Container lavender needs a sunny position and a root environment that drains well. Inspect the compost before watering; neither constant dampness nor prolonged neglect is a useful rule.
Light, and the one season to filter it
Give it sun through winter and spring. Lavender is a long-day plant, and light drives both the growth and the aromatic oil in the foliage. Weeks of shade produce soft, pale stems, few spikes and a plant that starts showing disease. A south window or an open, unshaded balcony is the right address.
High summer is the exception. Heat and humidity together are more than it can take, so during the worst of a July, filter the midday beam with a 50 per cent shade cloth and give the plant movement of air. A cool spot with a breeze beats a warm, sealed one every time.
Pot and mix, because the mix decides
Drainage matters more than pot style. Terracotta, plastic and porcelain all work; a heavy peat-based compost in any of them will still drown the roots. Nursery lavender is raised in gritty, open material for exactly this reason, and the same logic is behind granular mixes. Rosemary is the same argument in a different plant; the pot soil written up in its container mix is close to what this wants.
Watering on the plant's terms
The old rule is the correct one: do not water until it is dry, then water properly rather than sipping at the surface. Bone-dry pot soil stunts the plant, and permanently damp soil kills it. The leaves tell you when. The first faint wilt on a warm afternoon means the surface has gone dry, and a good soak at that moment is what the pot needs.
In summer, water in the morning or the evening while it is cool. Pour around the edge of the pot and keep the water off the foliage and the flowers, because wet leaves in hot weather invite rot. The volume of soil in a pot is limited, so timing counts for more in July than in March.
Feed it less than you think
Lavender grows willingly in poor ground and asks for almost nothing. When you do feed, use a dilute water-soluble fertiliser at one to two per cent, urea or a balanced compound. From June through August keep nitrogen down, since excess makes long soft growth that flops and flowers badly, and switch to a compound weighted toward potassium. At the winter repot, stir bone meal and a phosphate such as superphosphate through the fresh soil.
The cut after flowering
Uncut lavender goes woody and bald from the base upward, which is why every nursery stock plant looks clipped. When the spikes fade, take the plant back to roughly two thirds of its size and cut into leafy growth rather than the bare stems lower down. Old wood cannot be relied on to push again, and a hard cut into it during hot weather can finish the plant.
Through the year, remove dead stems and anything rubbing or straying. In late summer, shorten the new growth lightly to shape it into a dome, a ball or a squared hedge edge, whichever you are keeping it to. That late cut produces the fresh shoots the plant carries into winter.
Root rot, aphids and mites
Root rot is the main disease, and hot wet air is the cause. Ventilate, keep the air dry around the foliage, and tip any water out of the saucer after rain. If the stem base browns at soil level, cut back the watering, move the pot somewhere with airflow, and drench with a biological fungiser at label strength once a month until the browning stops.
Two pests turn up: aphids on the growing tips, and red spider mites under the leaves when the air is dry and still. This piece on aphids in pots runs through the treatment options in detail. The part worth remembering is that one spray fixes nothing. Repeat weekly for two or three weeks to catch the hatching eggs, and aim the wet at the undersides of the leaves, where mites actually sit.