How to grow zinnias

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The best zinnias I’ve ever seen were lining a path to a primary school in Sri Lanka. They were flowering in February in the fort town of Galle, on the south coast of the island, thriving in an environment of deluging, monsoon rain, interspersed with hot sun and high humidity. They loved it.

The plants, spaced 18in (46cm) apart, had merged into a hedge standing over 4ft (122cm) high and were massively flowery and luxuriant. I did a double take before realising they were just the common or garden zinnia, ‘Dahlia Mix’.
That hedge taught me what zinnias truly like – not always easy to provide in Britain, but still possible. In 2013, with four times the number of sunny, summer days in Sussex compared with 2012, the zinnias looked magnificent.
There’s no better late-summer plant, with a brilliant range of colours and flowers that look as though they’ve been cut from velvet-coated cardboard. Whenever I touch the petals, I’m straight back to Fuzzy Felt as a child. The texture is just the same.
So how do we get the best out of zinnias?
One important thing to know about them is that, from the moment of germination right through their life cycle, they’re particularly prone to damping off and botrytis. In a damp, cool year, with high air humidity, plants can get a plateau of mould, which sits on the growth tip and rots down into the heart of the plant. For this reason, you’ll find that lots of zinnia seeds are sold ready-coated with fungicide.
Zinnias famously hate root disturbance, so it is best to sow directly into freely drained soil with a fine tilth. Sow two or three seeds 2in (5cm) apart at 12in (30cm) spacing. Thin out the seedlings, so you end up with one plant every 12in (30cm) or so. You can also sow into modules (I use coir Jiffy’s), so there’s no pricking out, and plant out when still small with only a pair or two of true leaves. That way you minimise root handling.
Don’t sow too early. Remember the heat of Sri Lanka where they fare so well, and wait until the nights are warm enough to eat outside.
If the evening temperatures are chilly enough to have you reaching for the blankets to eat supper in the garden, then it’s still too cold. If you live in a part of the country where supper outside is a once-every-five-years experience, then sadly zinnias are not for you, although you could try growing them in a greenhouse or polytunnel.
Plants are best supported by individual stakes or, if being grown in a veg or cutting garden for picking, they look fine pushing up through nylon clematis or wire sheep netting, stretched horizontally one foot (30cm) or so off the ground and tied taut onto a series of canes. Even if your plants are not going to reach Sri Lankan heights, they benefit greatly from growing straight early on. If they collapse over, they’ll never grow or flower as well as when vertically supported.
The other key fact I learnt in Sri Lanka was that, as well as plenty of sun and heat, zinnias love a good amount of water, and this summer I’ve lavished mine with a good dousing every week at their roots – how they’ve loved it. Warm, moist soil and warm, moist air is what you’re looking for, and if we don’t get it naturally, that’s what you’re aiming to fake.
A cut above the rest
As cut flowers, zinnias are hard to beat, and in a cut-flower trial I visited at The National Cut Flower Centre near Spalding, Lincolnshire, the Benary’s range (so-called after the German plant breeder) were reigning supreme.
With this selection, the necks are strong. Zinnias have hollow stems below the flower, so weak necks, which bruise and break off all too easily as you arrange them, can be a problem. This can make zinnias tricky to lace into a hand-tied bunch and is one of the reasons you often see them arranged just on their own.
With most zinnias, you’ll get at least a week in a vase if they’re kept cool. They last best on a short stem, with one flower arranged in a single-stem vase, or poked through a wooden grid to support the flowers just out of the water. The smaller-flowered forms such as ‘Sprite Mix’ and ‘Zahara Fire’ – which, as its name implies, opens red and gradually changes to orange – are the best for arranging on this scale and, cut short, have a vase life of about 10 days.
Scatter the flowers into similar coloured vases. Coloured glass is a good match to the zinnia look and feel, and they also suit bright ceramic jugs of red, turquoise or acid-green.
Most important, especially when you’re arranging, is to restrict the colour range. The green forms, such as Zinnia ‘Envy’ or larger-flowered ‘Benary’s Giant Lime’ go with everything, but this is the only shade that does. Stick all the saturated, stained-glass colours together (either in the garden or picked to make a bunch), but keep these well away from the pales. Shoved in close together willy-nilly, zinnias can all too easily end up looking like dolly mixtures and lose their way on style.
A cream variety such as ‘Oklahoma Ivory’ is beautiful mixed with the green (‘Benary’s Giant Lime’) and a white (‘Benary’s Giant White’), but the pales look all wrong mixed with dark reds and oranges. Similarly, the red and purple tones (‘Benary’s Giant Purple’ and ‘Benary’s Giant Wine’) are lovely with the contrast of a splash of rich vermilion-orange (‘Benary’s Giant Coral’), magnificent against petrol blue or rich, dark green; and the rich pinks (such as ‘Oriola’ or ‘Benary’s Giant Pink’) are marvellous with any of the reds and oranges, calmed down by ‘Envy’ green.
With your colours carefully chosen, zinnias will give you the best ever September bunch of flowers.
This article first appeared in The Daily Telegraph on 31st August 2013.