The anticipation and excitement that come from sowing seeds for a favorite flower or vegetable can lead to disappointment weeks later when many, if not all, of those seeds have failed to germinate. While many factors affect germination — from the age and quality of the seed to the depth of planting — the most challenging for gardeners and often least understood is soil temperature. Learning the best soil temperature for seed germination will go a long way towards your seed-starting success.
Warmer temperatures speed up chemical reactions and, conversely, cooler temperatures slow them down. Those chemical reactions help break down the protective seed coat and tell the seed that it’s time to wake up and start growing. For a cool-season leafy crop like spinach, the ideal temperature for germination can be as low as 50 degrees Fahrenheit. For heat-loving tomatillos, the minimum desired soil temperature is 80 degrees.
While there is can be variation from one seed type to the next, most seeds for warm-season edibles prefer soil temperature between 70 and 80 degrees Fahrenheit (21.1 and 26.7 Celsius). When the soil temperature in your garden is too cool — or too hot — seeds may take longer than expected to germinate or will never germinate at all. Whether direct sowing seeds in the garden or starting seeds indoors, achieving the optimal soil temperature before planting will greatly increase the germination rate and also result in more vigorous plants.
Want to know what the ideal soil temperature range for optimal germination for the most commonly grown edibles? Download our free pdf that puts that information right at your fingertips for ready reference.
Indoors, seeds often require warmer temperatures than the average house is heated to in spring. Outdoors, even after your last frost date of spring, the soil will continue to remain cooler than the air temperature until it has had weeks to heat up. But before using the tricks to raise soil temperature, its important to know where you are starting from.
Checking Soil Temperature
All it takes to find out the temperature beneath the surface of your garden is an inexpensive soil thermometer. A soil thermometer typically has a 6-inch probe, but 6 inches is deeper than necessary for seed starting. Stick the thermometer between 1 and 2 inches into the soil and wait close to 30 seconds to get the most relevant and accurate temperature reading.
Take a reading in both the daytime and the evening, and calculate the average. If you have multiple plantings areas — especially if some get more hours of direct sunlight than others — don’t assume that they will all be the same temperature. Record the temperature of each.
Raising Soil Temperature Outdoors
A south-facing garden will heat up faster than others, and a raised bed garden or container garden is typically warmer than ground level. Wherever your garden is sited, if you find that your soil temperature isn’t where it needs to be, you can heat it up with plastic sheet mulch. Clear or black plastic mulch will take advantage of solar energy to raise the temperature of soil by several degrees and will hold that heat overnight.
To increase both the soil and air temperature, use a cloche or a cold frame. Both trap heat like mini-greenhouses and are excellent for extending the garden season. They may be made out of glass or plastic. The lids of easy-to-build homemade cold frames are often made from up-cycled window sashes.
Soil covered in mulch (wood chips, bark, shredded leaves, straw, etc.) will take longer to heat up in the spring, so rake away and remove the mulch after the last hard frost. (You can always reapply mulch after your seeds have sprouted or your transplants are in the ground.)
Raising Soil Temperature Indoors
Indoors, you may be starting your seeds in a soilless potting mix, but the same principles apply: For the most successful germination, get the growing medium into the ideal range for the seed.
A bag of soilless mix that got cold sitting outdoors or in an unheated garage isn’t an issue, because that is quickly resolved by adding warm water into the mix when preparing your cells or soil blocks. Still, room temperature will not be warm enough for many seeds to perform their best and achieve a high germination rate.
Also known as germination mats, seed-starting heat mats placed under seed trays will raise the temperature of the soil by approximately 10 degrees. So in a house kept at 65 degrees, the seed trays will get up to about 75 degrees — the ideal temperature in most cases.
The mats are thin, waterproof and come in many sizes. The most sophisticated seedling heat mats include thermostats to reach a precise temperature. These are helpful in a cool basement where an extra 10 degrees won’t be enough.
Online Gardening Academy™ Seed Temperature Chart
To learn the soil temperature range for optimal germination for common vegetable seeds, click to download the Online Gardening Academy™ Seed Temperature Chart.
Links & Resources
Episode 091: Starting Seeds Indoors: The Non-Negotiables for Success, Pt. 1
Episode 094: How to Start and Care for Seedlings Indoors: My Steps for Success
Episode 110: Why Mulch Matters in Every Garden: What You Need to Know
joegardener Online Gardening Academy™ Three popular online courses on gardening fundamentals; managing pests, diseases & weeds; and seed starting!
joegardener Online Gardening Academy: Master Seed Starting
*Disclosure: Some product links in this guide are affiliate links, which means we would get a commission if you purchase. However, none of the prices of these resources have been increased to compensate us. None of the items included in this list have any bearing on any compensation being an influencing factor on their inclusion here. The selection of all items featured in this post and podcast were based solely on merit and in no way influenced by any affiliate or financial incentive, or contractual relationship. At the time of this writing, Joe Lamp’l has professional relationships with the following companies who may have products included in this post and podcast: Rain Bird, Corona Tools, Milorganite, Soil3, Park Seed, and Exmark. These companies are either Brand Partners of joegardener.com and/or advertise on our website. However, we receive no additional compensation from the sales or promotion of their product through this guide. The inclusion of any products mentioned within this post is entirely independent and exclusive of any relationship.