Last Updated: April 7, 2023
Another growing season is soon to begin! Sowing tomato seeds is incredibly exciting, and watching them sprout is even better. But before you go planting seeds willy nilly, there are a few important things to know.
In this article, I will explain how to germinate tomato seeds properly. I’ll go through what a seed needs to sprout, when to plant seeds indoors, planting steps, and what to do after germination.
What Triggers Tomato Seed Germination?
Let’s start with a bit of basic understanding around what initiates germination in tomato seeds. In order for a dormant tomato seed to begin sprouting, it will need 3 basic environmental conditions.
The first and most important factor that a seed requires is water. Without enough moisture, a tomato see will not sprout successfully. For this reason, you must take steps to ensure your tomato seeds do not dry out while they are germinating.
Next up, the tomato seeds require warmth to germinate in a timely manner. While the seeds will sprout within a wide range of temperatures, the ideal temp should be between 75-85°F (24-29°C). This will lead to the quickest and most reliable germination rates.
Lastly, your seeds will need oxygen. This is least important because thankfully, oxygen is fairly common. Planting the seeds at the correct depth ensures the seeds are not suffocated, and fanning out seed trays daily helps as well.
There are a few other factors like soil medium and lighting, but these are much less fussy. Tomato seeds will germinate with normal indoor ambient light and in unconventional mediums (like a paper towel).
However, for the best germination rates, I’ll share my recommended supplies.
I’ll break this down into two basic supplies lists: Essential and non-essential supplies. If you want to keep it simple and are growing just a few plants, there is no need to go overboard buying expensive products. But if you grow dozens of tomatoes each year, it can save time to have a better seed sprouting setup.
Essential Seed Starting Supplies
- Seeds. Of course! Pick your tomato varieties wisely. I like to grow indeterminate varieties mostly, but determinates can be good for container plants or smaller growing spaces. See my recommended places to buy tomato seeds online here.
- Seedling trays. I always recommend starting seeds in small pots and transplanting the plants once they outgrow them. This helps avoid soil moisture issues and promotes faster growth.
- Soil medium. I use seed starting mix to plant my tomato seeds. These soil mixtures are light and fluffy, typically made up of peat moss (or coconut coir), vermiculite, and perlite. They are finely textured to encourage strong early root development.
- Spray bottle. I like to use a simple, cheap spray bottle to ensure the seed starter mix remains moist while the seeds are germinating. Again, this can be made using normal household items, but one can be bought for a few dollars.
Non-Essential Seed Starting Supplies
- Seedling heat mat. While these certainly increased my germination rates, it is less important for tomatoes. I mostly use this for starting pepper seeds which can be more stubborn to germinate.
- Humidity dome. This helps keep moisture from evaporating too quickly from the seed starting mix. Most seedling tray kits will come with an included humidity dome, however you can definitely use alternate items. Leftover plastic to-go food containers work well as makeshift seed starting humidity domes.
- Thermometer. I use a cheap thermometer/hygrometer to keep an eye on the temperature. This is the most important factor for a speedy germination.
- Gloves. Some gardeners don’t like to get soil under their finger nails, others don’t mind so much. It’s up to you!
With all of the required supplies on hand, you’ll be ready to plant your tomatoes. All that is left to do is determine exactly when you should be sowing seeds indoors (in your specific climate).
When to Plant Tomato Seeds Indoors
It is important to get the timing right when planting tomatoes. If you plant too early, the plants will outgrow their indoor containers before the weather is warm enough outside. Plant too late, and you may end up with smaller yields.
In short, tomato seeds should be planted indoors 4-6 weeks prior to the last risk of frost. This allows enough time for the plants to grow a strong root system indoors, and increases the growing season in cooler climates.
To determine your specific last date of frost, try a simple Google search. Or, use the free Almanac tool if you live in the US.
How To Germinate Tomato Seeds (Steps)
With your planting date set and supplies in hand, all that is left is to sow your tomato seeds! This is our particular method, though there are many other options out there for you to try.
I get near 100% germination rates with this method, but I always encourage you to try new methods each year and see what works best for you. Some indoor climates may see better results with different technique. Let’s get started planting tomato seeds!
Planting Tomato Seeds
- Gather supplies.
You’ll need your seeds, seedling trays, soil medium, and water at a minimum. If you want to keep a consistently warm temperature, a seed heating mat works wonders.
- Pre-moisten soil.
Pour enough of the seed starting mix into a mixing container – I use a large planting pot to mix the soil. Add a bit of water to the soil and mix thoroughly. Be sure to reach down to the bottom to moisten the soil evenly. The soil is at the right moisture level when it sticks together when squeezed, but does not drip water.
- Fill planting containers with soil.
Add the pre-moistened soil to your growing containers, pressing down gently until you feel some resistance. Fill to within 1/4″ of the surface of the containers and level the surface of the soil. I like to use seed cell trays for planting to save space. After a few weeks of growth, I’ll up-pot the plants into larger containers.
- Label seedling cells with tomato variety.
Label your seed cells with the plant variety before sowing the seeds – this helps avoid any confusion about which is which later on!
- Plant seeds 1/8-1/4″ deep.
Place 1-2 seeds in each container and press them about 1/8-1/4″ deep. Cover them gently with soil. Don’t plant too deeply as this can make it difficult for the seedling to emerge from the mix.
- Mist soil surface with water.
Spritz the surface of the soil where the seeds were planted to ensure they are thoroughly moist. Remember, water is one of the essentials to initiate tomato seed germination!
- Cover with humidity dome.
With the seeds planted, cover the trays with a humidity dome to keep in moisture. This essentially creates a miniature greenhouse for your seeds.
- Place in warm, dimly lit location.
Tomato seeds germinate best in very warm temperatures, between 70-80°F (21-25°C). Find a spot in your home that is warmest, like above the refrigerator or near your computer. I use a seed heating mat to maintain high temperatures, but this is optional. Light is not required until the seeds actually sprout, so don’t worry too much about lighting!
- Fan and mist daily.
While the seeds are germinating, lift the humidity dome daily to allow some fresh air in. Mist the surface of each seed cell to prevent the seeds from drying out – this is critical!
- Once sprouted, move seedlings to bright location.
After a week or so, you should see the beginnings of your new tomato plants. As soon as they sprout, you should remove the humidity dome completely and move the plants to a sunny or very bright location. I recommend grow lights for best results, but your plants will do just fine in a South-facing window (for Northern hemisphere folks).
Tomato seeds usually germinate pretty quickly, but if you are having trouble, remember to keep it warm and moist!
I monitor the temperature of my growing space with an affordable thermometer/hygrometer like this on Amazon.
What To Do After Tomato Seeds Sprout
Once the tomato seeds sprout, you should immediately move the seedlings to a bright location. Like I said, I use a grow light to keep all of my plants growing strong, but it is definitely not necessary for tomatoes.
If you plan to grow them in a sunny window, be sure to choose the brightest window you have. In the Northern hemisphere, this would be a south-facing window with as few obstructions as possible. Look for trees or buildings outdoors that may block the sun’s light.
Also, rotate the plants daily to prevent the plants from bending and reaching towards the sunlight. If you leave them facing the same direction, the stems will end up curving towards the light.
Fertilizing Tomato Seedlings
After 1-2 weeks of growth, I give a light application of fertilizer to my tomato seedlings. A 1/2 strength application is enough to give the young plants what they need to grow fast and strong before transplanting into potting mix.
After about 3 weeks of growth in seedlings cells, tomato plants should be outgrowing the small containers. At this point, I will transplant the tomatoes into larger pots and transition to potting mix with nutrients built-in.
The plants should be strong enough now to receive full-strength fertilizer as well, so I encourage you to use a high-nitrogen fertilizer like this one for healthy growth. This will prepare the plants well for the transition to the outdoors.
Learn more about transplanting tomatoes here.
Do Tomatoes Need Light To Sprout?
Many people wonder whether light is requires when planting seeds. This can vary depending on what you are growing, but for most nightshade plants (like tomatoes and peppers), it is easy.
Simply put, tomato seeds do not require any light to begin germination. However, once the seeds sprout, they should be moved to a bright location right away for strong growth.
I hope this article helped you with germinating your tomato seeds. Planting tomato seeds should be an enjoyable experience!
Every spring I look forward to my planting date, and I wish you the best of luck with your plants this year. Let me know if you have any questions below 🍅.