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Tomatoes from Seed: My 10 Most Important Seed-Starting Success Factors

Time flies when you are having fun – and when you are busy. It slips through my fingers, and a week turns into a month. My early March MOTHER EARTH NEWS blog target is but a memory – but better late than never, they say! I hope some of you benefit from my years of experience starting lots of seeds.

How to Start a Few – Or a Lot – of Tomato (or Pepper or Eggplant) Plants from Seed

I start a lot of seedlings each spring – far more than I can fit in my garden (why start so many?  That’s another story). We don’t have a greenhouse. We don’t have much room – this is our home, after all, not a nursery. Yet, with a sunny south facing window, some inexpensive heat mats, a garage with suspended shop lights and sunny driveway, I start hundreds (in some years, thousands) of seedlings successfully. Here are what I consider to be a list of the most important details to pay close attention to.

1. Timing is Critical

Here’s a rough guide that you can use and adjust as needed: I plan on one month from seed to the time to transplant to a larger container, and one month more until the plants are ready for the garden. I, therefore, work back two months from the last frost date to determine when to start seeds.

2. It’s OK to Plant Seeds Thickly

I’ve found that, for my needs, I need to use a small area to produce a lot of seedlings. My seed-starting container of choice is a 50-cell rigid plastic plug flat, with the cells a bit over one inch in each direction.

I plant up to 50 seeds in a cell – this gives me a potential for more than 2,000 seedlings taking up a 1-by-2-foot area. This method also works well for peppers, eggplant, tomatillos, lettuce, herbs, beets, most flowers, greens. Then again, I really find the act of separating seedlings and transplanting to be very therapeutic.

Of course, planting one or two seeds per cell is just fine as well. I just need to be very efficient with my space.

planting seeds

3. The Nature and Quality of the Planting Mix is Very Important

Avoid potting “soil” and look for a sterile seed starter, which is typically a soil-less mix of materials such perlite, vermiculite, peat, compost – the finer, the better.  Good, quick drainage is essential. The enemy of germinating seeds is typically fungal issues such as damping off.

4. Barely Cover the Seed, and Don’t Let the Surface Dry Out

I fill the flats, level off, water well – then surface-sow the seeds. I then sprinkle fine planting medium over the top until the seeds are no longer visible – this is very shallow compared with many planting directions.

I then gently spray and put a dome or loosely draped sheet of plastic over the flat and place on a heat mat in front of the south-facing window. Tomato seeds planted this way germinate in an average of 4 days. I flip the plastic sheet daily, and remove it when the majority of cells show germination.

5. Keep Seedlings Turning to Counteract Phototropism

The seedlings will, of course, grow toward the light – I turn the flats 180 degrees daily. The time on heat mats and in front of the window is kept as short as possible so that the seedlings don’t become thin and stretched, aka “leggy”.


6. Put Seedlings Under Grow Lights, or into Sun, as Soon as Possible

I have an unheated garage that does not go below freezing, with a table over which I’ve suspended shop lights with ordinary fluorescent tubes. I lower the lights so that the top of the seedlings are an inch or so from the tubes.

It is important to watch to ensure that the flats don’t dry out, especially if you plant as thickly as I do.

under lights

7. Hardening Off is a Must

If the temperature is warm enough (between 40 and 70 degrees Fahrenheit is fine), it is effective to start easing the germinated seedlings into the sun, a little at a time – perhaps 30 minutes the first day, an hour the second – aiming for full sun growing within a week or two.

Windy days (especially with a chilly wind) and wind-driven rain will make the tiny plants very unhappy. Move the plants in and out as needed.

8. Transplant into Deep Pots!

Seedlings are tough, and respond well to “tough love,” and, therefore, they survive separation and transplanting into individual containers just fine. But plant them deep! The cells of seedlings will pop out of the plug flat and can be eased into individual seedlings; a few roots will be lost but no worries.

I dry-fill strips of 3.5-inch square pots with the same seed-starting medium, place a seedling on each, and gently press at the stem near the root junction until the entire stem is under the surface – the seedling is planted deeply up to the leaves.

Any part of the stem under the planting mix will form roots, making a stronger plant. After the plants are situated in their new homes, water them well.

9. Transplanted Seedlings Need Rest Before Spending Days – and Nights – Outdoors

After I move my seedlings into individual containers, they rest in my garage for a week or so. This adjustment period will allow for the roots to begin to develop and the plants to take up water effectively again. Choose a cloudy, mild day to move them out into full fun.

hardening off

10. Watch the Thermometer

Tomato seedlings are tender, and frost will kill them. If you have a night forecast where 32 degrees is a possibility, move them back under the protection of a garage or into your house. Another method I’ve used when the number of flats is greater than I wish to relocate is to apply a double layer of frost cloth (aka floating row cover), draping it over the seedlings

I’ve experienced overnight temps down to 29 degrees (and lost some sleep knowing it was likely to happen), but sleep is the only thing I lost – the plants were all fine the next day. Be sure to wait until the air temperature is well above freezing before removing the covering.

I go into all of this in great detail in my book Epic Tomatoes. I’ve also got some rather amateur but informative videos on my website – look for the resources tab.

Craig Lehoullier is an heirloom tomato expert (and amateur plant breeder). He currently is on book promotion tour for Epic Tomatoes, setting upcoming tomato workshop events, updating his website and blog, devising a totally new, all-heirloom weekly podcast, shooting a tomato know-how video series and pondering topics for future books. He is co-leading the Dwarf Tomato breeding project to put 36 new dwarf-growing, open-pollinated tomatoes in the hands of various small seed companies. Read all of his MOTHER EARTH NEWS posts here.

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