How do I grow zinnias in New Zealand?
Sow zinnia seeds from late September, cover them lightly with soil, and keep them somewhere warm and bright. Plant out into full sun after your last frost, around Labour Day in most of New Zealand, spacing them 20 to 30cm apart. Pinch the growing tip out when the plant is 15 to 20cm tall, then cut often. You will have flowers roughly eight to twelve weeks after sowing.
Zinnias are the flower I recommend to every single beginner, and they are a workhorse on my farm too. I grow Benary's Giant, which is the series most cut flower growers land on, because the stems are strong enough to actually use and the blooms are enormous.
Why are zinnias so good for cutting?
Because they want to be cut.
A zinnia is trying to set seed. Every time you take a flower off it, the plant reads that as a failure and sends up more. So the person who cuts armfuls every week ends up with far more flowers than the person who leaves them alone to look nice in the garden. It is the most satisfying feedback loop in gardening.
They also flower for an extraordinarily long time. Mine go from early summer right through until the first frost knocks them down in autumn. That is four or five months of picking from one sowing.
When should I sow zinnia seeds in the Waikato?
From late September, into trays, somewhere warm. They are frost tender so they cannot go outside until the cold has finished with you, which around Hamilton means late October, Labour Day as the traditional marker.
You can also direct sow straight into the garden once the soil has properly warmed up, from about November. Slightly slower to flower, but it works and it is less faff.
I would sow twice. One lot in late September and another in early November. The second sowing comes into its own in late summer when the first lot is starting to look tired, and it stretches your season by a good month.
How do I sow zinnias, step by step?
- Fill a tray with seed raising mix and dampen it.
- Press the seeds onto the surface and cover with a fine layer of mix. Zinnias want darkness to germinate, unlike some seeds.
- Cover the tray with a dome or cling film.
- Keep it warm and bright. They germinate fast in warmth, usually within five to ten days.
- Remove the cover once they are up.
- Harden off at 5 to 8cm tall by putting the tray outside during the day for a week.
- Plant out in full sun after the last frost, 20 to 30cm apart.
- Pinch the top out when the plant reaches 15 to 20cm.
Why should I pinch zinnias, and does that not feel wrong?
It feels completely wrong and you should do it anyway.
When the plant is 15 to 20cm tall, snip the growing tip off, taking it back to a set of leaves. You are removing what would have been your first flower.
What you get in return is a plant that branches from the base instead of running up a single stem. More branches means more stems, and longer ones, because each branch grows up to reach the light. One pinch in November turns one flower into a dozen.
I do it to every zinnia on the farm and it is still the bit that makes visitors wince.
How much sun and water do zinnias need?
Full sun, at least six hours a day, and more is better. Zinnias grown in part shade get leggy, flower less, and are far more prone to mildew.
Water at the base rather than over the top of the leaves. Zinnias are prone to powdery mildew late in the season and wet foliage overnight is how it takes hold. Water in the morning, at the roots, and give them a good soak rather than a daily splash.
Feed weekly with a liquid fertiliser once they are established. Seaweed based ones are excellent.
When and how should I cut zinnias?
Cut in the cool of the morning, and use the wiggle test.
Zinnias do not keep opening once they are cut, so timing matters. Grab the stem about 20cm below the flower and give it a gentle wiggle. If the stem is floppy and the head wobbles about, leave it. If it stays firm and upright, it is ready. That firmness means the stem has matured and the flower will last in the vase.
Cut deep, down to a set of leaves low on the plant, and put the stem straight into water. Cutting deep feels brutal and it is exactly what encourages the long stems you want next time.
Change the vase water every couple of days and they will give you a good week.
What can go wrong?
Powdery mildew, a white dust on the leaves late in summer. Space plants for airflow, water at the base, and do not panic. It usually arrives near the end of the season anyway.
Leggy plants with few flowers, which is almost always not enough sun, or skipping the pinch.
Short stems, which comes from cutting timidly. Cut deeper.
Seedlings collapsing at soil level, which is damping off from overwatering. Keep the mix moist, not wet, and get the cover off once they are up.
Frequently asked questions
When do you plant zinnia seeds in New Zealand?
Sow from late September in trays, and plant out after the last frost, around Labour Day in most regions. You can direct sow into warm soil from November.
How long do zinnias take to flower from seed?
Eight to twelve weeks from sowing. A late September sowing typically flowers by Christmas.
How tall do Benary's Giant zinnias grow?
Around 90 to 100cm, which is what makes them a proper cut flower rather than a bedding plant.
Should I pinch zinnias?
Yes. Pinch the growing tip out at 15 to 20cm tall. You sacrifice the first flower and gain many more stems, all of them longer.
Do zinnias come back every year?
No, they are annuals. They flower until the first frost and then finish. Save seed or sow fresh each spring.
Why are my zinnia stems so short?
Usually because they were not pinched, or because they are being cut too shallowly. Cut deep into the plant, back to a low set of leaves.
