How To Grow Tomatoes From Seed

Although many tomato varieties are available from stores in the spring, there is nothing quite as rewarding as growing your own tomatoes from seed.  Also, if you peruse spring seed catalogs, you will note hundreds of tempting varieties of tomatoes are available from seed, as opposed to the few varieties you can find to purchase as plants.  I grew up in a home where seeds were chosen when the seed catalogs first arrived, planted in March, and moved from sunny window to window until hardened outside and planted in the garden.  It just wasn’t spring without watching the tomatoes grow!

It is very simple to give this a try.  Tomatoes are quite sensitive to frost, and should not be permanently planted outside until after the last frost date.  In the Waterloo area, that is generally considered to be about May 15.  It is thus recommended that seeds be planted 6 to 8 weeks before that last frost date.  We never plant before St. Patrick’s Day, March 17th, or plants may become too leggy.

Tomato Varieties

First, select the varieties you desire to try.  Consider how you want to use the tomatoes: slicers to serve, plum types to cook or for sauce, cherries for salads or snacks?  Heirloom tomatoes may have unique tastes and colors to try.  New hybrids may be stronger or more disease resistant.  Also, consider the grown size of the plants and your available growing area.  Bush type (or determinate*) tomatoes are available for containers.  Indeterminate* tomatoes can grow up to 6 feet long during Iowa summers and may need support in your garden.


Growing Medium

Next, select your growing medium and containers.  “Soilless” sterile potting mix is recommended, although any seed starting mix could be used.  If you don’t plan to repot prior planting outdoors, you should use a container at least 3-4 inches tall and wide, so the roots do not become constricted.  Fill the container to about ½ inch from the top, place a couple seeds on top, and cover with ¼ inch of the potting mix.  If more than one sprouts, pull out the smaller one.  Seeds can also be planted in cell packs, which is what this author likes to do, and replanted in a bigger pot after the top leaves have developed.  This author has not had good luck with biodegradable containers.  They don’t seem to degrade in the soil fast enough and may constrain the roots. However, I admit I haven’t tried them for many years and they may have improved. If using, I suggest you at least score the container when placing into the ground to free the root system.  Be sure to label each plant—you will want to know when growing and eating which varieties you will want to use next year, and which you don’t like as well.


Growing Conditions

Tomatoes are a tropical plant and need at least 60-degree temperature (70-75 is better) to sprout.  You can place them in a sunny window for warmth or use an inexpensive and easily acquired seed starting mat (like a heating pad, but not as hot) under your seeds.  If you cover your seeds to maintain heat, be sure to uncover as soon as they have sprouted, or they can quickly “damp off” and die of a fungal infection.  Good air circulation is important.  They can do well in cooler 65 degrees temperatures after they have sprouted.



Plant Light in a Sunny Window


It can be difficult in Iowa to have sufficient sun from your windows in late March and through April as you are raising your seedlings.  A plant light can be used.  This author uses a simple fluorescent tube plant light that is on a homemade stand about one foot high.  The plants can easily be placed close to the light when they are small (with a shoebox raising them up) and lowered as they become higher.  If the sun is out strong, they can be placed directly in windowsills, but I recommend moving them out at night if the weather is very cold.

A note on replanting:  Tomatoes are one of the few plants that develop root systems along their stem if they are planted deeper.  Most other types of seedlings should be replanted at the same depth at which they have started.  However, a tomato can actually benefit from being planted deeper, and I feel much of our seedling success can be attributed to our first replant up to the depth of the first two “seed leaves”, and again even deeper when transplanted to the garden. Always be sure there is plenty of room for roots to develop in the container.  It is good to use weak water –soluble fertilizer once per week after the plants have sprouted.  Keep soil moist, but not too wet.


“Hardening Off” & Outdoor Planting

Last but certainly not least, be sure to “harden off” your seedlings.  They have been growing in your home, away from the elements.  Once it is warm and sunny, the pots can be placed on a tray, and taken outside in a protected area to get used to the wind and direct sun.  This should be done slowly and carefully, first placing the plants in a protected and shady area.  Then move to direct sun, but be sure plants are protected from too much wind (we have used old windows propped beside them).  If you use one of those popular small portable greenhouses covered with clear plastic, be sure you don’t “cook” your plants—temperatures can quickly rise inside, just like inside a parked car.



You can safely plant your tomatoes outside after danger of last frost.  We have always planted them out a bit early, depending on weather, and planned to cover them if frost threatens.  However, the soil must be warm—at least 60 degrees is best, they can be planted outside at a 55-degree soil temperature but then growth will be very slow.  We place a few protective shingles around each plant when first placed in the garden, and remove once the small plant is strong.  We also use granular fertilizer in a ring around each plant for a good start.  Save a few extra plants for replacement in case of early loss due to squirrel, rabbit, or frost damage.


Lastly, some persons enjoy checking the moon signs for when to start seeds or plant.  If you garden by the phase and position of the moon, the Old Farmer’s Almanac claims that vegetables that bear crops above the ground should generally be planted during the waxing of the moon to the date it is full, and below ground vegetables during the waning of the moon.  The theory is that just as the gravitational forces of the moon affect the tides, that moon position can affect moisture in the soil and plant growth.  This year the almanac has favorable tomato seed planting on March 29-30 and April 2-3rd.  (A bit later than this author would start).

rany with tomatoes.jpg

Good luck!  There is nothing more satisfying than growing and eating your own tomatoes!


*Determinate means that a tomato will grow to a genetically predetermined size (should be on the seed packet), and indeterminate means the plant will continue to grow and bear tomatoes until killed by frost (in Iowa).  Determinate varieties tend to bear the fruits close in time (handy if you want to can or preserve).  Also, consider if the tomato is rated as disease resistant.  You may want to choose at least one variety that is resistant to verticillium and fusarium (designated by a V or F after the variety name).


Article by Pat McGivern


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