Important Factors that Affect Growing Tomatoes from Seed

When Do You Plant Tomatoes and Other Useful Tips

Important Factors that Affect Growing Tomatoes from Seed

It’s getting to be that time of year where everyone is forming their spring to-do lists and planning their upcoming vegetable garden. Why not add growing tomatoes from seed to your gardening repertoire? Starting seeds is something that may be viewed as difficult or intimidating for those who have never done it. But, with a little knowledge on the key factors that affect germination, anyone can be successful.

The first step toward growing tomatoes from seed is figuring out what varieties you want to grow. When choosing your varieties, you might want to consider how long does it take for tomatoes to grow? How long is my growing season? Will they preserve well or are they best fresh? I choose to exclusively grow heirlooms varieties, each for a specific purpose. Some of the varieties I grow have been passed down and I am now the fourth generation to grow them. In addition, I am trying some new varieties such as ‘Aunt Ruby’s Green’, ‘Black Cherry’, ‘Japanese Black Trifele’, and ‘Martino’s Roma Tomato’.  So, what are heirloom tomatoes? There are varying opinions on what truly makes an heirloom an heirloom. Simply put, they are open-pollinated (non-hybrid) breeds that have been passed down over generations. To me, this means a natural tomato unaltered or influenced by man, and more importantly, a more delicious fruit.

Once you have your varieties picked out, the next step is determining when to sow your seeds. Planting schedules by zone can easily be found online as a guide, but the general rule of thumb is to start the seeds indoors six weeks before the last frost. In my neck of New England, I am in Zone 5b. I start my seeds sometime around the second week of April so they are ready to hit the garden by end of May/beginning of June.

In my opinion, a successful start comes down to a few important factors when growing tomatoes from seed. If you make sure the proper container, soil, water, light, and heat requirements are met, then you can rest assured that you will have germination. These recommendations are based on my personal experience and have proven to work for me.


Containers come in many different sizes and materials. I have used some with small cells, larger cells, plastic material, and peat pots. For me, it usually comes down to what is readily available at the nursery when I’m shopping for my supplies. Some containers will come with a plastic cover if not, saran wrap works just as well. Each container has its own pros and cons and ultimately comes down to a matter of personal preference.

The main difference between starting seeds in a container with small cells (1.5” x 1.5”) versus larger cells (3” x 3”) is that there is an additional transplant required when using the smaller ones. The seedling’s roots will reach the limits of the container before it’s strong enough to be transplanted outside. So if these are used, they will need to be transplanted into larger individual containers, such as peat pots, while they finish growing indoors. Seeds sowed in the larger cells have enough space for the roots to grow until the seedling is able to be transplanted outdoors. However, I have found that they do tend to make for a more difficult transplant and you risk damaging the seedlings as the stems can become tangled. As far as material goes, I have used both plastic and biodegradable containers without any issue or transplant shock. If I am able to find them, I typically prefer peat pots as they can be directly planted into the ground, container and all.


Picking out soil at the store can be overwhelming. Organic versus non-organic, seed starting versus potting not to mention the endless list of fertilizers. It can be downright confusing and make your head spin. So, which is the best soil for tomatoes? Because all of the nutrients that a seedling needs in order to grow are actually contained within the seed itself, I try not to get too distracted by labels. It isn’t until sometime after germination that additional nutrients are required, which is why I like to focus on the quality of the soil as nutrients can always be added later.

Seeds started in small 1.5″ x 1.5″ cells with regular potting soil.

Soil specifically designed for seed starting is ideal. It’s fine in texture and aerated which provides drainage and allows the seedling to pop through without resistance. It can also be expensive, and sometimes difficult to find, which is why I’ve used regular potting soil in the past too. If this is the route you choose to go, just be sure to remove any large sticks or bark either by picking it out or by sifting the soil yourself. I also recommend adding perlite to the sifted soil for proper aeration and drainage. Whichever soil type you choose, be sure to use new, unused soil to prevent any kind of disease and never use dirt you dug out of the ground.


Over and under watering has significant effects on seed germination as well as the seedling once it has sprouted. Before filling the containers with soil, premix the soil with water in a large bucket to make sure that it is properly saturated. The soil should not be so wet that water drips out when it’s squeezed. The soil should hold its shape. Once the seeds are planted, water trays can be placed underneath the containers that allow the soil to absorb the water as needed through the drain holes below. I prefer to use a spray bottle rather than a tray, that way I can control soil moisture and I get in the habit of checking on my seedlings. I strongly discourage using a watering can as it does not allow enough control, and can easily lead to over-watering. This results in rotting, mold growth, and stem tapering at the soil line.


Light is a critical environmental factor for growing tomatoes from seed. A light source is not needed for germination, but once they have sprouted and a seedling has emerged, it becomes critical. Grow lights can be used if there isn’t a natural light source available, but I prefer to use a south facing window. This is how my grandfather taught me, and I have never had to resort to artificial lights. Because the natural light casts in a window from the side, as opposed to an artificial light fixed above, the seedlings have a tendency to “reach” or grow toward the light. To prevent this from happening, rotate the container 180 degrees every other day, which will allow the seedlings to grow upright.


Depending on the ambient temperature of your house, an additional heat source may be required when growing tomatoes from seed. In order for a seed to germinate it needs warm, damp soil. My house is chilly compared to most and usually hovers in the low 60’s during the day dipping down into the mid-high 50’s at night. This temperature range does not make for ideal growing conditions, so I have to provide an additional heat source directly. There are specialized seed mats that are designed to keep your soil warm. But, these are expensive so I prefer to use a regular heating pad and it works just the same.

A strong, healthy seedling ready to be directly transplanted into the garden.

No matter the variety of tomato you’re growing if you provide the basic essentials a seed needs to flourish into a mature plant you will be successful. Whether you are growing cherry tomatoes in pots or beefsteak tomatoes in a raised bed, a bountiful harvest all starts with a strong, healthy seedling. So, what varieties of tomatoes will you grow this year? Is this your first time starting your own seeds indoors?

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