Some seed types germinate best in warmer soil. This articles shares creative ways home gardeners make use of warmth around the house to help speed up and increase germination rates.
Also, see this printable list of the best soil temperatures for seed starting.
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What Seeds Like
Did you know every seed has its own preferred soil temperature for germination? Most will sprout under a fairly wide range of temperatures, but each has a sweet spot where the germination rate (percentage of seeds that sprout) is most successful.
Cool and Cold Crops
In general, our traditional spring and autumn crops including peas, and salad greens (see a longer list here that you can print out), that grow nicely in cold or cool conditions, are the same crops that like moderate soil temperatures for seed sowing. Very generally, this group germinates best with soil in the 50-68F | 10-20C temperature range.
The tender, warmth-loving crops like tomatoes, eggplant, and peppers (see a longer list here that you can print out) prefer warmer soil to sprout. Again, very generally, they will do fine if the soil temperature is 68-79F | 20 -26C.
Best Soil Temperatures for Sowing Seeds lists more of the vegetables and herbs that fall into to these two groups.
You can measure soil temperature with a kitchen meat thermometer. Take a few readings at different times of day, over a few days, to be sure of your results.
Tips for Warming Soil to Speed Up Seed Germination
Outdoors, soil temperatures can be warmed by adding a few inches of compost to your garden bed to keep heat in, using plastic mulch to warm the soil for days or weeks prior to sowing, or positioning cold frames in a sunny location. Let the sun work for you.
Indoors, gardeners come up with all sorts of tricks for speeding up seed germination. Because most seed types do not require direct light until the seeds have sprouted, you can place the seed tray elsewhere in your house where heat is available to warm the soil from below until the seeds sprout. You do have to be careful not to let the soil dry out, though. Seeds need both moisture and warmth to germinate.
You can measure the temperature of your soil or seed starting medium with a kitchen thermometer to ensure you are keeping it in the desired range.
Use a Seedling Heating Mat
If you want a product made just for this purpose, consider getting a proper seedling heating pad. Check the size of the mat you are buying. You may need a few to accommodate all of your seedling trays.
These ideas are hacks, meaning they are unconventional solutions. They are simply ideas, not advice, and anything you try is entirely at your own risk. If something seems unsafe, don’t do it.
Use what you have:
- Top of toaster oven
- Top of cooking range
- On a waterbed heater
- On a heating pad
- On string lights or fairy lights that radiate some heat-place them in a roasting pan with seed trays over top
- Over pots of hot water (cooling after cooking)
- In a chick incubator
- On top of the fridge
- Over a steam radiator
- Above a heat register
- Over a slow cooker or crock pot of warm water
No matter what you choose, the idea is to gently warm the soil from below, not above, and to do it in a safe manner.
In many of these examples, you are making use of leftover heat when the appliance is OFF (after use). Some place a baking sheet with a dish towel on top above the warm surface and place the seed starting tray on top.
Even with just a couple of hours of slow heat, some seeds respond quickly (within a day or two) to added warmth.
A device like a slow cooker partially filled with water can offer continuous heat for several hours. Seeds like peppers germinate much faster with warmth like this. You can see an example of this in in tip #1 here: speeding up seed germination with a slow cooker.
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Other Germination Tricks
- Always read your seed packets and follow the specific sowing instructions. If the seed company knows a good trick, it’s usually listed right there.
- Some seeds with tough coats like delphiniums benefit from presoaking in damp paper towels for a few days prior to sowing.
- Some seeds require scarification (scratching of the seed coat to allow the embryo to emerge) or stratification (varying the temperature), such as pre-chilling seeds in the fridge, or introduction to warmer soil, as we have discussed here. Again, these tips will be listed on your seed packet if applicable.
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- Check your soil temperature with a thermometer to be sure it’s in the optimum range for your seeds.
- In general, warmth-loving plants favour warmer soils, and cold-loving plants like it a bit cooler.
- If you are sowing seeds indoors and need warmer soil, look around for existing heat sources that you might be able to make use of (safely) for warming seed trays.
Happy growing, and be sure to get the list of best soil temperatures for seed sowing here.
~Melissa the Empress of Dirt ♛