown successfully, most citrus trees are grafted on to a rootstock. The main rootstock used in New Zealand is Trifoliata - it is vigorous, allowing the tree to grow to 4-5m. It is also tolerant of heavy and wetter soils and creates increased frost hardiness. By trimming or growing in a pot, citrus shrubs can be kept at 1.5-2.5m.
Meyer lemons and Tahitian limes can be successfully grown on their own roots. These are particularly suited to pots and small gardens, as the plant vigour is less than that of a grafted tree of the same type, with trees reaching 1.5m if left untrimmed. They still fruit prolifically from a young age, the trees just don't grow as big.
Citrus are gross feeders and thrive in good soil with regular feeding of a specialised citrus fertiliser. Plants which are showing yellowing of the foliage should in addition be given a top-up of magnesium. Yates Liquid Magnesium Chelate is highly recommended to make nutrients readily available to the plant. Where soils are lighter and sandy, particularly in parts of Springvale, Gonville and Castlecliff, an extra dose of Epsom salts is recommended more often. In lighter soils particularly, an application of mulch around the base of the tree at the start of each summer will also be of benefit in conserving soil moisture and helping retain nutrients.
Pruning is only required for shaping and plants are better left untrimmed from a fruit-yield perspective. Avoid any pruning between the early spring to midsummer period to reduce the risk of attack from borer beetle. If you do prune, be sure to seal cuts with pruning paste. The tell-tale sign of a borer attack is sawdust piles on and around the plant from holes in the stems/trunk where the beetles are active. This can be controlled with the use of No Borer Spray Injector. They can be difficult to control, so prevention is better than trying to fix later.
Any shoots from the rootstock should be removed as this will reduce vigour from the tree and subsequent fruiting potential.
A long, hot summer when trees are well watered will result in better fruit production, followed by the cooler months which promotes the change in skin colour of the fruit from green to yellow. When the summer is cooler the crop yield, size or quality tend to suffer.
When growing citrus in pots and containers it is important to use a "premium" potting mix such as Tui Pot Power, and fertilise monthly or bi-monthly using a specialist citrus fertiliser that is suitable for pots and containers, such as the Osmocote variety made especially for citrus. The addition of SaturAid re-wetting granules to citrus growing in containers is highly recommended. This product should be applied annually - it channels water to the root zone where it is needed most. It promotes even water distribution so there is less water run-off and fewer dry spots in potting mix and soils. It makes watering, rainfall and fertilisers more effective. It can also be used in the garden even in sandy, clay or compacted soils.
The most common problem with citrus is usually sooty mould, a black sticky substance on the leaves and stems. This is a secondary problem caused by the presence of scale and aphids which while sucking the goodness from the tree, secrete a sugary substance upon which the mould grows. The sugary substance is also attractive to ants. The good news is this is easily controlled with a spray of a suitable insecticide such as Growsafe Enspray 99: this is an organically certified spraying oil. If you are unsure, take some sample leaves into a garden centre for advice.
• Gareth Carter is general manager of Springvale Garden Centre.