How to grow zinnia

What are zinnias?

Zinnias are members of the daisy family native to Mexico and South Western USA. They are found naturally in scrub and dry grassland. While in the UK we grow zinnias as half hardy annuals, there are over 20 species in central America including shrubs, perennials and annuals.

Flowers are carried singly at the top of upright stems, with bright green, bristly opposite leaves. Plants are usually well branched, varying in height from dwarf bedding forms just 20-25cm (8-10in) high up to varieties over a metre tall. The taller forms make excellent cut flowers. The number of colour forms are extensive, from strong reds, oranges and pinks to whites and even green flowers. Some are two-toned.

Choosing the right zinnia

Zinnias are popular with lots to choose from so which should you grow? There are 4 main criteria to consider:

Colour and mixes

• Seed mixes offer a jumble of colours or a 'curated' look so you can have rich jewel colours and brights or pale green and whites
• You can also buy as single colours to make monochrome blocks
• Some cultivars also have bi-coloured flowers with striped petals or differently coloured tips

Flower style

• Small flowered singles look more slight and natural, perhaps for pairing with grasses. One example is Zinnia 'Red Spider'. Single flowers also attract pollinating insects
• Double flowers have a clownish, childlike element to them and often have larger size blooms
• Novelty flowers - the Zinderella Series has anemone centred blooms; other choices are more spikey petalled, like cactus dahlias or chrysanthemums

Height

This may be a consideration with bedding and pots. Opt for dwarf cultivars such as the single Z. 'Profusion White' at around 30cm (1ft). But if you want length for cut flower stems, there are many taller cultivars such as Z. elegans 'Benary's Giant Salmon Rose' at around 1m (3ft). It's often wise to grow these through netting or at close spacing for support.

Disease and cold resistance

If you garden in less than ideal conditions for zinnias or have been disappointed in the past, it may be worth trying plants from the Profusion Series, hybrids which performed better than many others in Yorkshire in a 2002 RHS trial.

Most of the annuals we grow come from the species Zinnia elegans but more slender or delicate ones like Z. angustifolia, Z. haageana and Z. peruviana offer cultivars and hyrbids that show improved disease resistance. 

Buying zinnias

Zinnias are most commonly grown from seed, but seedlings and garden ready plants can be found online or by mail order.

Purchase young plants in bedding packs at garden centres.

When to plant

Zinnias won't tolerate cold so need to be sown in spring not autumn. Late spring sowings help avoid frosts. Plants should be planted outdoors only when all danger of frosts is past, normally June.

Where to plant

Soil type and pH isn't critical but zinnias like shelter and warmth, sunshine and moisture to grow prolifically. In cold places or on heavy soils which stay cold and don't drain well they fail to thrive. The season will also have a bearing on success – the better the summer, the happier your zinnias will be. They grow prolifically and lushly in polytunnels too.

If planting in containers, choose a sunny site.

How to plant

Growing from seed

Don't be tempted to start off your seeds too soon. It's best to wait until late April or early May as planting out young plants when nights aren't yet warm will only lead to sulking and vulnerability to disease. They should flower around 12 weeks after sowing.

Sow in modules indoors at 20^oC, then as they grow move on into 9cm (3in) pots. Be careful not to overwater. Grow on at around 15-18^oC.

Just before they're ready to plant out, harden off (acclimatise). 

Check the seed packets for spacing, but setting plants at around 30-40cm (1-1.5ft) apart is about average if growing in a block, for example as cut flowers on an allotment. 

You can sow zinnia outdoors directly into the place where they are to grow in early June, but this is only really worth doing in reliably warm locations and on light soils.

Growing from seedlings and young plants

If your plants arrive via mail order, pot on into larger pots like 9cm (3in) when the roots have filled the little modules. Carry on caring for them as with home-sown seed - as above.

Growing from bedding packs

Again, hold back on planting out in the garden until early June in the south of England or mid-June if further north. Keep in a frost-free greenhouse until then. Harden off before you plant out at approximately 30-40cm (1-1.5ft) apart if growing in a block, but the pack's label should advise on spacing.

In containers

Zinnias aren't especially fussy about compost type. Just go with a quality peat-free potting compost.

Watering

Keep your zinnias watered in hot dry weather but try not to get the foliage wet as this can encourage fungal diseases.

Feeding

Feed every other week with a high potash feed like tomato feed.

Support

It's worth netting to support taller cultivars. Push in some canes at each corner of you block of zinnias and stretch over a sheet of taught pea netting or metal pig wire fencing. Depending on the eventual height of your zinnias secure the grid of support at around 30-40cm (1-1.5ft) above the ground. The plants will grow up and through. This works well when growing as a block, for example as a cut flower crop on an allotment. For little groups as summer border fillers, try pea sticks or woody brash for support.

Pinching out

It pays to pinch out zinnias. By removing the central growing tip when plants are around 20-30cm (8-12in) above a pair of leaves. This encourages branching and therefore more flowers. This can sometimes mean pinching out or sacrifing the first flower, but you will get more flowers in the long run.

Deadheading

While individual flowers can last a long time, their vibrancy can fade. When a flower starts to loose colour, snip back to another branch to allow its successor to develop and open. This will encourage more fresh flowers to develop.

Cutting

Handle stems carefully when harvesting as the hollow stems can easily crush, limiting their vase life.

Many zinnias are F1 hybrids so won't come true from seed. In the UK zinnias don't normally produce seed, so buy fresh seed every year.

For details of how to grow zinnias from seed - see 'Planting' above.

Especailly in cold, wet weather, zinnia are prone to fungal diseases such as damping off as seedlings or botrytis (grey moulds) and alternaria blight on older plants.

Slugs can ravage seedlings.

Get more gardening advice

If you’re a member of the RHS, you can use our online Gardening Advice Service, via MyRHS, for any gardening problems or queries.