Plums and Green Gages (Prunus domestica)

How well established is this crop?

This crop has been grown commercially in the region for over 10 years. Commercial returns, current growers and some level of regional based research is available to help those new to this industry.

Background

Until recently, plums have been available in New Zealand only during the summer months. Now with imports coming from California in our winter they are available for a lot longer. There are a large number of varieties within New Zealand, many of which were imported during the 1980s as breeding stock by the then DSIR. Some European types of plum such as the green gage are more commonly found in the home garden where the trees are prolific fruiters every year.

Climate

Most areas of New Zealand that grow this type of summerfruit are characterized by cool winters, hot dry summers with plenty of sunshine hours and low humidity. The Hawkes Bay and Central Otago provide this environment perfectly. The recommended growing degree day figure for plums is similar to that for peaches, Nectarines and Apricots and is at least 800 GDD at a base of 10°C, although a few are able to successfully fruit in regions with only 600 GDDs.

Winter Chilling and Frosts

A large number of plum selections are available in New Zealand. Trialling on a specific site or talking to a local grower will quickly establish which of these suit the winter chilling and climate found in a specific area.

Frosts do not generally harm a plum tree during the winter period but frosts over flowering can devastate a crop. Flowering can happen anytime from September and is dependant on variety. Both the flowers and young fruit are susceptible to this frost and can all be killed if temeratures drop below 0 degrees C. Traditional frost control measures such as smoke pots have been surpassed by over-head sprinkler frost fighting and now in some cases wind machines.

Rainfall and Irrigation

Rainfall by itself does not tend to harm plum trees but it does raise humidity and increase the chances of fungal and bacterial diseases such as spot and blast.

During periods of dry weather plums will benefit from irrigation, especially before harvest as it allows the fruit to grow. Under tree irrigation is recommended to keep both leaves and fruit dry, reducing the chances of bacterial disease.

Wind

Even though plums grow into medium sized trees, they still benefit from some wind protection, especially when they are young. It reduces the chance of bruising on the fruit and reduces the drying effects of wind over the summer period.

Soil

Plums prefer a sunny well drained soil if they are to do well. They are slightly more tolerant of damp conditions than apricots but do not like wet feet. A gentle slope also brings benefits as it can cause the frosts to be less severe during the sensitive flowering period in the spring.

Fertility

Plums require reasonable levels of soil fertility as they tend to be cropped heavily. Regular soil and foliage tests will quickly identify a deficiency and this can be easily corrected with the addition of fertiliser. Companies such as Hills Laboratories have a datasheet with the recommended soil and leaf nutrient levels for this crop. See the ‘Contacts’ section at the end of this datasheet.

Weed Control

Weed control in Plum blocks tends to be straightforward. When young trees are planted, plastic spray-guards are put around the trunks. This prevents rabbit damage and allows herbicides to be sprayed right up to the tree. Weeds growing up inside these guards are removed by hand. As the trees mature the bark on the trunk hardens and herbicides can contact them with no damage to the tree.

If growing organically, straw mulches, specialist low growing plants and mowing are used to control weed growth around the bottom of the trees.

Varieties

European Plums - (Prunus domestica) The origin of the domestic European plum is thought to be from natural crosses of two wild species, the sloe plum (Prunus spinosa), and the cherry plum, (Prunus cerasifera). These are usually, but certainly not always, yellow fleshed blue skinned plums, that are often naturally drier than the Japanese plums and with a more delicate and refined flavour.

European plums are usually late flowering and have a higher winter chill requirement than Japanese plums. As a general rule, European plums are better adapted to temperate, rather than warm temperate areas.

Some examples of European plums include; Angelina Burdett, Coes Golden Drop, Green Gage, Luisa, Stanley and Victory.

Japanese plums -(Prunus salicina). Some varieties are crosses with various other plum species, but are nevertheless regarded as Japanese plums. This species is actually Chinese, but became known from seedlings introduced from Japan, so the misnomer stuck. This species blooms early, and can be damaged by early spring frosts. This makes them more generally suited to warm temperate rather than temperate areas, unless good frost control methods are in place.

Some examples of Japanese Plums include; Billington, Queen Rosa, Santa Rosa, Burbank, Sultan, Doris, Late Doris and Fortune.

Cherry plums – These plums are very small cherry-like plums, red or yellow, with thin skin, pleasant sweet fruit. Most cherry plums in New Zealand are probably Prunus cerasifera or interspecific hybrids of this species. Cherry plums were sometimes crossed with Japanese plums (Prunus salicina)) to obtain better sized early dessert plums.

Examples of Cherry Plums include; Heard and Pernel. Both varieties are better for making jams than fresh eating.

Plumcots - A plumcot is a cross between an apricot and plum (
Prunus armeniaca x Prunus domestica). They flower early, around September, and over quite a long time. The trees have very showy quite large white flowers, and are an attractive early spring ornamental in their own right.

Kerby is one commonly named Plumcot, with more being imported by breeders from overseas programs.

All plums and plumcots are grafted onto a rootstock. A large number of these have been used with ‘Mariana’ being the most common. Some new dwarfing stocks are being trialled in an effort to keep the final tree height at around 2-3m.

Pests and Diseases

Plums suffer from a range of insect and fungal pathogens. Research work in Otago has shown that some of the diseases are going to be very difficult to manage by organic principles leaving agrichemicals the best method of control.

Insect pests include leafrollers, mites, thrips and scale. The disease pathogens include bacterial blast and bacterial spot, silver leaf, brown rot and a range of storage rots.

Layout

Most Plum blocks are planted on a reasonable scale and allow for tractor access between rows. This is usually 4.5 -5.0m. Within the row, spacing is usually 2-3m. Final tree height is anywhere from 4-5m.

General Management

Ground preparation is the key to developing a plum block. Irrigation installation and any drainage work is completed in the period before planting. Trees are delivered from the specialist nursery ‘bare-rooted’ in winter. This means they have been dug and shipped without any soil around the roots. They usually arrive in bundles of approximately 10 trees. These are usually ‘heeled in’ meaning the roots are covered with soil or sawdust until they can be planted. Tree rows are usually ripped with a deep blade to free up the soil. Holes are dug and the trees planted, usually with the graft union pointing into the direction of the prevailing wind. This gives the tree more strength as it grows. Weed control using herbicides, mowing, spraying and irrigation are needed from this time. Some pruning and tree shaping is also done, generally in the winter. The traditional shape for a mature tree was the vase shape. However, more recently training the tree into a centre leader has become popular. It is important to seal the cuts made at pruning with a fungicide/bacterial spore killer as the trees are susceptible to infection through these wounds. Fruit is generally removed from the tree in the first two seasons to ensure the tree grows vegetatively.

Pollinator trees are recommended when growing Plums, even though some trees are said to be self-fertile. But even with the best pollenisers, cold wet springs and low bee numbers can cause poor fruit set in some years.

Harvest

Picking is done by hand with care being taken not to bruise the fruit. They are placed in crates in the orchard before being taken to the packhouse for processing.

Equipment

Because most blocks of plums are planted on a reasonable scale, a tractor is required along with spray units, mowers, forks for transporting bins and maybe a truck to get fruit to a packhouse.

Returns

Returns are usually quoted on a per kilogram basis in the export and domestic markets. Howver it does depend on variety and the value of the $NZ at the time of sale.

In general a fully developed block should produce between 10-15/t/ha with prices ranging between $1-2/kg.

Contacts

If you are thinking of growing this crop, talk to local growers about varieties.

For fertility information contact Hills Laboratories. www.hills-labs.co.nz

Industry based grower organisation www.summerfruitnz.co.nz