Throwing seed bombs in winter?

The question turns up every November: can seed bombs be thrown in winter, or do you have to wait for spring? The short answer is that late autumn is one of the best windows of the whole year — deep winter is not. The difference comes down to one idea: cold germinators.

What cold germinators are

Many native flowers only germinate after their seed has spent a long spell in cold, damp conditions. Botanists call it stratification; gardeners say cold or frost germinators. It is a built-in clock. The seed drops in summer and could sprout immediately — and the first frost would kill the seedling. So it stays dormant while it is warm, and only once it has had weeks of cold does it read the signal: winter is over, it is safe now.

For seed bombs that has a practical consequence. Throw in October or November and you hand that job to the weather. The ball sits outside all winter, softens, the seed gets its cold spell, and in spring it germinates on its own — often one to three weeks earlier than a ball thrown in March.

Why deep winter still does not work

• Frozen ground absorbs nothing. The ball sits on top as if on a table. When the thaw comes it floats off with the meltwater, usually into the nearest dip or drain.
• Birds and mice are hungry. In January a softened ball of soil full of seed is one of the few reachable food sources. In October it disappears among beech mast and rose hips; in January it is a laid table.
• Snow is a blanket, not soil. A ball on snow has no soil contact. Once the snow goes, it is somewhere else and its seed is spread across the whole area instead of one spot.

Winter, month by month

• October

  The best autumn window. The soil is still warm and receptive, there is enough rain, and the ball makes contact before it turns properly cold.

• November

  Still good as long as the ground is not frozen. On heavy, very wet soil it is better to wait — permanent waterlogging rots some seed.

• December

  Keep them rather than throw them. Exception: a mild, frost-free week and open, crumbly ground — then it works as in November.

• January

  The worst month. Frost, snow, hungry birds. Balls stay indoors.

• February

  From the second half, once the soil feels crumbly and no long frost is forecast, you can start.

• March

  The big spring window. Everything collected over winter goes out now.

The two-second soil test

Push your thumb into the soil. If it goes in one or two centimetres without effort, the ground is ready. If the surface feels like wood, it is frozen and the ball stays indoors. If the soil smears like wet clay, it is too wet — wait.

What to do with the balls until then

Nothing, except keep them dry. Seed bombs keep through the winter under three conditions: dry, cool, dark. A box in an unheated hallway, a storeroom or a garage is ideal. Wrong: the shelf above the radiator, a cellar with condensation, and a sealed tin where residual moisture cannot escape.

The windowsill version for the impatient

If something has to happen in January — usually the case with children — do it indoors: one ball in a pot of seed compost, pressed in lightly, watered, kept bright but not too warm. Seedlings appear after ten to twenty days. Two caveats: the true cold germinators stay dormant because they have missed their cold spell, so only part of the mix comes up, and the plants stay softer than outdoor ones.

The proper version is to put that pot outside from December to February — balcony, window ledge, garden — and let frost and rain do the work. That is stratification in a flowerpot, and it works for any mix.

Which mix suits an autumn throw?

For October and November, the perennial native mixes are the ones to use, because that is where the cold germinators sit: Wildflower meadow and Bee mix. The Butterfly buffet works well too. Pure summer annuals such as Sunflowers make little sense in autumn — save those for April.