Fertilizing
Feed every 2–4 weeks from leaf-out in spring until late summer, using a balanced bonsai fertilizer. From midsummer, switch to a feed lower in nitrogen: too much nitrogen gives lush foliage at the expense of next year's flower buds.
- Do not feed for about four weeks after repotting.
- Stop feeding in autumn and do not feed during winter dormancy.
Placement
Keep your hawthorn bonsai outdoors all year in full sun, ideally with six hours of direct sun a day; too little light means few flowers and berries. In regions with very hot summers, light shade during the hottest hours prevents leaf scorch.
Good airflow helps prevent fungal diseases, so avoid crowded, stagnant corners. The tree is fully hardy, but protect the roots in the pot when temperatures fall below -10°C (14°F), for example by sinking the pot into the ground or placing it in an unheated shed.
Pruning
Hawthorn flowers in late spring on short spurs that form during the previous summer. Let new shoots grow, then cut them back to two or three leaves after flowering; avoid trimming in late summer, when it removes the next year's flower buds.
- Do structural pruning in late winter or early spring while the tree is dormant, accepting that it costs some flowers.
- Thin out dense areas to improve airflow and remove some of the berries if the tree is weak.
- Wire young branches in spring or summer; older wood is stiff and brittle, and thorns that get in the way can be removed.
Repotting
Repot young trees every 2–3 years and older trees every 3–5 years, in early spring as the buds begin to swell. Hawthorn dislikes heavy root pruning, so trim the roots moderately and remove only dead or overly long ones.
- Use a free-draining mix such as akadama, pumice and lava rock; hawthorn tolerates neutral to slightly alkaline soil (pH 6–7.5).
- Water thoroughly afterwards and keep the tree in light shade, out of the wind, for one to two weeks.
Watering
Hawthorn likes consistent moisture but is sensitive to waterlogging. Water thoroughly when the surface of the soil feels dry and let excess water drain freely; never let the root ball dry out completely.
During the growing season, and especially in summer heat, you may need to water daily. In winter the tree uses little water, so keep the soil only slightly moist.
Seasonal care
- Spring: Repot as the buds swell and start feeding once the leaves have opened. Enjoy the blossom in late spring and keep the flowers dry to reduce disease.
- Summer: After flowering, cut new shoots back to two or three leaves and switch to a low-nitrogen feed. Keep the tree in full sun and water as soon as the surface dries.
- Autumn: The berries ripen and the leaves colour and fall; stop feeding. Remove fallen leaves and fruit to reduce fungal problems.
- Winter: Leave the tree outdoors to go dormant and protect the roots below -10°C. Keep the soil slightly moist and do structural pruning at the end of winter.
Common problems
Hawthorn belongs to the rose family and shares many of its pests and diseases.
- Fire blight: Shoots and flowers suddenly turn black-brown, as if scorched, often in warm, wet weather around flowering. Cut affected branches well back into healthy wood, disinfect your tools after each cut and destroy the clippings.
- Rust: Orange spots on the leaves and fruit, especially when junipers grow nearby, as the fungus alternates between both hosts. Remove affected leaves and keep the tree away from junipers.
- Powdery mildew and leaf spot: White coating or brown spots on the leaves in humid weather or a dense canopy. Improve airflow, water the soil rather than the leaves and clear up fallen leaves.
- Aphids and caterpillars: Aphids distort young shoots and caterpillars can strip leaves in spring. Rinse aphids off with water or insecticidal soap and pick off caterpillars by hand.
- Few or no flowers: Usually too little sun, too much nitrogen or trimming in late summer. Place the tree in full sun, feed less nitrogen and prune mainly after flowering.