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When To Use A Heatmat For Germination Of Plants

When To Use A Heatmat For Germination Of Plants

Raymond Meijer |

I would like to use a heat mat for germinating my seeds. How does this work and what are the benefits? In this blog, we will look together at the effect of heat but also at the principle behind the natural process of germinating seeds.

How do seeds germinate?

Each seed needs specific conditions to germinate. Some seeds germinate in spring, and other seeds only when it gets really warm. When you start sowing seeds yourself, it is important to know which seeds require which conditions in order to have as many seeds germinate successfully as possible.

Based on the moment of germination, we can divide the seeds into different groups. When you want to use a heat mat it is important to know this. In this blog, we will mainly focus on the cold germinators, and the warm germinators.

In nature, seeds fall to the ground at the end of the summer when the flowers have finished flowering. The seeds are at rest because winter is coming and they have to wait for the right moment to germinate because in the most favourable conditions they have the greatest chance of survival. Temperatures drop in autumn, and in winter it even freezes for a few days. The seeds cool down to below freezing point. At this time, no seed will germinate. After the winter, however, the earth begins to warm up, the seeds feel the warmth of the sun, and often the light as well (another very important element). This is a sign that winter is coming to an end. For the seeds, this is a sign that favourable conditions are coming.

Fun fact; did you know seeds have a sense of gravity and use this to sprout towards the top?

It is time to germinate!

We are going to sow these seeds ourselves. The seed growers harvest the seeds at the end of the summer and by spring we buy these seeds in sealed bags in the shops. Full of enthusiasm, we sow these seeds on the windowsill and wait patiently. Nothing happens. We wait a little longer, perhaps we are too impatient. Maybe they need a little more water. We give more water and things start to go mouldy. No success with these seeds unfortunately, it must have been bad seeds. Disappointed, we move on to other seeds, or buy new ones. If seeds don’t germinate we probably did something wrong.

Had we known when to sow these seeds and what conditions they needed to germinate, our work and waiting would not have been in vain. Besides, we are often disappointed and demotivated when something doesn’t work out. A great pity when you know it’s so easy to solve!

So how can we sow these seeds?

All we have to do is imitate the natural process. When sowing delphiniums, for example, we put some of the seeds we are going to sow in a bag in the freezer for a week or two. Then we take the seeds out and let them germinate at room temperature in seed trays.

You can also let nature do its work and sow the seeds in February, for example, under glass or in a plastic grow tent. The seeds will germinate on warmer days due to temperature fluctuations. However, you must be patient and make sure the seeds do not get too wet and go mouldy. Sowing under glass, in a grow tent or in a propagator ensures that the seeds are not washed away or become mouldy when it rains a lot. The sun will also ensure that it warms up a little faster. We have now discussed seeds that germinate after it has been cold. What about seeds that need heat?

So why would you use a Heatmat?

The packets of seeds give an indication of how long it takes for them to germinate. Beet seeds, for example, germinate at 7 degrees Celsius. Germination takes between 4 and 10 days. Quite a difference! In cold conditions, starting at 7 degrees Celsius, you will have to wait until about day 10 to see seedlings emerge. When we germinate the seeds with the help of some warmth, for example in the greenhouse with a heat mat, you will see seedlings emerge after just 4 days! Almost a week faster than if you let them germinate at lower temperatures. In addition, the germination of seeds will also be lower, and the chance of the seeds going mouldy in the seed trays will also be lower. If we now look at plants that really need heat, such as tomatoes, peppers or certain flower seeds such as zinnias, we will see that you need to add heat to have any germination at all.

Imagine, we are going to sow tomatoes in the greenhouse in March or April. We need to start pre-sowing early because the plants will need a long time. Our climate in the Netherlands means that we have a relatively short summer, so we have to pre-sow them. We sow the seeds in the greenhouse or in a small container with a lid. It is still very cold outside in February or March and no tomato seed will think that conditions are favourable for germination. If we now use a heating mat, the temperature of the sowing and cutting soil will rise to around 20 degrees. In addition, the warmth of the mat will also protect them from low temperatures at night. So besides the fact that by using a heat mat you can speed up the germination time of certain seeds considerably and the percentage of germinated seeds will be higher, it can also be essential for some seeds. Plants that are not naturally suited to our relatively cold climate with short summers can be germinated and grown in a greenhouse, in a small grow box or in a plastic grow tent with the help of some extra heat.


When germinating seeds it’s good to know the right tempratures for germination. Many seeds will germinate best around 20 degrees celcius. For a full list of the lowest, average and best germination temperatures check out our blog on growing temperatures. Using a heat mat can greatly improve the gemination speed and also help with germinating seeds that need warmth to start out. It’s also great to create a warm environment for plants that are not native to your area and are usually grown in hot climates. We will continue to use heat mats ourselves to speed up the process and extend our growing season.

Our favorite Heatmat

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