Want your seeds to sprout faster? Have a look at the creative ways home gardeners make use of household items like heating pads and Instant Pots to help speed up sprouting and increase germination rates.
Also, see this printable list of the best soil temperatures for seed starting.
How to Help Seed Germination
Provide Optimum Conditions
While you can’t fool nature into moving any faster than it naturally capable of, providing the right conditions gives seeds their best chance.
Seeds will sprout the fastest when you start with fresh, viable seeds and provide the optimum conditions for each specific species or variety. Yes, it varies.
- Each seed type has a range of preferred temperatures that, along with moisture, encourage germination.
- 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) or just take a very long time.
- For best results, we provide soil temperatures within that sweet spot.
How can I measure soil temperature at home?
You can measure the temperature of your soil or potting mix using a basic digital kitchen thermometer. It’s ideal to choose a thermometer with a long probe so you can check the temperature at various depths.
For outdoor seed starting, it’s helpful to measure the soil temperature at sowing depth and repeat this several times a day over a period of several days. Assuming it’s the right time of year, when the temperature is consistently within the desired range, it’s sowing time.
Related: Seed Starting for Beginners | Sow Indoors Grow Outdoors
Two Types of Seeds
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—with exceptions, this group germinates best with soil in the 50-68° F (10-20° C) temperature range.
2Warm Crops
The tender, warmth-loving crops like tomatoes, eggplant, and peppers prefer warmer soil to sprout.
Again, very generally—with exceptions, they will do fine if the soil temperature is 68-79° F | 20 -26° C.
Some in this group, like peppers and squashes, can be particularly responsive to somewhat warmer temperatures.
Seed Sowing Temperature Charts
Tips for Warming Soil to Speed Up Seed Germination
Seeds need both moisture and warmth to germinate—but not too much.
Warm up the soil before sowing your seeds.
Outdoor Seed Sowing
Start at the right time of year with a location suited to the plant’s light, soil, and drainage needs.
From there, 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.
Indoor Seed Sowing
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 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 and keep on top of watering. More is not more.
If you want to test seeds prior to sowing in soil, use this seed viability test.
Use a Seedling Heating Mat
If you want a product made just for this purpose, consider getting a proper seed starting heat mat with a thermostat. This explains when a heat mat can help if you don’ t have other options (see below). The usefulness will depend on your indoor conditions.
Check the size of the mat you are buying as well as product reviews and refund policy. You may need a few to accommodate all of your seedling trays and they are a bit pricey.
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 residual heat produced in your home.
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 (this is only true for older appliances that radiate heat 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
- In an Instant Pot—see instructions below.
A food dehydrator is not recommended because, by design, it’s likely to dry out the seeds which will defeat the purpose. Seeds need to retain their inner moisture to remain viable.
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.
The modern version of slow cooker seed sprouting uses an Instant Pot on the lowest (yogurt) setting.
How to Germinate Seeds in Instant Pot
After using a slow cooker for seed starting for years, I tried the same process in my Instant Pot (with built-in air fryer) as soon as I got it. I think I started seeds before I cooked anything!
While I wouldn’t buy one for this purpose, I have to say it has been an excellent addition to our kitchen—I use it nearly every day for food, and now and again for seed starting. It’s particularly good for stubborn pepper seeds.
How It Works
Running on the lowest setting, the Instant Pot gently warms moistened seeds to encourage germination.
Choose seeds that both prefer warmer soil temperatures to germinate and sprout within a few days, otherwise the use of electricity probably outweighs the benefits.
Recommended seeds to try: peppers, zinnia, squash, rosemary, and eggplant.
Make sure you have supplies ready to transplant the newly sprouted seeds. Some are ready in just a day or so.
Steps
- Fill Instant Pot inner pot with 1 to 2 cups water and add a metal trivet (2 to 3 inches tall).
- Place seeds between layers of moist (but not dripping wet) paper towel or fabric scraps and place in food containers, Ziplock bags, or sheets of foil. Lids are optional.
- Label each type of seed.
- Place containers (food bags, or foil) on trivet. It’s fine to experiment with stacking them and/or using lids.
- Turn on Instant Pot to lowest temperature setting (91°F / 32°C on many machines). Note the start time. Some machines will automatically shut off after a set time and need restarting. Put a reminder in your phone if needed.
- Check daily for sprouted seeds and to monitor temperature. Add or remove lids to adjust temperature as needed.
- Remove and transplant any germinated seeds into potting mix. Note how long germination takes for each seed type for future reference.
Notes
- The number one tip is to never let the seeds dry out or the water in the pot run dry. You also don’t want to create mold so some air flow is recommended.
- I find the best temperature range (measured on the moist paper towel) is 80° to 90°F (26° to 32°C).
- Whether you need lids on the seed containers and/or the Instant Pot will depend on your temperature readings. Add or remove lids as needed to maintain the recommended range.
- Look up the “days to germination” for each of the seeds you are starting to know when to expect sprouting. If your seeds are fresh and viable, they may germinate a bit sooner than expected.
Resources
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 cold climate seeds with tough coats like delphiniums benefit from cold and 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 | Protecting seeds from around the globe.
Ebook
Seed Starting for Beginners
Sow Inside Grow Outside
by Melissa J. Will
NEW EDITION | Everything you need to get started with indoor seed starting for indoor and outdoor plants. Grow what you want—any time of year!
About This Ebook | Visit Ebook Shop
This ebook is a digital file (PDF format) you save to your device. It is not a physical product.
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Summary
- 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.
- If you are sowing seeds indoors and need warmer soil, look around for safe, existing heat sources to warm newly sown seeds.
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 ♛
Melissa J. Will
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