Seed germination temperature is critical for getting your seeds off to a great start. That tiny seed is the miraculous beginning of a whole new plant that will give you food and beauty throughout its season. But the only way to be sure your plants reach their potential is by giving them the conditions they need.
To begin, a seed is in a dormant state and may seem like it’s not even alive. Guess what? That tiny seed is absorbing oxygen, giving off carbon dioxide, and preparing to grow as soon as mother nature says GO! One signal that a seed needs is to have the right seed germination temperature.
This article will help you start your seedlings off right. I’ve included convenient charts to help you identify the proper germination temps for your seedlings.
What Is Germination, Anyway?
First, let’s clarify what seed germination is. Germination is the emergence of the seed from dormancy. The seed itself must go through several steps to awaken.
The first step is absorbing water – a process called imbibition by botanists. Keep in mind that even a dead seed can absorb water, so this should not be used as a viability test.
Then, as the seed swells, the pressure of the water breaks open the seed coat. At the same time, the seed is preparing to use its stored food to make energy. This energy will produce a root that begins to search for food and establish the plant.
What Germination Requires
Your seed has all that it needs to sprout and make its way above ground, except for three important things. The right seed germination temperature, plenty of oxygen, and the right amount of moisture. As soon as it has those three things, it moves from dormancy to germination.
You can’t change the amount of oxygen in the air. However, you can provide a loose growing medium that allows air to move around the seed. You can also provide a fan to increase airflow in the grow room or greenhouse.
What you can control is the seed germination temperature and the humidity levels in the room.
In some cases, your seeds need stratification. This means they need a chilling period before they go into a warm soil bed. We do this by placing them in the refrigerator and making them think it is winter.
Temperature And Why It’s Important
So what is the right temperature? When we start our seeds indoors we try to provide the perfect growing conditions. After all, mother nature won’t always provide the perfect day for ten days straight.
We all know that some plants like cool weather and some like hot weather. Your lettuce grows well in the cool rainy days of spring, while your tomato plants won’t grow at all during those conditions.
Germination temperatures are different than growing temperatures. Your flats of seeds waiting to grow require higher temps than mature plants.
The seeds of plants are genetically programmed through a series of chemical and biological reactions to “know” when its time to start growing. This isn’t to say plants don’t get confused by the weather just as we do. A late warm spell this fall had one of my apple trees blossoming in October.
Temperatures that are too low or too high tells the seed to remain dormant. The seed can also recognize swings in temperature. Ions in the soil are giving the seed clues to when it should break free and start to grow.
Even as tiny seeds your plants have preferences. Seeds such as lettuce and peas germinate in cooler weather. Things such as peppers, tomatoes, and watermelons need warmer temps to germinate.
Temperatures for Common Plants
The best range for most vegetable plants is between 75-85°F and flowers should be around 70°F. Fruit varies wildly, and many fruit seeds require a period of cold stratification. It’s important to remember that is SOIL temperature, not air temperature. There are several ways to moderate your soil temperature which we will talk about later.
Optimum Seed Germination Temperature for Veggies
| Plant Variety | Temperature | Humidity | Days to Germination |
| Arugula | 65-75°F | 50-60% | 5 |
| Beans | 65-85°F | 50-60% | 7 |
| Beets | 60-85°F | 45-65% | 4 |
| Broccoli/Cabbage | 75-85°F | 50-60% | 4 |
| Carrots | 70-85°F | 45-65% | 6 |
| Corn | 75-95°F | 60-70% | 3 |
| Cucumber | 75-95°F | 60-70% | 3 |
| Eggplant | 80-90°F | 60-80% | 6 |
| Kale | 65-75°F | 50-60% | 4 |
| Lettuce | 60-70°F | 50-60% | 3 |
| Onion | 65-85°F | 50-60% | 6 |
| Peas | 60-80°F | 50-60% | 6 |
| Pumpkin | 70-90°F | 60-80% | 4 |
| Radish | 65-85°F | 50-60% | 4 |
| Spinach | 55-75°F | 50-60% | 5 |
| Squash | 80-95°F | 60-80% | 4 |
| Swiss Chard | 75-85°F | 50-60% | 4 |
| Tomato/peppers | 70-90°F | 60-80% | 6 |
| Watercress | 55°F | keep wet | 7-10 |
Optimum Seed Germination Temperature for Fruits
| Plant Variety | Temperature | Days to Germination |
| Apple | 60-75°F | 5 |
| Cherry | 65-85°F | 90+ |
| Honeydew | 70°F | 7 |
| Lemon | 65-75°F | 7-14 |
| Pear | 32-40°F | 90+ |
| Pomegranate | 75-80°F | 30-40 |
| Strawberry | 40-60°F | 14-21 |
| Watermelon | 75-90°F | 4 |
Optimum Seed Germination Temperature for Flowers
| Plant Variety | Temperature | Days to Germination |
| Aster | 65-75°F | 7-10 |
| Bachelor’s Button | 65-70°F | 7-10 |
| Coleus | 70-75°F | 10-14 |
| Cosmos | 70°F | 5-7 |
| Geranium | 70-75°F | 7-12 |
| Globe Amaranth | 70°F | 14 |
| Impatiens | 70-75°F | 10-14 |
| Lisianthus | 75°F | 10-14 |
| Nasturtium | 65-70°F | 10-14 |
| Petunia | 75°F | 7-10 |
| Zinnia | 70°F | 5-7 |
Planting Seeds
To start with, purchase a soil thermometer. You can use this in your indoor and outdoor planting areas. A good soil thermometer helps you monitor the temperature to make sure you’re in the right zone.
It’s a good idea to set up your growing area the day before you plant the seeds. Put your seedling starter soil in your containers and water well with warm water. Then, let that water drain out for a day.
In the meantime, put the trays on a grow mat or a warm space such as the top of the refrigerator. Use your handy dandy soil thermometer to check the soil and make sure it is falling in the optimum range.
After everything has stabilized, plant your seeds. Continue to monitor the temperature of the soil and make sure it stays moist.
What Happens in Less-Than-Perfect Conditions?
You may not have a lot of equipment for germinating seeds or the perfect location. Will your seeds still germinate?
Seeds have a wide range of temperatures that they’ll germinate in, so don’t fuss if you don’t hit it exactly. Just remember this: seeds take longer to germinate or may have poorer germination rates if they don’t have optimum conditions.
Seeds have a lot going on before we even see any evidence of a plant! Check out this video and focus on everything that goes on in the first seven days.
How To Provide The Right Soil Temperature
Putting a heater and lights in a small room can transform it into a grow room. You can heat the air to a nice 75°F and have full-spectrum bulbs ready for your emerging seedlings. But this may not entice germination.
Germination needs soil heat, and heat coming from the bottom is the best way to warm up your soil mix. You can accomplish this by putting heating mats under your seed trays or pots.
The soil temperature is more important than the air temperature. If your air temp goes down to 50°F during the night but the soil temp remains at 70°F, then your plants will still be happy.
Seedling mats come in a variety of sizes. The nicer ones have a controller to keep your seedlings at the optimum temperature.
They are typically a foot wide and can be between 18-inches and six-feet long. They are made to be able to fit your flats on them lengthwise. It’s best to put them on metal shelves or tables. You don’t want them to get too hot on wood furniture.
DIY Options
Seed heat mats can get pricey – especially the ones with built-in thermostats that can be preprogrammed to meet a certain temperature. Don’t worry if you are on a budget, there are some clever DIY solutions.
- Build a lightbox to keep seedlings warm
- Use a heating pad made for people set on low (use caution when watering as they are not made to get wet)
- Rope lighting rated for outdoor holiday lights are perfect for warming the bottom of trays
Other Factors
Humidity and Germination
Humidity is also very important. When you sow your seeds, you undoubtedly add warm water to your seedling mix. But making sure that moisture sticks around is important to the developing seed.
Most seeds need moisture but don’t want to be wet. If you have central forced air heat or a wood stove, you may not have enough air moisture in your home. Plants suffer when the humidity falls below 30%.
Misting your seedbeds versus using a watering can help provide moisture to the soil and the air. If you heat with a wood stove, place a tea kettle of water on the top of the stove. The wood heat causes the water to evaporate into the air. This is good for humans as well.
You can also use a humidifier to add moisture to the air. In addition, you can cover your trays with a plastic or glass top, or just wrap them in saran wrap.
Air Circulation
The flip side of adding humidity is also making sure you have good air circulation. A fan can help keep the moisture moving and keep unwanted mold spores or funguses from developing.
Be careful that you don’t allow moisture to build up. If you see the plastic has condensed droplets take it off and let the tray air. Otherwise, you may cause mildew or mold problems.
Take Away
Growing your own plants from seed is fun and fulfills that winter need to garden. When starting from seed you give yourself so many more options than just buying seedlings at the local megastore. In addition, it is much cheaper to grow your own.
