Soil Temperature and Seed Germination

Paul Alfrey - Balkan Ecology Project
Thursday, 2nd March 2017

Learn how to get the right soil temperatures for the best seed germination.

A few days ago we sowed the tomato seeds for this season's market and home garden. It never ceases to amaze me what little indoor space you need to rear thousands of seedlings. We use two 50 cm x 30 cm x 15 cm trays to germinate approx 150 seedlings from 10 cultivars. When they get bigger we move them into two 1.3 x 8 m beds covered with polythene to rear them before they take their permanent positions in the gardens in early - mid April.

Many of the plants we grow I prefer to sow directly outside and one of the most important things to consider when sowing is that the temperature of the soil is high enough for the seed to germinate.

Other important considerations include:

  • Whether the seed requires any pre-treatment before it will germinate, i.e stratification and scarification (mainly relevant for perennial plants particularly trees and shrubs).
  • How deep you sow the seed - too shallow is better than too deep. 
  • That the correct moisture levels are kept constant during the germination phase - not too wet, not too dry and with the ideal moisture levels similar to that of a wrung out cloth.
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Elaeagnus commutata -  Epigeal germination
We will focus on soil temperature for germinating seeds, why this is important, how to take soil temperature, and I've included a table showing the minimum and preferred soil temperatures for germination of some common plants.

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Eruca sativa - Rocket germinating

Often you will find a monthly guide on a seed pack indicating when to sow seeds and this generally works okay, but can be misleading. If you have a long cold winter and the soil is cold, germination will be delayed and in some cases the seeds may rot in the ground.  On the other hand, if the soil is unusually warm in the spring, it's possible to seed earlier. Being able to tell the soil temperature and being aware of the preferences of each plant will result in more or your seeds germinating.

Measuring Soil Temperature 

You want to measure the temperature at seeding depth and this will differ for each seed you sow. The general rule is sow to a depth of no more than twice the diameter of the seed, but like I said above it's better to go too shallow than too deep.

Any thermometer that will measure temperature at a specific depth can be used to measure soil temperature. Insert the thermometer into the area where the seeds will be sown and wait a few minutes before you take a reading.

Bear in mind that each area of your garden will probably have a different temperature. The soil temperature is influenced by the following factors:

  • Bare soil warms much faster than mulched soil and vegetated soil.
  • Dry soil will be warmer than wet soils.
  • South facing soils will be warmer than north facing, and the amount of shade cast on the soil will affect the temperature considerably.

Warming up the Soil

As the air temperature starts to warm up in early spring you may like to get a head start with your sowing and accelerate the warming of the soil. If you have a mulch on your soil for the winter you can temporarily remove the mulch. The dark coloured soil will absorb all wavelengths of light and convert them into heat, warming the soil much faster. Another alternative is to leave the mulch on and cover the bed with a plastic sheet or glass pane. On a sunny day this will provide considerable heat. Of course you can also remove the mulch and use the sheet or glass on the bare soil and this has the added benefit of germinating any seeds in the patch that can be pulled before you start sowing.

Here's a table providing the minimum and preferred soil temperature for a number of crop seeds and the estimated time it takes the seeds to germinate.

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