Low potassium level in the soil: Potassium (potash) deficiency is most likely to occur on light sandy soils. Where it occurs, leaf and shoot growth may be vigorous, but the shoot growth may not ripen satisfactorily in autumn, staying soft and green and liable to frost damage. Potassium is active in ripening and hardening the wood and encouraging flowering and fruiting. A deficiency can easily be remedied by feeding with sulphate of potash at about 15g per sq m (1/2 oz per sq yd), applied in early spring when growth begins, when the soil is moist, to an area extending at least 1m (3¼ft) beyond branch spread. A repeat application can be applied if necessary in the following year.

Lack of vigour: If the shrub or tree is apparently quite healthy but small-leaved, lacking in vigour and making very little annual growth, this could indicate generally low levels of nutrients in the soil. Flowers and berries are often borne only on stronger, young growths and plants only making thin, weak annual growth may remain without berries. If this happens, the plant may benefit from an application of a general fertiliser such as Growmore in early spring at a rate of 100g per sq m (3oz per sq yd). Apply as a surface dressing to the root area and follow with a layer of mulch. Other causes include root damage through disease, poot planting technique, shade and waterlogging.

Frost: Late spring frosts can damage flowers, blackening and destroying the central reproduction organs and a plant’s ability to produce berries or fruit. Flowers may appear undamaged until examined more closely. Apple, pear, plum and oranmental crab apples (Malus) species are particularly susceptible to this type of damage.

Drought: This often occurs where climbers or shrubs are growing against a sunny wall in a dry border and can result in premature drop of the fruit (berries) even though pollination has been successful.

Poor pollination caused by weather: This can occur in cold weather in spring, particularly where plants are being grown in open situations and exposed to strong winds that can deter insects from visiting plants in flower. It can also occur if plants flower during wet or frosty weather when pollinating insects, such as bees, are not active.

Shade: Shrubs in particular may be shaded by nearby trees and walls and not flower unless more light can be arranged.

Genetics and the part they play in compatability is a major factor in producing a good crop of berries. The following should be considered, particularly if you have tried to improve the cultivation and environmental factors.

Lack of a pollination partner: Many trees and shrubs are naturally dioecious (male and female flowers produced on separate plants), so you need to make sure you grow both male and female plants to pollinate each other. In the nursery or garden centre, plants of different sexes are often stated on the labels, for example:

• Ilex aquifolium ‘Madame Briot’ female
• Ilex aquifolium ‘Silver Queen’ male
• Skimmia japonica ‘Rubella’ male
• Skimmia japonica ‘Veitchii’ female

Some female cultivars of Ilex (holly) may produce a few berries even when isolated but, for regular and full berrying, a male cultivar is required nearby. Unfortunately some plant names can be misleading; the holly‘Golden King’ is actually female and 'Silver Queen' is male! Check in the RHS Plant Finder to be sure.
   
Self-incompatibility: Euonymus europaeus and its named clones have variable self-incompatibility (are partially or fully self-sterile) and, therefore, one plant on its own may often not produce berries. To be sure of obtaining berries, it is necessary to plant two plants reasonably close together so that there can be cross pollination. However, it is also necessary to plant separate clones (the genetically identical off-spring of a single parent, produced by vegetative propagation).

Sterility: Some plants such as Rosa moyesii and Rosa moyesii ‘Geranium’ (which is more compact and larger-fruited) are often sterile when raised from seed so will not produce their usual crop of hips. These plants should always be propagated by budding or grafting onto a rootstock.

Seed-raised plants: If trees or shrubs have been grown from seed rather than bought from a nursery, their fruiting potential may vary considerably from named cultivars, which are propagated vegetatively from selected plants with a known capacity for good berry production. Where seed-raised plants give a poor performance, they are best disposed of, and stock obtained that has been propagated in a nursery by budding, grafting or from cuttings that will have the good flowering and fruiting qualities of their selected parent plant. Remember also that seed-raised trees and shrubs have a juvenile period that may last several years, during which they grow often vigorously but only settle down to their productive (i.e. flowering and berrying) phase later. Plants propagated by cuttings or grafting tend to flower more quickly.