Want your seeds to sprout faster? Have a look at the 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.
How to Help Seed Germination
Here’s the key things to know:
- Each seed type has a range of temperatures from low to high that encourage germination (along with moisture).
- Within that range there is a minimum temperature, a sweet spot, and a maximum temperature.
- Too cold or too hot and the seed either cannot sprout (cold) or may sprout and die (hot).
- So, for best results, we learn the sweet spot and provide it for best germination rates.
1Cool and Cold Crops
In general, our traditional spring and autumn crops including peas and salad greens, 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 50-68° F (10-20° C) 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 68-79° F | 20 -26° C.
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
Seeds need both moisture and warmth to germinate.
Warm up the soil before sowing your seeds. And warmth from below the seed tray is best, without allowing the potting mix to dry out.
OUTDOORS Soil temperatures can be warmed by adding a few inches of compost to your garden bed to keep heat in, using natural or 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. I’ve listed some ideas below.
Keep in mind that high temperatures will kill seeds or dry out seedlings, so stay in the desired temperature range. More is not more.
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.
Are These Seeds Still Good?
Seed Viability and Germination Test
- First, follow any recommend preparations for sowing such as scarification (roughing up seed coat) or stratification (e.g. pre-chilling).
- Place 10 seeds an inch apart on moist paper towel, coffee filter, or cloth. Fold over and insert in labelled plastic bag.
- Keep in warm, dark place. Most need a 65-75°F | 18-25°C temperature range. Do not allow paper towel to dry out.
- After expected germination period (number of days), check how many seeds sprouted (e.g. 8 out of 10 = 80% germination rate). You can sow any seeds that have germinated. If nothing sprouted after an extra week, the seeds are probably toast.
How to Keep Seedling Trays Warm
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.
A regular digital kitchen thermometer or meat thermometer works fine for measuring soil temperatures.
Household Hacks For Warming Seed 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.
The idea is to make use of heat produced in your home for other purposes and have the seedling tray nearby.
SAFETY FIRST: USE THESE IDEAS AT YOUR OWN RISK
Use what you have:
- Top of toaster oven (after use, with cooling rack on top: no direct contact with seedling tray)
- Top of cooking range (again, after use, no direct contact)
- 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 (some old fridges get warm up top)
- Over a steam radiator
- Above a heat register (beware of dry air-must be humid)
- 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.
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 pre-soaking 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.
Related: The Global Seed Vault in Norway | Learn how it protects seed species from around the world
- Check your soil temperature with a thermometer to be sure it’s in the optimum range for your seeds.
- In general, warmth-loving plants favor warmer soils, and cold-loving plants like it a bit cooler but the key is always to have soil temperature in mid-range for successful seed sowing.
- 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.
If you would like help with seed starting, see Seed Starting for Beginners for all my top tips.
~Melissa the Empress of Dirt ♛