Why Aren’t My Tomato Seeds Germinating? 5 Causes and Fixes
If you find yourself staring at a tray of tomato seeds wondering if the seedlings will ever appear, you’re not alone. Farmer Briana Yablonski will cover common reasons your tomato seeds aren’t germinating.
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Growing tomatoes is a rewarding pursuit that provides many benefits. It enables you to cultivate rare varieties that are difficult to find as seedlings, helps you save money, and gives you the satisfaction of watching plants grow from seed to ripe tomatoes.
However, you’ll only enjoy these benefits if your tomatoes germinate. Tomato seeds should sprout within two weeks, so something is wrong if you don’t see your seedlings emerge within this timeframe. Poor seed quality, an unsuitable environment, and incorrect planting practices could all be to blame.
I’ll cover five reasons why your tomato seeds aren’t germinating and provide information that will help you have success the next time you try to grow tomatoes from seed.
Too Cold
Tomatoes germinate best when the soil remains between 65 and 85°F (18-29°C), with the optimum end of the range being ideal. When the soil is adequately moist and kept within this range, you should see seedlings emerge within two weeks.
The seeds can sprout at lower temperatures, but their germination will be slower and more sporadic. Temperatures below 50°F (10°C) and above 90°F (32°C) will likely prevent germination from occurring.
If you’re starting this crop indoors, you may think that your home is warm enough. However, air temperatures below 65°F can stall germination. So, if you’re starting in a basement, garage, or other cool area, the ambient air temperature probably isn’t warm enough for 100% germination.
Increase the Soil Temperature with a Heat Mat
Fortunately, you don’t have to crank up your heat to keep your tomato seeds happy. Heat mats are a reliable way to increase soil temperature. These waterproof mats heat up to temperatures between 70 and 85°F (21-29°C) and warm the seedling trays to similar temperatures. Some mats have adjustable thermostats, but many remain at a suitable, stable temperature.
Using a heat mat is easy. Simply sow in whatever tray or container you like, water, and place the tray on top of the heat mat. Adding a humidity dome on top of your tray helps trap heat and moisture, but it’s not necessary.
Since these mats will warm the tray to the ideal temperature for germination, you should see seedlings emerge within a week. Remove the humidity dome, but keep the seedlings on the heat mat until all the seedlings germinate.
Other Ways to Increase Soil Temperature
If you don’t want to invest in a heat mat, there are other ways to increase the temperature. First, try placing your seedlings in the warmest room of your home. If that area is still below 65°F (18°C), you can utilize other tricks.
The top of refrigerators is often warm, so placing your seed trays on top can help speed up germination. Just make sure to check the trays regularly and move them to a bright location once they germinate. Other places that are often warmer than the rest of the house include water heat closets and areas near south-facing windows.
Too Dry
All seeds, including tomatoes, require moisture to break dormancy and germinate. That’s why seeds in a dry seed packet or bag never sprout. If you allow your tomato seeds to dry out during germination, the seedlings may never appear.
The solution to this problem is simple: keep the soil moist! However, completing this task is often more complicated than it sounds. Since your tomato seeds are just under the surface, focus on keeping the top half-inch moist.
Start by choosing a seed-starting mix that holds moisture while allowing excess water to escape. A mix that includes peat moss or coco coir is often a good option.
After you plant, gently water the soil. You can either top water them with a watering can or hose, or bottom water them by placing the seedling trays in a solid tray filled with about half an inch of water.
Keep the Soil Moist
After the top of the soil is wet, there are a few tricks to keep it moist. First, place a humidity dome over the tray to trap moisture. You can also use plastic wrap if you remove it as soon as the first seed germinates.
Adding a fine layer of vermiculite to the top of the tray also helps trap moisture and prevents algae growth. Simply sprinkle a light dusting of the material on top when you’re finished sowing.
Even if you implement one or both of these tricks, check the moisture twice a day. If the top of the soil feels dry, add more water. Since life can get busy and our seeds aren’t always on the top of our minds, don’t be afraid to set timers to remind you to check the moisture.
Too Wet
Tomato seeds require moisture to germinate, but wet soil can prevent germination. Saturated medium can cause seeds to rot before they have a chance to sprout. Even if the seedlings germinate, excessive moisture can kill them, a phenomenon known as damping off.
You can take a few steps to ensure your mix isn’t too wet. First, choose a well-draining mix designed for seed starting. Never use native soil, straight compost, or another material that holds onto excess water.
Second, ensure the seedlings’ container has drainage holes. If you plant in a yogurt container or other pot with a solid bottom, the medium will remain wet.
Finally, make sure to water only when it needs it. It’s easy to water on a set schedule, but this isn’t the best practice. Instead, look at and touch the top of the soil at least twice a day, and water only when it’s dry.
Planted Too Deep
Since tomato seeds are small, you should plant them about an eighth of an inch under the soil surface. Planting an inch underground won’t always prevent germination, but it will inhibit the seedlings from breaking through the soil surface.
After the seed coat softens, the seed completes the necessary steps to prepare to germinate. If all goes well, it will eventually send out a small root (the radicle), followed by a small stem and leaves (the epicotyl). The radicle grows down to anchor the plant while the epicotyl begins reaching toward the surface.
The seed’s endosperm contains a small energy reserve to sustain it throughout this process. Once it uses up the food in the endosperm, it relies on light for energy. If the sprouts don’t reach the surface in time, they’ll die. Tomatoes have a particularly small endosperm, which is one reason it’s crucial to plant them near the surface.
Planting too deep also increases the odds that the seed will rot before they have a chance to germinate. Potting mix often remains moist an inch lower than it does near the surface. This constant exposure to moisture can lead to rot and prevent you from ever seeing tomato sprouts emerge.
Seeds Aren’t Viable
If you believe you did everything correctly and your tomato seeds aren’t germinating, the problem may be seed viability. All companies that sell tomato seeds must guarantee a germination rate of at least 75%. If you collected and saved seeds, you can perform a germination test to check seed viability and germination rate.
Tomato seeds remain viable for four years, but their germination rate drops each year. If you’re planting ones that are three years old, overplant to compensate for lower germination rates.
Factors Affecting Seed Viability
Although time naturally reduces seed viability, several factors can accelerate this decline. Temperature and moisture are the primary elements to pay attention to.
You should always store tomato seeds in a cool and dry place. An area that’s between 35-40°F is ideal, so keeping them in a refrigerator is a good option. However, you can also keep them in a cool pantry or garage. Never leave them in a sunroom, a shed that heats up during the summer, or another hot location.
Moisture can damage seeds, so I recommend placing them in a sealed plastic bag or container to keep them dry. This way, you won’t have to worry about water spills or other accidents.
