The sowing calendar: when seed bombs really belong in the soil

The question I get most often is: “Can I throw these already?” The honest answer is a little longer than yes or no – but not much.

The short answer

There are two windows in the year when seed bombs work reliably: March to May and September to October. In between and after that it is possible too, but it takes far more attention – or it simply does not work.

Why the date matters less than the soil

Calendars are convenient, but a plant does not follow the date – it follows soil temperature. Most summer flowers in our mixes start to germinate at around eight to ten degrees; sunflowers would like a few degrees more. A mild March reaches that earlier than a cold corner of the countryside – and a balcony box against a house wall is often two weeks ahead of a lawn.

Month by month

• January & February

  Time for planning, not for throwing. The soil is too cold, and any seedling would freeze at the next frost. Keep the balls dry and dark in their bag.

• March

  As soon as hard frost is over and the soil feels crumbly, you can start. Sowing early means deep roots before the first dry summer day.

• April

  The best month. The soil has warmed through, rain still falls regularly, and the whole season lies ahead. Sunflowers want to go out exactly now.

• May

  Still good. After mid-May the last late frost has passed. From now on, water more often on dry days.

• June

  Possible, but work. Young seedlings dry out within two days in a hot week. Only throw if you can water daily.

• July & August

  The worst time. Throw now and you will be watering for six weeks – and often still lose half of it. Better to wait for September.

• September

  The second window. The soil is still warm, the air cooler, and rain returns. Ideal for everything perennial.

• October

  Late autumn sowing. Much of it will not germinate now but overwinter inside the ball and start in spring. That is exactly what many wildflowers want.

• November & December

  Over for this year. Any balls left keep easily until next spring when stored dry – no need to rush.

Spring or autumn – which is better?

It depends on what is inside the ball. Annual summer flowers such as calendula, cornflower or sunflower want spring: they germinate, flower and set seed in the same year.

Many perennial wildflowers, on the other hand, are cold germinators. Their seeds need a cool, moist phase before they are ready at all – a built-in protection against starting in warm late summer and then freezing. For them, sowing in October is ideal: the ball spends the winter outside, gets its cold spell, and starts all the stronger in March.

If you cannot decide, split the bag. Half in April, half in October – and next year you will know which works better on your patch.

Why July and August are so difficult

The problem is not the heat but the dryness right after germination. A seed inside the ball survives drought easily – it simply waits. The critical part is the first two weeks afterwards: the root is a few centimetres long and depends entirely on the top layer of soil, which dries out fastest. Two hot days without water are enough.

If it really has to be high summer: throw in the evening, water thoroughly and keep the spot moist every day for two weeks. Partial shade helps. And expect that not everything will come up.

Three special cases

• Sunflowers are fussier than the rest: April to May, full sun, and only one ball per spot – they need room. See the sunflower mix
• Balcony boxes warm up faster than garden soil, so you can often start two weeks earlier – but you also have to water more often, because a box dries out just as quickly.
• Giving is not throwing. A bag in an advent calendar or an Easter basket can be handed over any time of year. The throwing then simply happens in spring. Stored dry, nothing happens to the seed in the meantime.

What to do with leftover balls

Nothing special: store them dry, dark and cool, ideally in the bag they came in. Not a damp cellar, not a sunny windowsill. That way they stay viable at least until the next season. Balls that have got wet and started to fall apart should go out soon – they have already had their starting signal.

In short

After that the rest is as always: place or throw onto loose soil, water thoroughly once, and do not let it dry out for the first two weeks. That is all the tag says, too.