Gardening: How to grow citrus in Whanganui

Whanganui has a climate that grows good citrus. Photo / 123RF

Autumn is a great time of year to be planting citrus fruit.

The new plants from the nurseries are lush and full because of good summer growth and garden soil is moist from autumn rains. And with winter approaching, the prospects are good for further moisture to aid root establishment.

Citrus are one of the favourite fruit plants in Whanganui, for small and large gardens as well as container production, with eventual tree sizes being controlled by the use of dwarfing rootstocks.

The most popular citrus grown in Whanganui are lemons and mandarins. Though all the others are grown too.

Here in Whanganui we have a climate that 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 to 20 years to reach four or five metres on a larger rootstock and two to three on dwarf rootstocks.

For a smaller garden, they can be contained and kept smaller with pruning or growing in a pot, which will restrict the size.

Where the location is sheltered from wind and warmer, the trees will grow and produce more fruit faster.

Citrus trees will generally try and 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 two to three 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 approximately a 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 no fruit or low fruit numbers and a year of heavy fruit production.

To be grown successfully, most citrus trees are grafted onto a rootstock. This plays a major part in determining the ultimate height the variety will grow, as well as having an effect on the hardiness of the tree.

There are two main rootstocks used in New Zealand - 'trifoliata' and 'flying dragon'. Trifoliata is the most widely used - it is vigorous, allowing the tree to grow to four or five metres in 15 years. It is also tolerant of heavy and wetter soils and creates increased frost hardiness.

When grown in pots the trees tend to dwarf themselves to the size of the pots and can be successfully maintained at 1-2m and produce well for many years.

'Meyer Lemons' and also 'Bearss (Tahitian) Limes' and 'Finger Limes' can be successfully grown on their own roots.

These plants are particularly suited to pots and small gardens as the plant vigour is less than that of a grafted tree of the same type.

They still fruit prolifically from a young age, the plants just don't grow as big.

Citrus are gross feeders and thrive in good soil, with regular feeding of a specialised 'citrus fertiliser'.

Plants showing a yellowing of the foliage should in addition be given a top-up of magnesium ('Tui Epsom Salts') or 'Yates Liquid Citrus Cure', which is a plant tonic of zinc and manganese. Where soils are lighter and sandy, particularly in parts of Springvale, Gonville & Castlecliff, an extra dose of 'Yates Liquid Citrus Cure' or 'Tui Epsom Salts' is recommended on a more frequent basis.

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 & helping retain nutrients.

Pruning is only required for shaping and plants are better left untrimmed from a fruit yield perspective.

Pruning is best completed in early spring before October when the borer beetle starts to lay its eggs.

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 promote 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 "top of the line" potting mix such as 'Natural Bark brand Potting Mix', 'Ican Premium Potting Mix' or 'Tui Pot Power'.

The use of a 'cheap' potting mix will really doom your efforts to failure.

Citrus are a 'hungry' plant and I recommend you fertilise monthly or bi-monthly using a specialist citrus fertiliser that is suitable for pots and containers such as 'Tui Enrich Fruit & Citrus', or 'Yates Acticote Fruit & Citrus'.

The addition of SaturAid re-wetting granules each summer to established 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 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 actually a secondary problem caused by the presence of particularly scale and other insects which while sucking the goodness from the tree secretes 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 'Yates Mavrik' mixed with the addition of 'Grosafe Enspray 99' to give maximum effect.

If you are unsure then take some sample leaves into a garden centre for advice.

As mentioned above, avoid any pruning between the early spring to midsummer period to reduce the risk of attack from borer beetle.

The telltale 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' into the holes.

They can be difficult to control, so prevention is better than trying to fix later.

Some more unusual citrus to look out for;

Tangor Kiyomi: For something different, try this hybrid citrus fruit – it's a cross between a mandarin and orange.

It has large fruit like an orange, with the easy-peel of a mandarin. It's very juicy, thick-skinned and seedless when self-pollinated.

Lemon Lemonade: A very juicy, lemon-like fruit with a mild, refreshing grapefruit-like flavour. Fruit can be eaten fresh or juiced. Fruit has a very strong scent. A heavy cropper.

Lemon Ponderosa: Extra-large fruit of this citron-lemon hybrid is a favourite among collectors of novelty citrus.

Fragrant blooms produce gigantic seedy lemons throughout the year, with good lemon flavour.

Kumquat Meiwa: Round, spicy, sweet fruit that is excellent for marmalade, preserves and candied fruit.

A showy smaller shrub that is excellent for container growing. The hardiest of all citrus.

Bay Sweetie: NZ-bred mandarin hybrid with easy-peel, sweet and juicy fruit. Few or no seeds.

It ripens from winter to spring. High health, attractive foliage and showy, perfumed flowers. A great home garden variety. It grows approximately 2.5 x 2.5m. Full sun position in well-drained soil.

Tolerates light frost. Water regularly during hot, dry spells. Fertilise regularly from spring to mid-summer.

Some of the favourites:

Lime Bearss: A hardier selection of Tahitian lime with small, thin-skinned, deep green seedless fruit which turns lime-yellow at maturity. Protect from frost. Tree habit is vigorous and spreading.

Orange Harwood Late: NZ selection of Valencia orange and called NZ orange. Sweet juicy thick-skinned fruit, ripens throughout the summer. Excellent eating or juicing. A reliable cropper.

Mandarin Satsuma Okitsu Wase: Do you love those sweet-tasting, seedless mandarins with the soft puffy easy to peel skin?

Then plant a mandarin Okitsu Wase. This is an early ripening Satsuma variety that has thick-skinned, easy-peel, sweet juicy fruit with segments that easily separate. This variety grows well in cooler areas too but needs protection from regular heavy frosts (-2C and cooler).

*Gareth Carter is General Manager of Springvale Garden Centre