Fertilizing
Citrus are hungry trees, and a lack of nutrients quickly shows as yellow leaves. Feed every 2 weeks from spring to early autumn with a citrus fertilizer, or a balanced fertilizer with added micronutrients, at about half the label strength.
- Choose a fertilizer that contains iron, magnesium and manganese, since citrus are prone to deficiency chlorosis.
- Feed once a month in winter if the tree stays warm and keeps growing; stop if it is kept cool and resting.
- Never use fertilizer spikes meant for citrus in the ground, as they burn the roots in a small pot.
Placement
Citrus need as much light as possible, ideally at least 6 hours of direct sun a day. From late spring until autumn, keep the tree outdoors in full sun, introducing it gradually to avoid leaf scorch after winter indoors.
Bring it inside before night temperatures fall below about 5°C (41°F), and never expose it to frost. Indoors, choose the sunniest window or a bright, cool room at 10–15°C (50–59°F), where the tree rests and drops fewer leaves. Keep it away from radiators and draughts, and raise humidity with a humidity tray.
Pruning
Prune citrus in spring, before or just after the flush of new growth, and trim new shoots during the growing season. Cut shoots back to 2–3 leaves once they have hardened, and remove crossing, inward-growing and overly vigorous branches.
Citrus flower on the current season's growth, so moderate pruning does not prevent flowering, but constant hard pruning reduces the crop. Wire with care, as the bark marks easily, and watch for thorns on many varieties. Thin the fruit so the tree carries only a few at a time, especially on young trees.
Repotting
Repot citrus every 2–3 years in mid to late spring, when temperatures are warm and the roots are active. Older trees can go 3–4 years between repottings.
- Prune roots moderately, removing no more than a quarter to a third of the root mass.
- Use a free-draining, slightly acidic mix, such as akadama and pumice with some fine bark, at around pH 6–6.5.
- Keep the tree out of strong sun and wind for 2–3 weeks afterwards, and wait about a month before feeding.
Watering
Water thoroughly when the top 1–2 cm of soil feels dry, letting excess water drain away. Citrus dislike both waterlogged roots and complete drying out, and overwatering is the more common mistake, especially indoors in winter.
Use rainwater or filtered water where tap water is hard, as lime in the water causes yellow leaves over time. Water more often outdoors in summer and much less when the tree is kept cool in winter.
Seasonal care
- Spring: Repot when it is warm, start regular feeding and move the tree outdoors once nights stay above 5°C (41°F), acclimatising it to full sun over two weeks.
- Summer: Keep it in full sun, water as soon as the surface dries and feed every 2 weeks. Trim new shoots back to 2–3 leaves and thin the developing fruit.
- Autumn: Bring the tree indoors before nights fall below 5°C (41°F) and check it for pests first. Reduce feeding as growth slows.
- Winter: Keep it at the sunniest window or in a bright, cool room at 10–15°C (50–59°F), water sparingly and avoid moving it around.
Common problems
Most citrus problems come from low light, water and nutrient issues:
- Yellow leaves: Pale leaves with green veins point to iron or magnesium deficiency, often worsened by hard water or cold, wet soil. Use a citrus fertilizer with micronutrients and soft water.
- Leaf drop: Sudden leaf fall usually follows a move to a darker spot, a draught or overwatering in winter. Give more light, keep conditions stable and let the soil dry slightly.
- Scale insects: Small brown bumps and sticky honeydew on stems and leaves. Scrape them off and treat with horticultural oil.
- Spider mites: Fine webbing and speckled leaves in dry indoor air. Shower the foliage and raise humidity.
- Root rot: Wilting despite wet soil and a sour smell come from waterlogging. Repot into a free-draining mix and water less.