Vernalisation and Stratification: What They Are, Why They Matter, and How to Do Both

If you've ever sown seeds that simply refused to germinate — despite doing everything right — vernalisation or stratification might be the missing piece. These two processes sound technical, but once you understand what's happening, they make complete sense. Plants are just following nature's rules.

What is Stratification?

Stratification is the process of exposing seeds to a period of cold and/or moist conditions before sowing. In nature, this happens automatically — seeds fall to the ground in autumn, sit through winter in cold, damp soil, and germinate when spring warmth arrives.

Many seeds are designed to require this cold period as a safety mechanism. It prevents them from germinating during a brief warm spell in autumn, only to be killed by the frosts that follow. Stratification essentially tells the seed: "Winter has passed. It's safe to grow now."

Seeds that commonly require stratification include:

• Hellebores
• Primulas
• Many native wildflowers
• Some clematis species
• Certain tree and shrub seeds

What is Vernalisation?

Vernalisation is similar but applies more broadly — it refers to the requirement for a sustained period of cold temperatures to trigger flowering (rather than just germination). It's most commonly discussed in the context of biennials and perennials.

A classic example is the foxglove (Digitalis). Sown in summer, it grows vegetatively through its first year, experiences winter cold, and then flowers the following spring or summer. Without that cold period, it may never flower at all — or flowering may be significantly delayed.

Vernalisation is essentially the plant's way of synchronising its flowering with the right season, ensuring it blooms when pollinators are active and conditions are favourable for seed set.

What's the Difference?

In short: stratification gets the seed to sprout, vernalisation gets the plant to flower.

How to Stratify Seeds Naturally

The simplest method is to let nature do the work:

1. Sow seeds in autumn directly into the ground or into pots left outside
2. Allow them to experience the natural cold of winter
3. They'll germinate when conditions are right in spring

This works beautifully in climates with a genuine cold winter — like much of New Zealand's South Island and higher altitude regions in the North Island.

How to Stratify Seeds by Human Methods (Cold Stratification)

If you want more control — or if your climate doesn't get cold enough — you can replicate winter in your fridge:

1. Moisten a small amount of seed-raising mix, vermiculite, or paper towel — damp but not wet
2. Mix your seeds through it or fold them into the paper towel
3. Place in a zip-lock bag or small sealed container and label it with the date and seed variety
4. Pop it in the fridge (not the freezer) at around 2–5°C
5. Leave for 4–12 weeks depending on the species — check your seed packet or species sheet for guidance
6. Sow as normal once the stratification period is complete

Check the bag every couple of weeks. If you spot any seeds starting to germinate, get them sown immediately!

How to Vernalise Plants

For most gardeners, vernalisation happens naturally — you sow biennials in summer, they grow through autumn, experience winter, and flower in spring. Job done.

But if you're growing in a warmer climate or want to speed things up, you can vernalise young seedlings artificially:

1. Grow seedlings on until they have a few true leaves (they need to be past the seedling stage to respond to vernalisation)
2. Move them to a cold frame, unheated greenhouse, or sheltered outdoor spot where temperatures drop to 2–10°C at night
3. Keep them there for 6–8 weeks
4. Bring them back into warmer conditions to trigger flowering

This is particularly useful for cut flower growers wanting to time flowering for specific market dates.

A Note for Cut Flower Growers

Understanding vernalisation and stratification gives you a real edge in production planning. Many of the most beautiful and sought-after cut flowers — hellebores, ranunculus, anemones, delphiniums, and sweet peas — have specific cold requirements that directly affect when and how well they flower.

Getting these requirements right means better germination rates, more reliable flowering windows, and ultimately a better crop. Our species sheets cover germination temperatures and cold requirements for each variety in detail — well worth having on hand when you're planning your sowing calendar.

The Bottom Line

Both vernalisation and stratification are simply plants doing what they were designed to do — using temperature as a reliable signal for the right time to grow and flower. Once you understand that, working with these requirements (rather than against them) becomes second nature.

Cold is not the enemy. Sometimes, it's exactly what your seeds need. 🌱