Fertilizing
Feed from bud break to late summer, every two weeks with a liquid fertilizer or once a month with solid organic fertilizer. Use a balanced fertilizer in spring.
- Once the fruit has set, switch to a fertilizer lower in nitrogen and richer in potassium, such as a tomato feed, to support fruit instead of long shoots.
- Stop feeding in autumn and do not feed in winter.
- Too much nitrogen gives long internodes, large leaves and few fruits.
Placement
Keep the grapevine outdoors all year in full sun, at least six hours a day, with good air circulation to keep mildew away. Small pots can get afternoon shade during heat waves to prevent leaf scorch.
- It is frost-hardy, but in a pot protect the roots once temperatures drop below -10°C (14°F), for example in a cold frame or unheated shed.
- Protect the young shoots from late spring frosts.
- Never keep it indoors: it needs the winter cold to rest and bud properly.
Pruning
Do structural pruning in early winter, while the vine is fully dormant. Grapevines bleed heavily if cut in late winter or early spring, when the sap starts to rise.
- In winter, cut last year's shoots back to one or two buds; the fruit forms on the new shoots that grow from them.
- In the growing season, let shoots grow five or six leaves and cut them back to two or three, or to one or two leaves beyond the last fruit cluster.
- Remove unwanted tendrils, thin crowded shoots for airflow and keep only one or two bunches of grapes on a small tree.
Repotting
Repot young vines every 2 years and older ones every 3–4 years, in early spring before the buds open. Grapevines root vigorously, so the pot fills up quickly.
- Use a well-draining mix, such as akadama, pumice and lava rock with some organic matter.
- Trim no more than about a third of the roots.
- Keep the vine out of strong sun and wind for a few weeks afterward.
Watering
Grapevines in leaf use a lot of water. Water thoroughly as soon as the top of the soil feels dry, which in summer can mean once or twice a day, but never leave the pot standing in water.
- Water the soil, not the leaves, and do not mist: wet foliage encourages powdery and downy mildew.
- Water most generously while the fruit is swelling.
- In winter, water only enough to keep the soil slightly moist.
Seasonal care
- Spring: Repot before the buds open if needed, and protect the new shoots from late frosts. Start feeding when the buds break.
- Summer: Keep the vine in full sun, water generously and switch to a low-nitrogen, potassium-rich fertilizer once fruit sets. Cut shoots back regularly and thin the foliage so air can move through it.
- Autumn: Enjoy the fruit and autumn colour, then stop feeding. Remove fallen leaves to reduce fungal spores.
- Winter: Leave the vine outdoors for dormancy and protect the pot below -10°C (14°F). Do structural pruning in early winter, well before the sap starts to rise.
Common problems
Most problems with grape bonsai are fungal diseases and pests that thrive in still, humid air.
- White or grey coating on leaves and fruit: Powdery mildew, favoured by poor airflow and warm, humid weather. Thin the foliage, avoid wetting the leaves and treat with a sulfur-based or other suitable fungicide.
- Yellow spots on top of leaves, fuzz underneath: Downy mildew, which spreads in wet weather. Remove affected leaves, keep the foliage dry and treat with a suitable fungicide.
- Sap dripping from cuts: Bleeding from pruning in late winter or early spring. It rarely kills the vine, but do major pruning in early winter in future.
- Aphids, scale and mites: Aphids collect on soft shoot tips, while blister-like galls on the leaves are caused by tiny erineum mites. Rinse off aphids, remove galled leaves and treat with insecticidal soap or horticultural oil.
- Few or no grapes: Too little sun, too much nitrogen or cutting away the fruiting shoots. Give full sun, feed less nitrogen and keep a few buds on last year's wood in winter pruning.