How to sow seeds that need cold to germinate outdoors – a quick guide

1. Why some seeds need cold to germinate

Some plants have seeds that need to go through a cold phase – also known as cold stratification – before they can germinate. These seeds contain special substances to inhibit germination that can only be broken down by the effect of low temperatures over a period of several weeks. Only after this cold stimulus are the seeds ready to sprout when temperatures start to rise in spring. This adaptation ensures that they don’t germinate too early in autumn and then fail due to frost damage.

2. The best time to sow

The perfect time for sowing seeds that need a cold phase is late autumn through to early winter, generally between November and January when temperatures are low enough to provide the necessary cold stimulus. This gives the seeds enough time to go through the cold phase during winter, so that they can germinate perfectly when temperatures start to rise in spring.

3. Factors to consider when sowing

When sowing seeds that need cold to germinate there are a few key points to consider:

Pots rather than direct sowing: We recommend sowing seeds that need cold to germinate in pots or seed trays. Many plants that need a cold phase also need light to germinate. These are often very tiny seeds that are easily blown or washed away. If they’re sown in pots these problems are easier to manage – and pots are more easily moved around, too.
Substrate and location: Use a free-draining, humus-rich substrate and make sure it’s finely textured, so that your tender seedlings can develop optimally. A semi-shade location is ideal, so that the soil doesn’t dry out but your seeds still get enough light.
Protection from animals: Covering your potted seeds with a fleece, a fine wire mesh or a net will protect them from animals – mice and birds will be delighted to help themselves to your seed in winter. It’s important that this protection should be breathable to prevent moisture build-up and allow the cold to reach your seeds.
Creating the cold phase: When the seeds are first sown they initially need a short period (2-4 weeks) at temperatures between +5 and 15°C – so it might be best to place the pots in a greenhouse or in a cool location like a staircase or landing to start with. After this, though, they should be placed outdoors during the winter so that they go through their natural cold phase. Sowings work even better if the pots are set into the ground in a free space in your garden. All in all what’s needed here is patience: most plants that need a cold phase won’t germinate until spring when the temperatures start to rise again.