Agronomic principles for stone fruit

Cherries

Cherries are distinctly different from other types of stone fruit.

Sweet and sour cherries require 2-3 months for fruit development and thinning is impracticable and usually unnecessary. Trees can grow very tall but are generally trained to contain the size of the tree for ease of picking. Sour cherries are smaller and generally pruned more regularly to provide new wood and flowering and to open up the centre of the tree.

Cherries develop good root structures and deep well-drained soils are more productive. Heavier, wetter soils will restrict cherry yield. Sweet cherries are better in cooler drier climates where disease pressure, frost risks and rainfall at harvest are less of a problem. Hot temperatures during ripening will reduce yield and fruit size. Sour cherries prefer cooler, humid climates.

Both cherry types have a relatively high chilling requirement – 1000-1500 hours - and this precludes production in warmer climates. They are one of the last stone fruit types to flower, with sour cherry being slightly later than sweet cherry. Cold hardiness is better than in peaches but not as good as European plums. Temperatures of -3°C will damage buds.

Irrigation

Cherries have a high leaf to fruit ratio therefore there is a lot of competition for both sunlight and nutrients. This, combined with the relatively short growing season means they have a high nutrient demand in comparison to other stone fruit crops and any deficiency or nutrient stress is likely to impact on fruit quality and size.

Crops tend to be most nutrient deficient at petal fall because there is insufficient leaf area to support nutrient transfer. This is worse in cold springs when soil uptake is reduced. As a result, nutrient storage from the previous season is particularly important in cherries.

Cherries need irrigation or adequate soil moisture to ensure good fruit fill. Nevertheless, rainfall during bloom and just before harvest is a problem leading to disease that aborts flowers and causes skin cracking at maturity. Sour cherries are slightly less prone to cracking and thus suit slightly wetter climates.

Cherries do not ripen in storage and so must be picked ripe at the stage of maturity desired by the marketplace, particularly for fresh fruit consumption. The fruit can gain up to 30% of their weight and most of their flavour in the 7 to 10 days before harvest, so the timing of picking is critical.

Plums

Plums are the most diverse group of stone fruits.

European plums are more cold hardy and are similar to apples and pears in terms of temperature tolerance. Nonetheless, crops can still be damaged at temperatures of –4-5°C at bud swell, and –2-3 °C at blossom. They can bloom 1-2 weeks later and have a chilling requirement of 1 000 hours or more. European plums don’t produce commercial crops until year 3-4. They require up to six months to ripen and some canning or prune cultivars don’t ripen until the autumn.

Japanese plums have a chilling requirement of 550-800 hours and usually produce a small crop in year 2. Rainfall and high humidity encourage brown rots and lead to fruit cracking. Japanese plums require a short three-month period for fruit development. Thinning is essential, which can produce 100,000 flowers per tree. This is not always necessary in European plums, particularly prunes.

Varied rootstocks are used and there is a wide range of material available, some offering nematode and bacterial canker resistance. Tree types are more upright than peach and there is less need for light exposure to improve skin colour. Trees are vigorous and bushier at 4-8m height and pruning is less important than in other stone fruit types. Unlike most other modern stone fruit species, many plums require cross-pollination. Deep well-drained soils with a pH of 5.5 to 6.5 provide the best crops. Plums – particularly European plums – are more tolerant of waterlogging than other Prunus species and can be grown on heavier clay soils.

Plums are also more tolerant of drought than peaches and nectarines and some rootstocks are available for high pH soil production.

Apricots

Chilling requirements for apricots range from 400-1000 hours. They have a relatively short heat requirement, with the result that apricots bloom before most other Prunus species. Warm temperatures too early in the spring, will lead to bud break and place the crop more at risk of damage from late frosts. Temperatures of -4°C will damage swelling buds and -2°C will damage flowers.

Apricots prefer relatively dry climates. Rainfall close to harvest can result in cracking and rots. The fruit ripens in early to mid-summer. As they bloom early, apricots produce smaller fruit than peaches - around 4- 6cm in diameter. Apricot seedlings are the most common rootstocks, though peach rootstocks are sometimes used on acid soils or where irrigation is unavailable.

Deep fertile, well-drained soils are best, and apricots are more tolerant of high soil pH and salinity than other Prunus species. They are intolerant of waterlogging. Apricots are small to medium trees with spreading canopies. They are generally kept to 4-5m in height. Orchards require good light exposure for good fruit colour. Training is needed to provide open-centre trees and pruning and thinning is usually quite severe in order to produce quality fruit to suit the market.

Ripe apricots have a red blush. Flesh colour is important in apricots. Apricots have a spreading canopy structure.