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Tomato Germination And You: How It Works

Truly, the tomato is a favorite food in vegetable gardening, especially when it comes to growing tomato plants from seed. Seeds for popular varieties can be found at any major seed distributor. Learning about tomato germination and planting is important since gardening with tomatoes is so rewarding!

Gardening friends get together to discuss tomato plant cultivation just as spring arrives. They cultivate tomatoes easily indoors in grow tents, greenhouses, or hydroponic systems with proper amounts of light and the right temperature. Starting tomato seeds will give you ample experience in growing food. And the flavor difference between a grocery store tomato and one grown in the garden cannot be denied. 

Growing tomato plants from seed is excellent for anyone who wants to get a sense of the tomato plant life cycle. But what is happening in the tomato germination process? Let’s dive into the botanical aspects of gardening tomatoes from seed. 

Tomato Seed Germination

Tomato germination
Tomato germination is a fascinating process. Source: falequin

Before a tomato seed becomes a seedling, it has to go through an enzymatic process called germination. Here is a basic overview of that process. 

Tomato Seed Botany

A seed begins as a dormant cell called an embryo, surrounded and protected by a seed coat and an endosperm. In the Solanoideae sub-group, which hosts tomatoes and peppers, the seed coat is relatively thin. It is broken through exposure to the right temperature and moisture.  

The endosperm provides the seed embryo with starches that aid in germination. A cap covers the embryonic root, and as warmth and moisture break down the seed coat, the root and cotyledon break out of the seed. The cotyledon rises to pull in nutrients from above tomato seedlings via sunlight, and the root embryo dips below to pull in nutrients from the growing medium. Tomato seeds contain one cotyledon, which becomes the first leaves of tomato seedlings. The cotyledon absorbs nutrients and ultraviolet light to feed young plants.  

How to Germinate Tomato Seeds

Cherry tomato seedling
Tomatoes can germinate in peat or coconut coir, like these coir pellets. Source: detsang

There is no one particular way to germinate tomatoes. They can thrive outdoors, indoors, or even in a hydroponic system. Starting tomato seeds is a cinch!

Indoors

Start tomatoes in trays with potting soil. Seed starter pellets can also be used, but we highly recommend Epic 6-Cell Seed Starting Trays. Place on a heating mat. Begin by sowing about ⅛ inches deep and cover the tray or starter pot with plastic wrap to trap in warmth and moisture. Keep the mat heated within the range of 60 to 70 Fahrenheit. Seeds should germinate within one to one and a half weeks. If the seeds take a long time to germinate, this is often a sign that there is not enough heat for sprouting. Simply increase the temperature on your heating mat if you notice this. 

Place tomato plants in a sunny south-facing window in the northern hemisphere or a sunny north-facing window in the southern hemisphere. Try not to expose your seedling containers to too many hours of sunlight. A range of a few hours per day for seedlings at a range of temperatures at 50-70 Fahrenheit is ideal. Too much heat from the sun will burn plants. Note that sprouting in a warm windowsill could produce leggy seedlings. If possible, use a grow light instead.

Grow Tents

Grow tents are a great way to control the climate around your seedlings and ensure higher germination rates. Begin by starting seeds in potting soil in containers, and place them in your tent. You can also plant them in a soilless mix like coconut coir or a combination of vermiculite, perlite, and sand. Whatever mix you use, ensure it’s suited to tomato seedlings. Adjust the temperature and the humidity in your tent as needed. Then plant them in a larger pot when the cotyledons fall off. For instance, if the soil looks a bit moldy, this could be a sign the humidity is too high. 

Hydroponics

A tomato plant can also be grown in a hydroponic system. This way, you’ll get a good healthy yield from each plant, conserve water, and carefully control the climate. However, if the pH in your nutrient mix is off even by just a little, your entire effort can fail. For hydroponics, plant in a soilless mix. Then seedlings should be planted in your hydroponic system in the nutrient solution with a pH of about 4.5. 

Outdoors

If you live somewhere warm with a long growing season, you can plant seeds outdoors directly in the ground of your garden when the temperature is right. This is the riskiest method of starting seeds as they are fully exposed to the elements. Animals, insects, and heat can damage seeds quickly. Gardeners who start seeds outdoors know to observe carefully and use caution. 

Hardening Off

Seedlings that will be planted outdoors must be hardened off in their pots for a couple of weeks. To harden off your tomatoes, place seedlings in a partly sunny area of your garden for a couple of weeks. Check on them often. Then transplant them to a container or in the ground.

Storing and Preparing Tomato Seeds for Germination

Learning to collect and store tomato seeds will help you keep your favorite varieties around for years and years. Through the process of seed fermentation, you can decrease the likelihood of disease and promote growth. 

Seed Fermentation

In tomato flesh, there is a gelatinous sheath that protects seeds. Try planting seed directly from a tomato without removing this sheath, and you’ll find it doesn’t germinate as easily. To prevent seed diseases and help erode the seed coat, ferment them. 

Pull out all the seeds you’d like to save with the gelatinous sheath attached. Then immerse the mass in clean water, preferably in a glass jar with a lid. Place the jar somewhere out of direct sunlight in an area that has temperatures around 70 degrees Fahrenheit. 

Once or twice each day, stir the liquid to separate the seeds from the mass. This also prevents mold. After three days, move the mass into a new container at least three times the size of the original jar with three times the water. Then remove the gel as seeds fall to the bottom of your container. Those seeds at the bottom are most viable. Rinse the remaining non-seed material in a colander or sieve. Flip clean tomato seeds onto a cheesecloth or paper towel to be drained. They’ll need to dry for at least 5 days in an area with good ventilation. 

To prevent adherence, stir the seeds around on the cloth or paper towel each day. After the tomato seeds have dried, store them in a cool dry place or the refrigerator for up to six years. 

Vivipary 

Vivipary in tomato
Vivipary in a tomato. The seedlings can sometimes manage to penetrate the tomato skin while seeking light. Source: mykhal

Sometimes seeds begin the process of germination within the tomato flesh. This phenomenon is called vivipary and occurs when ripe fruit remains in conditions where tomato seeds are likely to germinate. They become a mix of dormant and sprouted seeds. Contrary to popular belief,  vivipary sprouts are edible, and they can be planted to start tomato seedlings, as long as the tomato hosting them isn’t moldy or damaged. However, you’ll have more success cultivating tomato plants from seeds you germinated yourself. 

Testing Tomato Seeds

Use a plastic bag to test tomato seeds saved from eons ago. Place seeds in between a folded, moist paper towel, and place them in a warm place out of direct sunlight. Leave the bag partially open to allow proper air to flow, and seeds will germinate within a week. 

Document which seeds sprout, which need to be discarded due to mold/damage, and which don’t respond at all. Divide the number of successful seeds with the total amount started, and that is your germination rate. Transfer tomato seeds to a growing medium once they sprout. 

Germination Conditions

Tomato seeds do not need light to germinate, but they do need proper warmth and moisture. Temperatures around 70 degrees are ideal. Temperatures between 50 and 75 also work. Tomato seedlings prefer well-draining sandy loam soil at a pH between 6.0 and 7.0. Use a pH tester to ensure you have the proper acidity for good growth. 

If you’re using soil-less media like coconut coir, begin by planting just as you would in a container. For hydroponics do the same. In a hydroponic system be sure the right mixture of nutrient solution is applied to your plants to keep the pH balanced. Tomato seeds need a proper planting depth to sprout. Too deep and the cotyledons will be unable to reach above the soil surface for nutrients. Too shallow, and roots will not form. 

Tomato plants do best when you plant them at the proper time, even indoors, or in a climate-controlled situation. Planting them in good soil in early April promotes proper tomato plant growth. 

Tomato seeds require a decent amount of moisture and humidity to grow. Keep the soil damp but not overly wet. Soilless media requires less water because it retains more water. Use your finger to test a couple of inches down into the medium. If it’s dry, it’s time to water.

Germination Problems

Newly emerged tomato seedling
This seedling tomato is so young its cotyledons haven’t spread open yet. Source: The Ewan

Most germination problems stem from improper temperature, soil, or too much light. Let’s cover a few of these and how to remedy issues that can arise when starting tomato plants. 

Old Seeds

Germination rate of tomato seeds decreases significantly if they are too old. A seed variety that originally had a germination rate of 90% could drop to 50% or lower. Most will remain at the same viability for up to six years. Use the test method from this article to determine the germination rate of old ones.

Improper Conditions

If the temperature is too cool or too hot, tomato seeds may not grow properly. A too-hot temperature will cause seeds to dry out and get burnt, ending their life before they begin. Cold temperature conditions won’t dry out the soil between waterings, causing seed rot. Germination also takes much longer at temperatures lower than 65 degrees. This is why a heating mat is a valued part of tomato seed germination, as it provides warm temperatures for ideal germination. 

Once germinated, keep an eye on your tomato seedling to make sure it doesn’t show any sign of becoming leggy from lack of light. They don’t need light to germinate, but they do need it once they’re above the ground!

Frequently Asked Questions

Newly germinated tomato seedlings
All tomato seedlings look similar when still showing cotyledons. Source: Superrad

Q: What is the fastest way to germinate tomato seeds?

A: The fastest way to germinate a tomato seed is through the test mentioned in this article. Place tomato seeds in between a folded, damp paper towel and place them in a plastic bag. Keep the bag just slightly open to allow air to circulate, and you’ll have tomato sprouts in about a week.

Q: Do tomato seeds germinate better in the dark?

A: Tomato seeds do not need light to germinate. And too much light can make your resulting seedlings spindly. Try germinating seeds in starting trays with a warming mat, or in a plastic bag with a paper towel. The plastic will help ensure there is enough moisture for seeds to grow. 

Q: Do I soak tomato seeds before planting?

A: You can but it’s not necessary for germination. Moist growing medium is enough!


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