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A zinnia goes from seed to first flower in about ten weeks and then flowers for another ten to twelve before frost or exhaustion stops it. Everything that matters happens in three short windows, and none of them is the one people worry about. Getting the sowing temperature right, taking out the first bud, and picking relentlessly account for almost the whole difference between a good stand and a poor one.
The plant is killed outright by the first frost. There is no overwintering, no dormancy and no cutting back for next year, so the season genuinely ends in one night. Knowing your average first frost date and counting back is the only planning a zinnia needs.
| Stage | When | The job |
|---|---|---|
| Sowing | Well after last frost, soil above 70F (21C) | Direct sow at stations, 1/4 in (6 mm) deep, 2 to 3 seeds each |
| Emergence | 4 to 7 days after sowing | Keep the surface damp, watch for slugs after dark |
| Thinning | At two true leaves | Snip surplus at soil level to one plant per station |
| Pinching | At 8 to 12 in (20 to 30 cm) tall | Remove the growing tip above the third or fourth leaf pair |
| Support and leaf strip | At around 18 in (45 cm) | Net or stake, and strip the lowest 4 to 6 in (10 to 15 cm) of leaves |
| First flowers | About 10 weeks from sowing | Start cutting deep and often. Do not let anything set seed |
| Peak and mildew | Late summer | Keep picking, remove dusted leaves, water only the ground |
| Seed saving | 4 to 6 weeks before frost | Stop cutting from two marked plants and let heads dry |
| First frost | The end | Pull and compost. Clear the bed of infected foliage |
Spring: do nothing, on purpose
The most useful thing you can do with zinnias in spring is resist sowing them. Cold soil rots the seed or gives you a patchy stand, and the head start you think you are getting evaporates within a fortnight. Spend the time preparing the bed instead: fork it over, rake it level, and hold off on heavy manure, which produces soft mildew prone plants.
Count back ten weeks from your first expected flowers and check that lands after the soil is genuinely warm. If your season is too short for that, sow into deep individual cells rather than trays, one seed per cell, and plant out at two or three true leaves. Never sow zinnias into a communal tray you will have to transplant.
Mark your station spacing in the bed before you sow. Deciding spacing with seed already in your hand always ends in plants too close together, and crowding is what brings mildew forward by weeks in late summer.
Early summer: the fortnight that decides the year
Between thinning and pinching, roughly the third to sixth week after sowing, you make the decisions that fix how the rest of the season goes. Thin to one plant per station at two true leaves and cut the surplus off rather than pulling it. A fortnight later, when plants are eight to twelve inches (20 to 30 cm) tall, take out the growing tip above the third or fourth leaf pair.
Both jobs feel destructive and both are correct. Thinning a station down to one plant when all three look healthy is hard, but three plants sharing one root run give you three weak ones. Pinching out a bud you have waited two months for is harder still, and it triples the flower count.
Get support in during the same window, while the plants are around a foot (30 cm) high. Netting or stakes pushed in later go through roots you have spent two months growing, and a tall zinnia that has once bent in a storm never straightens up.
If you only remember one thing about zinnias, make it this: take out the first bud. Nothing else you do to the plant returns as much.
Midsummer to frost: pick, strip, and sow again
From the first flower onwards the routine is unchanging. Go down the row every three or four days, cut everything open on a long stem back to a leaf joint, remove anything faded, and take any yellowed or mildewed leaf out of the bed. Water the ground and never the foliage. That is the whole job, and doing it consistently is worth more than anything clever.
Powdery mildew arrives in late summer regardless. When it does, strip the affected lower leaves, keep watering at the base, and accept that the lower third of each plant is finished. A second sowing made four to six weeks after the first comes into flower right at this point with clean foliage and carries the display into autumn.
Four to six weeks before your average first frost, stop cutting from two marked plants and let their flowers brown on the stem for seed. When the frost comes it kills everything overnight. Pull the plants and take the foliage out of the bed rather than composting it in place, because mildew and leaf spot both overwinter on debris and start next year's crop off early.
- Cut every three or four days Take stems back to a leaf joint 12 to 18 in (30 to 45 cm) down. Two shoots break below every cut, so hard picking is what keeps production climbing.
- Never let anything set seed One faded head left on the plant slows the whole thing down. Strip them whether or not you want the flowers, except on the plants you have marked for seed.
- Sow a second batch as insurance Sow again 4 to 6 weeks after the first sowing, provided you still have 12 weeks before frost. Young clean plants take over as the first block mildews.
- Cut tired plants back by a third When stems shorten and flowers shrink after about ten weeks of cropping, cut the plant back by a third and soak the ground. It flushes again within a fortnight.
- Clear the bed after frost Pull the dead plants and remove the foliage from the garden. Leaf spot and mildew both carry over on debris left lying under the plants.
Keep a note of your sowing date, the date of the first flower and the date of the first frost. Three numbers a year, and after two seasons you know exactly how late you can sow and still get a full run of flowers.