One of the questions I get asked most often is why some seeds germinate so easily while others seem to do absolutely nothing. The truth is that not all seeds behave the same. Some are ready to grow as soon as they're given warmth and moisture, while others need a little extra encouragement before they'll wake from their natural dormancy.
If you've ever come across terms like stratification, scarification, pricking out or hardening off and wondered what they actually mean, here's a simple guide to the seed sowing techniques every gardener should know.
Stratification
Some seeds, particularly many perennial flowers, need to experience a period of cold before they will germinate. This process is known as stratification.
In nature, these seeds fall to the ground in autumn, spend the winter exposed to cold temperatures, and only begin growing once spring arrives. This natural chilling period tells the seed that winter has passed and it's safe to start growing.
If you're sowing seeds indoors or you live in a warmer part of New Zealand, your seeds may never receive enough cold to trigger germination. Fortunately, you can recreate this process at home.
To stratify seeds:
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Mix your seeds with damp sand or vermiculite using roughly four parts sand to one part seed.
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Place the mixture into a labelled zip-lock bag.
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Leave it for around 24 hours so the seed can absorb some moisture.
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Store the bag in the refrigerator for the length of time recommended on the seed packet.
Check the seeds regularly. If you notice tiny white roots beginning to emerge, they're ready to be potted up immediately. Otherwise, once the recommended chilling period is complete, sow them as normal.
Scarification
While some seeds need cold, others have the opposite problem—they have an exceptionally hard outer seed coat that prevents moisture from reaching the embryo inside. This is where scarification comes in.
Scarification simply means gently weakening or scratching the seed coat so that water can penetrate and germination can begin.
There are a few ways to do this depending on the type of seed:
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Lightly rub the seed with fine sandpaper.
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Nick the seed coat carefully with a nail file or sharp knife, taking care not to damage the seed inside.
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For some species, soaking the seeds in warm water overnight is enough to soften the seed coat.
Not every seed should be scarified, so always check the growing instructions first. When required, however, it can dramatically improve germination rates and speed up the process.
Pricking Out Seedlings
Once your seedlings have germinated, they'll eventually need a little more room to grow.
Pricking out is the process of carefully lifting young seedlings from their sowing tray and transplanting them into larger pots or cell trays.
I usually prick out seedlings when they've been sown closely together, when it's still too early to plant them into the garden, or when I'm growing slower-growing perennials that need to become established before planting out.
The best time to do this is once the seedlings have developed their first set of true leaves.
The very first leaves that appear are called cotyledons. These are seed leaves and often look quite different from the mature plant. The next set of leaves are the true leaves, and these resemble the foliage the plant will eventually produce.
To prick out seedlings:
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Fill larger pots or punnets with moist potting mix.
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Use a dibber, pencil or even the handle of a spoon to gently loosen the seedlings from the soil.
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Always handle seedlings by their leaves rather than their delicate stems.
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Lower each seedling into its new planting hole and gently firm the soil around the roots.
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Water lightly after transplanting.
I then keep my seedlings in the greenhouse, or somewhere warm, sunny and sheltered, until they're ready to move into the garden.
Hardening Off
One of the easiest mistakes to make is planting young seedlings straight from the warmth of the greenhouse into the garden. The sudden change in temperature, wind and direct sunlight can cause transplant shock.
Hardening off is the process of gradually introducing seedlings to outdoor conditions before planting them permanently.
Start by placing your seedlings outside in a sheltered position for a few hours each day before bringing them back inside. Gradually increase the amount of time they spend outdoors over the course of a week, allowing them to adjust to the cooler temperatures, wind and stronger sunlight.
By the end of the week they'll be much better prepared for life in the garden.
That said, I often find that simply waiting until both the soil and the weather have warmed naturally can reduce the need for extensive hardening off, particularly with many summer annual flowers.