Growing Zinnias - From Seed to Seed
Zinnias have always been a part of our farm. From interplanting among tomatoes, basils, herbs and marigolds to growing them in our home cutting garden, these heat-loving, cut-and-come-again flowers are a delight in the summer garden.
Breeding new varieties has further refined our methods of starting zinnias from seed. Our breeding has focused on fully double, dahlia-like zinnias, and this work has taught us the importance of seedling raising.
Here is our method for growing zinnias—from seed, to field, to vase, and back to seed again—at Transition Farm.
Zinnias are a warm-season crop
As we are located in the cool temperate zone of Australia, our first challenge is not to start our zinnias too early.
Our own trials, together with research from universities and horticultural research centres, have highlighted how important the early stages of a zinnia's life can be. Zinnias are a major flower crop for cut-flower farmers, and, like tomatoes, they have attracted a considerable amount of research.
I'm not one to baby plants, but I do baby zinnia seedlings!
Take care with young seedlings
A publication from Utah State University notes:
“Double-blooming zinnias may revert back to singles if the transplant is root bound or if roots are heavily disturbed when transplanted.”
— Zinnia Cut Flower Production in Utah
In addition to root disturbance, we ensure that our zinnia seedlings are regularly watered. We do not try and drought stress our zinnia seedlings until they are well established in the field.
When to sow seeds
As we are usually starting our seeds ahead of the weather being settled enough for transplanting, we start our zinnias on a heat pad approximately four weeks before transplanting.
We base our transplanting date on when night temperatures are consistently above 10–15°C, together with suitable soil and weather conditions.
We use soil blocks
We use soil blocks for sowing our zinnia seeds. We adore soil blocks because they:
- Bring the zinnia roots into contact with our soil microbiome before transplanting.
- Provide the seedlings with enough food to grow for 4–6 weeks without supplementary feeding.
- Allow the seedlings room to develop without becoming root-bound, provided we transplant them before they have outgrown the block.
If we are transplanting into a protected area, we use 1½-inch soil blocks. If the seedlings are heading to the field and we want a little more room for growth, we use 2-inch soil blocks.
What if you don't have soil blocks?
Many growers use seedling trays and other propagation systems. We outline some of the choices for growing strong seedlings in our post about raising healthy seedlings.
Soil mix for seedlings
We make our own soil-block mix. You can find the recipe and instructions here.
TRANSITION FARM SEEDLING MIX RECIPE→
Optimum Germination Temperature
We sow 1–2 seeds per block, cover them with vermiculite and place them on heat pads set at 22°C.
The seeds generally germinate within 3–7 days. Once they emerge, we ensure they have a strong light source. With our very windy fields, we want to grow stocky, sturdy seedlings rather than tall, leggy plants.
If you are worried about not having the ideal temperature, you can also germinate the seeds inside. Zinnia seeds can be folded into a paper towel. Wet the paper towel thoroughly but not dripping - you can squeeze the rolled up paper towel to ensure it is damp but not soggy. Then place in a plastic bag or on a plate with a bowl on top. Check each day to ensure the paper towel stays moist. The seeds usually germinate between 3-5 days.
You can then gently tuck them into a seedling tray and grow them on in a well lit and warm place.
Direct sowing
If you are in a warmer climate region of Australia, direct sowing is another option.
Sow the seeds approximately 6 mm deep and protect emerging seedlings from slugs, slaters and other soil-dwelling pests that enjoy young growth.
After germination
After germination, we grow our seedlings in the glasshouse, where they experience fluctuating day and night temperatures. Nights may dip to 15°C, while days may rise to 28°C.
We provide a direct light source and use fans to encourage strong stems and prevent the seedlings from becoming leggy.
Hardening off
As we get closer to transplanting, we move our zinnia seedlings into a protected area outdoors and gradually expose them to the conditions they will experience in the field.
We begin to reduce watering slightly, allowing the seedlings to adapt to outdoor conditions. We do not deliberately drought-stress our zinnia seedlings at this stage. Our aim is to harden them off gradually without placing them under unnecessary stress.
Soil preparation and transplanting
Zinnias may be in your growing area for the entire warm season, so soil preparation is not an afterthought here.
We start preparing for our zinnia season in the autumn before transplanting.
Autumn preparation
In April, we sow an autumn green manure where the zinnias will be planted the following season. This may include:
- Oats
- Rye
- Fava beans
- Lupin
- White clover
- Persian clover
- Dunn peas
- Vetch
We may add slow-release soil amendments, such as rock dust, ahead of sowing the green manure. We may also add well-aged manure to help establish a healthy, robust green manure crop.
Once the green manure has germinated, we apply prepared biodynamic 500, a preparation we use to support our soil microbiome. The green manure grows through the winter.
Spring preparation
In spring, the green manure is chopped up and incorporated into the soil. We apply prepared biodynamic 500 again as part of our approach to supporting soil life and humus building.
Our aim is to build healthy, living soil that supports the zinnia plants throughout the growing season.
Supporting living soil
Soil tests can be useful. Numbers are interesting and can be illuminating, but looking strictly at soil chemistry is limiting. Soil biology - or building life in soil - allows for subtle connections and networking that occur in living soil.
A plant's relationship with living soil changes over the course of its life, and plants require different minerals at different stages of growth.
As biodynamic farmers, we support humus building, the maintenance of soil structure, strong bacterial and fungal soil biology, and healthy photosynthesis.
All of these processes contribute to a living soil that is dynamically changing. They encourage the innate intelligence of the ecosystem and allow plants the opportunity to express their own health.
Learning about and supporting living soils is a lifelong endeavour. The rewards are healthy plants, beautiful cut flowers, fewer pest and disease problems, and excellent vase life.
Transplanting
For cut flowers, we transplant our zinnias into a full-sun location, spacing plants 20–25 cm apart, with rows 30 cm apart.
We take care to transplant without unnecessarily disturbing the roots, particularly while the plants are young.
Growing zinnias
We grow our zinnias using both drip lines and overhead irrigation. Although we do drought-stress our zinnia seed crops, we do not want to stress our plants during their early growth. And unless you are. trying to save seeds from your strongest plant, drought stressing is not something we would recommend. That said, zinnias thrive in hot and dry conditions. But too much of either can cause the plants to revert to singles.
Studies have shown that they can recover in some cases and go back to producing the fully double, dahlia like flowers.
Support
As we are in a very windy location, we use the corral method to support our zinnias.
We drive stakes every 4-5 metres and wrap strong string around them to create a cage or string support for the plants.
You can also use horticultural netting, rolled out horizontally between stakes, to support the plants as they grow.
In a home garden, a zinnia may have enough support from its neighbours, or it may require a single stake and soft ties to prevent the limbs from breaking throughout the season. Do not underestimate how big side limbs can get on a plant.
Pinching and deadheading
Pinching the initial bud or bloom, followed by subsequent deadheading, can encourage branching and further flower production.
In the photo above, the plant has a main stem and laterals or side shoots forming from leaf nodes down the main stem. We remove the main stem, cutting it above a leaf node. We usually do this when the plants are about 30cm high.
We pinch to help develop strong plants with plenty of flowering stems for the cutting garden. We deadhead to keep the plants from going to seed. Once a plant starts to produce seeds, the flower production declines.
Flowering and harvest
If we are growing zinnias for cut flowers, we leave our blooms to form large, full domes. The longer we leave them, the more complex the patina of mature blooms and newly emerging petals becomes. This adds to the visual interest of the flowers.
For example, we would leave the flowers pictured above for awhile longer before cutting - just to get that marshmellow like domed bloom.
Especially our Transition Farm bred varieties benefit from waiting to harvest. If you are finding it hard to wait, just leave one flower so you can see the difference between waiting and not waiting.
The wiggle test
The wiggle test can also help determine when a flower is ready to cut.
Grab the stem and gently shake it. If the bloom is very floppy, it is best to leave it until the stem develops more strength. A firm stem indicates a better harvest stage. This helps to prolong vase life.
Harvesting and conditioning
We cut deeply into the plant to obtain a long stem, using sharp secateurs or a sharp knife and cutting just above a leaf node. More stems will emerge from that node, helping to maintain productive plants throughout the season.
We cut our blooms during the cool part of the day, remove any foliage that would sit below the waterline and place the stems immediately into clean, cool water.
We then move them to a cool, shady location to hydrate for a few hours. After conditioning, we change the water, add floral preservative and recut the stem ends to help extend vase life.
We do not refrigerate zinnias. Instead, we keep them in a cool shed, away from draughts and direct sunlight.
At home, some growers use a small amount of bleach in their vase water. Zinnias are sometimes described as “dirty” flowers because they can make the water cloudy. A small amount of bleach may help reduce this, although changing the vase water daily is a simple alternative.
Under suitable conditions, we often expect the petals to remain vibrant for 7–10 days. We have also had zinnias develop roots in vases when the water was changed daily.
Saving seed
If you are hoping to collect seed from your zinnia flowers, you need to ensure that pollinators are present in your garden or growing area.
Bees, flies and other flower-visiting insects move from plant to plant and can contribute to pollination and seed production.
For our breeding work, we pay particular attention to the plants and flowers from which we collect seed. Our aim is to continue selecting for the colour and form we are breeding towards. We also select seed from plants that show great vigour.
We ensure that our breeding fields are well isolated from other zinnia varieties as the flowers will cross pollinate.
Leave the flowers on the plant until they have dried. This allows the developing seed to mature. The flower in my hand (pictured above) is still not dry enough. We prefer to wait until the lower petals are fully browned.
Once the flower is dry, harvest it, dry it further if necessary, and then gently rub out the seeds - they are attached to petals (ray seeds) and buried in the head (disc seeds). Both sorts will contain viable and unviable seeds. We have found that the heavier the seed, the greater the viability - so we use wind to seperate viable seed from unviable seed and chaff. Colour and bendability also help people sorting smaller amounts of seed. Darker coloured seed that is thick and stiffer tends to be more viable. Still, shaking your seed through a small stream of wind may help you sorting the chaff from the seed - and the heavier seed will fall closer to the air source.
From seed to seed
Growing zinnias is a rewarding process. From preparing the soil in autumn and raising healthy seedlings through to growing, harvesting and saving seed, every stage contributes to the following season.
For us, zinnias can be both focal flowers and accompanying blooms in arrangements. Because they are so productive throughout summer, we take care to give them a strong start. In return, we are rewarded with an abundance of beautiful blooms that carry us through into autumn.
