A lot of wildflowers need to be sown in spring in order for them to receive all that the cold weather throws at them, to enable them to germinate in spring. This is called vernalisation (also known as stratification) and the process breaks the dormancy of the seed so that, come spring when the weather starts to warm up, the seeds will germinate.
This is quite a satisfying process from a gardener’s point of view – although nothing appears to be happening over winter and you fret and worry that the seeds have been killed by frost, one day you go out to check them and little green shoots have magically appeared after months of empty, barren-looking compost!
How to vernalise wildflower seeds
The vernalisation process starts in autumn. The seeds/bulbs you have chosen should come with details as to whether they need to be exposed to the cold. If using seeds, sprinkle the seeds into a small bag of damp compost and label it with the current date. Then seal the bag and place in the fridge – yes, in the fridge! This helps to speed up the process of cold exposure, particularly as UK winters are quite mild in some parts of the country.
Keep in the fridge for around 6 weeks and then pour the compost mix into a tray (topping up with more compost if necessary). Water lightly and label the tray with seed type and when sown. Then place outside somewhere to get the frost, cold and snow. You may want to cover with clingfilm or glass in order to stop little rascals such as mice from eating the seeds.
Come spring, little seedlings will appear and you can then transfer the seedlings to individual pots (once big enough to handle) and grow on until ready to plant out when big enough.
How to vernalise wildflower bulbs, such as Bluebells
Bulbs such as Bluebells need to be planted in autumn to a depth of about 4 inches. These bulbs are only available during autumn. To get a random, naturalised look, grab a handful of bulbs and throw them to the ground in the area you want them to grow. Then plant each bulb where it has landed. Bluebells can also be bought “in the green” in spring and summer, which means you buy them just as they are getting leaves, while thjey are flowering or after they have finished flowering, and you plant them staright away.
If you decide you want to grow your own Bluebell bulbs from seed, then be prepared for a long wait for flowers! The seeds need to be sown as per the seed instructions above – it is easier to sown them into plug trays, a few seeds per cell. In spring you will get tiny thin green spikes like grass. Individually plant these into small pots and grow on, or keep them in their plug tray cells until well grown. They will not flower until 4 years old when a bulb has formed.
What wildflowers need vernalising?
This is not an exhaustive list by any means but here are a few examples – Bluebells, Yellow Rattle, Cowslip, Cow Parlsey, wild Irises, Wild Clematis, Dog Violet, Sweet Violet, Yellow Archangel, Marsh Marigold, Wild Garlic (Ramsons). Good seed suppliers should mention on the seed packaging if vernalisation needs to be carried out. A lot of tree and shrub seeds need vernalisation and some may even not show any sign of life until a couple of years or so have passed! Dog Rose is one species that springs to mind.