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Home & garden.

Updated: 13 Jan 2022

How to grow zinnias

The bold, daisy-like blooms of zinnias will fill your borders with vibrant colour. Discover our best zinnia varieties and tips for how to grow them
CT
Ceri Thomas
Zinnias

Available in a riot of hot colours along with some muted hues, zinnias will flower all summer until the first frosts. They’re pretty easy to grow from seed and are a magnet for pollinating insects. Many varieties have long straight stems, so are great as cut flowers, plus some dwarf types produce a low mound of blooms, which are ideal bedding plants for the front of a border. Many come as mixes, giving you flowers in different colours.

Which? Gardening magazine grew a range of popular varieties to see which would give us the best displays.


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Key facts

PLANT TYPE Half-hardy annual

POSITION Full sun

SOIL Well-drained, fertile soil in a sheltered spot

How to grow zinnias: month by month

JanuaryFebruaryMarchAprilMayJune



SOWSOW/PLANTPLANT
JulyAugustSeptemberOctoberNovemberDecember
FLOWERINGFLOWERINGFLOWERINGFLOWERING

Best zinnia varieties

Which members can log in now to see the full results and which are our Best Buy varieties. If you're not a member, join Which? to get instant access.

Best Buy zinnias
What it looks likeVariety nameHeight x spread
50 x 50cm
This low, bushy, mound-forming zinnia put on an impressive show throughout summer. It was first into flower in early July and was covered in earthy-toned blooms for an impressive 15 weeks. The small double and semi-double, bicoloured blooms in deep maroon, yellow, orange, red and cream reminded us of marigolds, but were none the worse for that. They would be perfect for a long-lasting display at the front of a sunny border. Peak flowering: August-October
What it looks likeVariety nameHeight x spread
75 x 30cm
At 10cm across, the double flowers in carnival hues of bright lemon yellow, dark orange and deep raspberry were an arresting sight. Our compact, bushy plants grew a little taller than the expected 65cm and produced plenty of long flower stems, ideal for cutting. Peak flowering: August-September
What it looks likeVariety nameHeight x spread

How we test zinnias

We bought 25 zinnia seed mixes. We sowed seed singly in modules of Best Buy compost for seed sowing in mid-April. We kept them at 20°C in the greenhouse, where most mixes germinated well within four to seven days. We potted on into larger pots in mid-May, before pinching out the growing tips to encourage branching. We moved them to the coldframe to harden off at the beginning of June. By mid-June, our plants were large enough to plant out in a fertile, well-drained bed at the Which? Gardening magazine trial site. We planted each mix in 16-plant blocks (4 x 4) with a range of sizes from each variety. With mixes, one colour may grow more strongly than another, so if only ‘strong’ seedlings are used, you may end up with flowers of only one colour rather than the full range. After planting, the bed was covered with a layer of mulch to keep weeds down. Over summer, we recorded how well they flowered, whether they were affected by pests, diseases or bad weather and how long cut flowers lasted.

Sowing

Zinnias don’t germinate well when sown direct and dislike cold nights, so start off the seed in April in modules in the greenhouse or on a sunny windowsill. Use a Best Buy compost for seed sowing, and sow singly since zinnias don’t respond well to root disturbance. Cover the seeds lightly and keep moist. They need a constant temperature of 20°C and should germinate within seven days. You can encourage more branching and therefore more flowers by pinching out the growing tips. However, zinnias are prone to fungal infections, such as alternaria and botrytis, which can enter through areas of damage, so you may choose not to pinch out. 

Caring for your plants

Planting

Plant in a sunny position either in the ground or in containers from mid-May, once there’s no chance of another frost.

Cutting for a vase

The key to long vase life is cutting the flowers at the right time. Zinnias have hollow stems below the flowerhead that are soft and easily bent, but they stiffen up as the stems mature. Stems picked while soft and immature will quickly droop in the vase. A simple wiggle test will tell you whether it’s time to cut: hold the stem about 10-15cm below the flower and give it a gentle shake. If the stem feels bendy it’s immature. If it’s ready to cut, the stem will remain upright and firm.

Deadheading

Removing dead heads will keep plants looking neat, but otherwise they don’t need much attention. 

At the end of the season

Pull them up at the end of the season and put on the compost heap.

Common growing problems

Slugs and snails

Pick off any slugs you find and keep them away with organic pellets based on ferrous phosphate.

Read more about slugs and snails.