When to sow zinnias

Zinnias come from Mexico and stop growing in cold weather, so there is no point starting them early. April is the safest month to sow indoors; late March works if you have a bright, warm spot, and the RHS suggests waiting until late April or early May in the UK.

Plant out only when frosts are over and nights are mild: after the Ice Saints in mid-May in Germany, Austria and Switzerland, and usually in June in the UK, later in the north. Zinnias planted into cold soil simply stall.

You can also sow straight into the bed from late May, once the soil is warm. Direct-sown zinnias flower later, but you skip the indoor work.

Sow one or two seeds per module or small pot, 0.2–0.4 in deep. Sowing singly means you can plant out later without disturbing the roots. Keep them warm, ideally around 68°F and not much below 64°F; seedlings appear after one to three weeks.

Once they are up, move them somewhere bright and a little cooler, around 59–64°F, so they stay sturdy. Water from below or at the base, never over the leaves, because seedlings easily collapse from damping off.

Harden the plants off before planting them out 8–16 in apart: dwarf types closer, tall cut-flower types wider. At about 12 in, a 120 × 80 cm raised bed holds two rows of four, eight plants; compact varieties at 10 in fit twelve.

Zinnias need sun, warmth and shelter. In cold, windy spots or heavy, badly drained soil they sulk, and in a cool, wet summer they flower less. A sunny bed against a warm wall or a raised bed suits them well.

Keep the soil evenly moist but never waterlogged. Water in the morning and at the base of the plants: wet leaves, especially overnight, invite mildew and leaf spots.

In reasonable garden soil zinnias need little or no feeding. In pots, give a liquid feed every two weeks or so, ideally one rich in potassium such as tomato feed.

Zinnias flower from July into September, peaking in August and September, and in a mild autumn into October. They are true cut-and-come-again flowers: each stem you cut back to a side shoot or pair of leaves is replaced by new branches.

Cut for the vase once the flowers are fully open, and handle the stems gently: they are hollow and crush easily, which shortens vase life. Deadhead the rest of the plant regularly.

Bees, bumblebees and butterflies visit the open flowers.

Saving seed is hit and miss. Most modern varieties are F1 hybrids that don't come true, and in cool summers seed often doesn't ripen properly. If you want to try, leave a few flowers of an open-pollinated variety on the plant until they are brown and dry, then dry the heads indoors for two to three weeks.

Most garden zinnias are Zinnia elegans. Dwarf varieties stay at 8–12 in and suit pots and bed edges; tall cut-flower types such as the Californian Giants reach 31–39 in and are the ones to grow for the vase.

Slugs are the biggest danger to young zinnias and can strip a fresh planting overnight. Plant out sturdy plants rather than tiny ones and protect them for the first weeks.

Powdery mildew, a grey-white coating on the leaves, is common from late summer, along with leaf spots such as Alternaria and, in wet weather, grey mould. Space plants for airflow, water at the base in the morning, remove affected leaves and clear old plants at the end of the season. Don't compost badly mildewed plants.

Aphids and spider mites can appear. If you must spray, use soap-based products in the evening to spare pollinators.

Zinnias belong to the daisy family, like lettuce and sunflowers, but as a summer flower they play no real part in vegetable rotation. Planting them among vegetables to draw in pollinators is common and sensible; any effect on the crops next to them is unproven.