When to plant zinnia seeds – for a bold and beautiful show

Getting savvy about when to plant zinnia seeds can bring blooms in vivid shades of red, magenta, orange, and lime to the backyard from July into October. 

Fashionable in the 19th century and enjoying a resurgence in popularity, these annuals bring retro glamour, as well as exotic appeal. 

Once you know how to grow zinnias, they are easy to introduce and, started off from seed at the right time, will bloom over a long period, brightening the backyard and providing fresh cut flowers for the house, and our guide provides the optimum times to sow and plant these fabulous annuals. 

When to plant zinnia seeds

Some zinnia species grow wild in North America, but the most widely grown garden form – Zinnia elegans – is native to Central America. Hailing from a warm climate dictates when they should be sown.

‘Zinnias are wonderful because they are easy to grow, and they produce tons of beautiful flowers,’ says Catherine Kaczor of Hudson Valley Seed Co. ‘They are a joy to grow, and they come in so many colors, shapes, and sizes. Virtually anyone can grow them. Just pick a spot with full sun, and watch these beautiful flowers bloom all summer long.’

Zinnias might be part of your flower bed ideas and also do well in pots if you’re looking for container gardening ideas. As for when to plant zinnia seeds, they can be sown in April or May under cover. Alternatively, they can be sown direct or planted out as ready-grown plug plants in early summer.

When to plant zinnia seeds under cover

April or May is the best time to plant zinnia seeds in a greenhouse or cold frame or on a bright windowsill, for earlier flowering. Zinnias are half-hardy annuals that dislike the cold, so don’t sow them under cover too early in spring because they shouldn’t be planted out until the weather has warmed sufficiently. For cold regions, that might not be until early June. 

Zinnias don’t like their roots being handled or broken, so avoid sowing into seed trays. Module trays are a possibility, but there are some even better options. ’Because zinnias hate root disturbance, I use coir Jiffy pellets,’ says zinnia supplier Sarah Raven. ‘That way you minimize handling. You can also sow them into lengths of guttering so that the seedlings can slide straight out into their planting hole.’ 

Keep the modules, pellets, or guttering well watered, but never waterlogged to prevent the seedlings damping off. 

If you live in a very cold climate (such as north Scotland), don’t bother with modules or pellets. Instead, sow your zinnias direct into greenhouse beds or into the soil under a polytunnel and leave them to bloom there all summer, rather than sowing in modules or pellets to transplant out later. This is because zinnias need summer heat to flower well outside. 

When to plant zinnia seeds direct

Because zinnias dislike root disturbance, sowing direct often reaps the best results. But when to plant zinnia seeds direct? Wait until the weather is T-shirt warm, which might not be until early June in cool climates. 

‘Zinnias are very sensitive to the cold, so wait until all threat of frost has passed to sow or transplant,’ says Catherine Kaczor. ‘A rule of thumb for our area in the northeast US is to wait until around Mother's Day (8 May this year). Zinnias generally don’t start to grow until the soil warms up above 60ºF (15°C), so starting them from seed and planting early is unnecessary, as the zinnias won’t flower any earlier. So, save yourself some trouble and direct sow your zinnias.’

Shannie McCabe, horticulturist for Baker Creek Heirloom Seed Co echoes this: ‘Zinnias perform best when direct sown outdoors two to three weeks after the last average frost date or when the temperature of the soil has reliably warmed up, as zinnias do not like cold.’ 

Sow in fertile, well-drained soil in full sun. Choose gaps in perennial borders or – since zinnias are worth including when you’re planning a cut flower garden – sow colorful rows in the vegetable garden. Rake the soil very well before sowing, so the fussy roots can grow down easily. Then sow shallowly, around 0.1in (3mm) deep, and 12in (30cm) apart. 

When to plant zinnia seedlings

Zinnias bought as ready-grown plug seedlings or those you have grown yourself can be planted as soon as the weather has warmed in May or June. Choose a site in full sun that has fertile, well-drained soil and rake it well before planting. Because zinnias dislike their roots being touched or moved, try not to handle the modules or plugs much and hold them carefully when planting. 

When to sow zinnias in pots

If you want to enjoy zinnias in containers on the terrace, sow them direct into the pots under cover in April, and move the pots outside once the weather has warmed in May or June. The tall 3ft (90cm) zinnias will, of course, require a bigger container, but compact varieties can be grown in a 12in (30cm) pot. Ensure the container has drainage holes, and keep the compost moist, but not waterlogged. 

Can you start zinnias from seed?

You can start zinnias from seed and, as long as you plant the seeds at the right time, they are easy to grow. Choose between sowing the seeds in April or May under cover, or sowing them direct when the weather – and the soil – is warmer.

How long do zinnias take to bloom from seed?

Zinnias take around two month to bloom from seed, although the timing does vary according to the weather conditions. As zinnias are annuals, they won’t come back the following year, but you can collect the seeds from your plants and then follow our guidelines on when to plant zinnia seeds for a brilliant display next year.