How to Sow Zinnia Seed: Direct Sowing, Warmth and Spacing

Direct sow, because zinnias hate having their roots moved

A zinnia puts down a fast taproot in its first two weeks, and that root does not forgive being broken, coiled or teased apart. Plants lifted out of a tray and set into the ground stall for ten days to three weeks, sulk with purple tinted lower leaves, and frequently throw one small flower on a short stem and stop. Seed dropped straight into the ground where the plant will live has none of that check and simply keeps growing.

The practical test is easy to run yourself. Sow half a row directly and plant the other half from a tray on the same day. By midsummer the direct sown half will be taller, branched lower down and carrying more buds, and it will usually flower first despite starting later. This is the single biggest lever on how good your zinnias are, and it costs nothing.

If your season is genuinely too short to direct sow, start in deep individual cells or paper pots rather than a communal tray, sow one seed per cell, and get them into the ground while they still have only two or three true leaves. A zinnia that is already showing a bud in the pot is a zinnia that will never make a good plant.

Never pot a zinnia on. The check from one repotting is worse than the delay from sowing a fortnight later straight into the ground.

Wait for soil above 70F rather than watching the calendar

Zinnia seed germinates in four to seven days when the soil sits above 70F (21C), and it germinates fast enough that you can watch it happen. Below 60F (15C) it either sits and rots or comes up in scattered patches over three weeks, which produces an uneven row you will be fighting all summer. Cold wet soil is the reason most failed sowings fail.

Push a finger into the bed in mid morning. If it feels genuinely warm rather than merely not cold, you are close. A cheap soil thermometer pushed two inches (5 cm) down and read at the same time each morning removes all the guesswork. In practice this means sowing well after the last frost, when night temperatures have settled above 50F (10C) and the soil has had two or three warm weeks to catch up.

There is no advantage in being early. A sowing made two weeks after the soil warms will catch up with, and usually pass, one made into cold ground, because the early sowing spends that fortnight sitting still and losing seedlings to damping off.

A quarter inch deep, and thicker than you need

Zinnia seed is a flat arrowhead shape, big enough to place one at a time with your fingers. Cover it a quarter inch (6 mm) deep and firm the soil down onto it. Seed left on the surface germinates and then dies when the top layer dries out on the first hot afternoon, and seed buried an inch (2.5 cm) down exhausts itself before it reaches light.

Sow two or three seeds at each final station rather than scattering along a row. That way you thin down to the strongest without disturbing the soil around a plant you intend to keep, and there is no bare gap where a seed failed. Water the row gently and keep it damp until emergence, then move to less frequent, deeper watering.

Make a second sowing four weeks after the first and a third four weeks after that if your season allows. Zinnias flower for roughly ten weeks from a single sowing before the plants tire and mildew catches up with them, and staggered sowings give you clean young plants coming into flower just as the first batch declines.

1. Prepare a loose, well drained bed Fork the top six inches (15 cm) and rake it level. Zinnias want free draining ground and moderate fertility. Heavy manure or rich compost gives you a tall soft plant that mildews early and blows over.
2. Mark the stations before you sow Work out the spacing first and mark each planting position with a stick or a dib. Sowing first and spacing afterwards always ends up with plants too close, and crowding is what brings powdery mildew in early.
3. Sow two or three seeds per station Drop them a finger width apart at each mark, cover a quarter inch (6 mm) deep, and press the soil down. Water with a fine rose so the seed is not washed out of place.
4. Keep the surface damp for a week Water lightly every day or two until seedlings show. Once they are up, stop the daily sprinkle and switch to a thorough soak two or three times a week to drive the roots down.

Thin ruthlessly, and thin early

Thin to one plant per station when the seedlings have two true leaves. Snip the surplus off at soil level with scissors rather than pulling them, because pulling drags the root of the one you are keeping and you have just done the exact damage you avoided by direct sowing.

The spacing you choose here is the main thing standing between you and powdery mildew in late summer, so do not compromise. Crowded zinnias hold still humid air around their lower leaves every night, and that is precisely the condition mildew needs. Plants at honest spacing dry off within an hour of sunrise and often stay clean weeks longer than a crowded row two feet away.

Note the sowing date, the station spacing you used and the date the first seedling appeared. Comparing that against the date mildew first shows on the lower leaves is the most useful record you can keep, because it converts spacing from an opinion into a number you can act on next year.

Growing for cut flowers is the one case where slightly tighter spacing pays, because plants shaded on their sides throw longer straighter stems. Take that trade knowingly, and expect mildew a fortnight earlier.