Thinking about planting? Think about soil temperature…

Spring is coming (despite the forecast of snow for this weekend!) and it’s time to plan for planting.

When getting ready for spring planting, bear in mind that soil temperature is crucial for germination of seeds. Warm weather seeds are much more interested in whether the soil is warm than if the air is balmy.

In Canberra, most seeds need to be started in trays or pots inside where the raising mix can be kept warm – maybe on a window sill in the sun (as long as it’s protected from overnight extreme cold). If you use a glasshouse for germination, make sure the glasshouse retains enough heat and the seeds don’t freeze overnight. Many seeds can be started in a cupboard or next to a heater.

This is a rough guide to soil (or raising mix) temperature required for optimum germination. In early spring, it takes a while for the soil to warm up.

Often seeds that are planted (or tossed onto the soil) early will just sit and wait until the soil warms up and, as long as they aren’t eaten or washed away or frozen part-way through germination, a strong seedling will come up when the time is right. This happens every year for many gardeners with self-sown tomatoes and lettuces and pumpkins. Often, even though these seeds might germinate later than those started early inside, they will be stronger, grow faster and be more productive if they can set fruit outside of the peak heat times.

In Canberra, it’s tricky to rely entirely on garden-based germination for tomatoes, capsicum and zuchinis/cucumbers/pumpkins which need to reach the flowering stage before the weather becomes too hot in mid-summer so starting a few seeds indoors early can be a good way to insure against extreme heat in spring which will prevent flowers from setting fruit.

Why not try an experiment this season? Toss some tomato and pumpkin seeds into a corner of the garden and then compare with seeds started indoors. Which plants grow best? Do the plants which start later in the garden eventually catch up to those that start earlier inside or are they too vulnerable to the late spring frosts which it’s easy to protect seedlings inside from?

For seedlings in the garden, home-made cloches are easy to set up using plastic soft-drink bottles. Stay tuned for the next post to find out more – and check out some action photos.

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