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Growing Your Own Transplants
September 9, 2015

Lettuce, beets, and onion transplants
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There are lots of good reasons to grow your own garden transplants, and possibly almost as many for purchasing them at your local garden store. If all goes well in the transplant growing process, one may save a few dollars by putting in some hours into selecting seed, seeding, and caring for young transplants until they're garden ready. But you also wouldn't have to go many clicks from this feature story to find one of my rather expensive transplant growing debacles.

The obvious reason for using transplants is to extend the gardening season by getting a head start with healthy, sturdy plants ready to set out as soon as all danger of frost is past instead of direct seeding. Beyond any economics, growing your own transplants allows you to somewhat control when your plants will be ready to set out into the garden. But possibly the best reason for growing your own transplants is that it opens up an amazing array of plant varieties from mail order seed houses that you'll never see at your local greenhouse, garden or discount store.

The basics of seed starting are really pretty easy. You need containers, seed, and "clean" planting medium. Generally, you need a fairly warm location to start most seeds. Later on, you'll definitely need a fairly warm, sunny (or brightly lit) location to grow out your transplants. And of course, you'll need water.

Petunias growing in egg cartonsYour seed starting containers can be low tech egg cartons, old flower pots, small cottage cheese or dip containers, or even the half pint or third quart wax paper milk carton containers often used for school lunches. Or if you prefer, you can go with higher tech options including commercial seed flats and inserts that with care can be reused many times. The main thing about containers is they need to hold soil but still allow excess moisture to drain away. (I used a pencil to punch holes in the bottom of the styrofoam egg carton cells shown at left. Then I used the egg carton top to trap any drainage and keep it off our windowsill.)

Catalog coversThe easiest way to get seed would seem to be at a local garden center or discount store. Seed racks begin popping up in stores well before Valentine's Day and can be a lot of fun to peruse. But those seed racks generally include only the most popular (or most profitable) seed varieties. I still buy an occasional packet or two of seeds off a rack each year, usually on an impulse, but get most of our flower and garden seed from several reputable, trusted seed vendors. Most offer free, gorgeously illustrated print catalogs along with online catalogs and ordering.

Plant rackI used the phrase "planting medium" above, because when starting seeds, one needs a disease free material in which to plant seeds. Potting soil may have damping off fungus in it which attacks tender, young seedlings at their base, eventually killing them. There are soilless mixes on the market, things like peat pellets (peat moss doesn't normally carry damping off fungus), or one can sterilize regular potting soil to make a disease free planting medium by baking it in a covered container (holds in some of the odor) at 400o F for an hour or so in the oven.

A sunny windowsill that is brightly lit for a good bit of the day may provide all the light you need to grow your transplants. Often, however, there's just not enough sunlight through the late winter months in the best of window locations. Fortunately, transplants respond quite well to fluorescent lights, whether an expensive plant starter setup or the basic shop lights we use.

And since I mentioned water at the beginning of this section, let me add that it's better if you can use water that hasn't been through a water softener for your transplants. Soft water picks up a little salt in the softening process that probably doesn't do the plants any good. If soft water is all you have, it may do, but if you have a bypass around your water softener (or are lucky enough to live in an area with or served with naturally soft water), use it.

Let's Plant Something

Transplants we grow (in approximate order of planting)

Petunias, ivy leaf geraniums, vinca, and impatiens for early hanging baskets
More onions - shallots - leeks
herbs (parsley, sage, rosemary, and thyme...and others such as basil and oregano)
brassicas (broccoli, cauliflower, cabbage, kohlrabi, brussels sprouts)

sweet bell and paprika peppers
cucurbits (melons, squash, cucumbers)
other flowers, mostly annuals (alyssum, impatiens, dusty miller, vinca, marigolds, dianthus, snapdragons...)

more lettuce

in June
fall brassicas

in July and August
fall lettuce

At right is a listing of many of the things we start indoors each year under our plant lights. I won't attempt to cover all of them, as many have similar requirements to get started.

While our first plantings are usually of petunias (in egg cartons), onions, and geraniums, for this feature story I'd prefer to begin with America's garden favorite, tomatoes. We love tomatoes on our hamburgers, in BLTs, in spaghetti sauce and soups, in salads, and right off the vine, complete with red tomato juice dripping down ones chin. Even with our kids grown and just my wife, Annie, and I now at home, we still can and use about 15-20 quarts of whole tomatoes each year. Once one learns to successfully start tomatoes, a relatively easy task, most other stuff really isn't all that much more difficult.

Tomatoes are generally one of the last transplants we start in the spring. They take only six to eight weeks to grow from seed to an ideal size for transplanting into the garden. Many of the monster tomato plants one sees in the spring in stores are actually way too old for transplanting! While I'll give you a link later for a seed starting date calculator, backing up six to eight weeks from your last killing frost date will get your tomatoes started at about the right time.

Inserts - deep sixpack and fourpackWe usually start our tomato plants all at once in commercial inserts, fourpack or deep sixpack, either purchased from a greenhouse or garden supply store or left over from previous years. If we want only one or two plants of a variety, I often use 3" square plastic pots to start the plants. When trying to get some very old tomato seed to grow a few years ago, I dumped the remains of a packet that had been stored in our freezer for over 20 years in a large flowerpot and covered it with planting medium. (And the seed grew!)

Note that when reusing planting containers, be sure to thoroughly wash them out. Better yet, finish the wash cycle with a quick rinse in a chlorine bleach solution to kill off any disease organisms that might be living on the containers (more often living on unseen organic matter still clinging to the container). And of course, rinse the sterilized containers again with clear water after rinsing in bleach, as the bleach itself can harm ones transplants.

Seeding tomatoes and peppers
Seed on soil

I fill my inserts or 3" pots with a mix of potting soil that I've recently sterilized in the oven. Before planting, I thoroughly water the fourpacks or pots with warm water. If you're using a planting tray or seed flat, pouring the water into the tray and allowing the planting medium to absorb the water for a half hour or so should give the seed all the moisture it needs until it germinates.

Tomatoes and peppers covered with humidomes and on heat matsThen I make a small depression in each fourpack cell or pot, place a single seed in it, and then push a bit of soil over it, making sure each seed is completely covered. Tomato seed, like a lot of other vegetable varieties, doesn't need light to trigger germination. (Some flower and herb seeds do need light to germinate, but I'll cover that later on.) I also slip an extra seed or two at the corners of the inserts, as one generally doesn't get 100% germination.

Tomato seed likes warm conditions to germinate. A warm or room temperature shelf should do. Since we start our transplants in our basement which runs a bit cold in March and April, I usually start our tomatoes on a heat mat with the thermostat set at around 75o F. We also use a cover, often a commercial, clear "humidome" to hold in moisture and heat during germination. But one can easily start tomatoes in a warm spot in the house, being careful never to let the soil dry out, but also not to make the soil soggy so the seed rots. Within seven days (or less), you should begin to see tomato plants emerging. It's important to get the emerging plants into good light at this point, so that they won't become overly tall (leggy) searching for light and produce weak stems that topple over easily. (Note that if you plants get leggy, just lift the tomato seedling, roots and all, and plunge it deeper into the planting medium, almost right up to the leaves.) If using a heat mat, it's also important to get the new seedlings off the mat and into normal temperatures to allow them to toughen up a bit as they grow.

Cold frame on April 10, 2017That's about it for starting tomato plants. If using fluorescent lights, be sure to keep them at least an inch or two above the top of the tallest plants, varying that height as the plants indicate. (If they get leggy, lower the light. If the leaves begin to bleach out a bit, raise the lights.) A week to ten days before transplanting, you'll need to set your plants outside to let them gradually acclimate to outdoor conditions. When doing so, keep an eye on the young plants, as they harden off slowly and may need to be protected from full sunlight (and wind) the first few days. We manage hardening off plants under a cold frame and along the sunny edge of our back porch.

Most of the vegetable transplants we grow are started about the way we do our tomatoes. We often start our bell pepper plants side-by-side with tomatoes. Melons require bigger pots and more bottom heat (especially seedless watermelon), but the process is essentially the same.


The first vegetable transplants we start each year are our onion plants. We get them going sometime in mid- to late January so that they're at an ideal growth stage for transplanting in late April or early May. Besides the timing of the plants, growing our own starts allows us to choose varieties that do exceptionally well under our growing conditions and suit our tastes.

With seed for some of our favorite hybrid onion varieties disappearing from garden catalogs, we conducted what for us were extensive trials of thirteen onion varieties in 2014. We found a few new to us varieties that we liked, but also stayed with some other longtime favorites. For long term storage, we currently grow Milestone, Clear Dawn, and a few Yellow of Parma. Milestones are a somewhat sweet hybrid yellow onion. The open pollinated Clear Dawn variety was bred out of the hybrid Copra storage onion line. Yellow of Parmas are an heirloom yellow onion that do well for us, just not as well as Milestones and Clear Dawns.

Onions drying

For red onions, we love the Red Zeppelin hybrid (now discontinued), but also have added the open pollinated Red Creole, Rossa Di Milano, and Tropeana Tonda varieties to our growing mix at times. For sweet onions, we still grow the tried and true Walla Walla open pollinated variety, but they only store well for a month or two. That's not a problem for us, as we use them up quickly for canning with green beans and bread and butter pickles, and in our annual batches of Portuguese Kale Soup.

Note that most of the varieties we grow are long day varieties, suited to northern climates. Only the Red Creole variety is a short day onion, more suited for growing in southern climes. According to the Oregon State Extension Service, Walla Wallas were "originally a short day onion, now modified through years of selection by growers." Our Red Creoles produce small red onions several weeks earlier than our other varieties. Don't get spooked by the short, intermediate, or long day stuff. I've added some links a bit on down that explain all that.

Flat of sterilized potting soil Adding warm water
Sterilized soil in flat Adding warm water

Under dome, over heat matWe seed our onions to a large, standard plant flat. I now use a standard 1020 slotted seed flat inside a much sturdier (and more expensive) Perma-Nest tray to carry the weight of a flat full of damp potting mix. We just fill the tray with sterile potting mix, thoroughly wet it with warm water, use a ruler (or whatever is available) to make shallow (1/8 inch deep) seed rows, and put in and lightly cover the seed. I cover the flat with one of those clear domes that fit tightly to hold in moisture. If I have a heat mat available, I set its thermostat at around 70o F, as our basement runs pretty cold in January when we seed our onions.

We mark our onion rows and other plantings with plastic row markers that we reuse year after year. We clean off the permanent marker from them by soaking them a few days in a bleach solution.

Making row labels Making furrow Seed in row
Marking row labels Making furrow Seed in row

Once the onions emerge, the clear plastic cover comes off. We grow our onions and almost all the rest of our transplants under fluorescent shop lights on an old plant rack I built years ago. I keep the fluorescent tubes at least 2" above the tallest plants. Our bank of six 48" shop lights are now on a timer, but I've grown lots of transplants with 24 hour lighting with no apparent ill effects. For the onions, it's just a matter of keeping them moist and adjusting the height of the lights as the onions grow. If, no when, they get too tall, I give them a "haircut" with scissors to about three inches tall. Other than using old seed for onions (onion seed doesn't keep well from year to year), I've never had a failure with growing onion transplants.

Onions under plantlights

Okay, I have to take that last statement back. In 2013, either our cats or dogs decided our onion transplants hardening off on the back porch were an ideal spot to pee on. By the time I figured out what was happening, the onion plants were in pretty sad shape. To add insult to injury, after I transplanted the onions beside some direct seeded carrots, a couple of young dogs dug up most of the softbed the onions were growing in going after a mole.

Perfect size onion plantsStarting transplants in rows in a flat mimics garden culture, but isn't really suitable for most transplants. We've grown onions, beets, and asparagus plants this way, but use individual pots or the commercial plastic inserts used by garden stores for their plants for most of our stuff.

While you may find some good looking Walla Walla onion plants at your favorite garden store, the other varieties we like are pretty much seed-only in availability. And beyond that, I've never had a mail order of onion plants arrive in acceptable planting condition at the right time.

When it comes time to transplant homegrown onion plants, I find that most of the soil drops off the onion plant roots when removing them from the tray. Fortunately, onion plants tolerate being bare rooted into the soil pretty well as long as you keep them moist until they get their root systems established again.

Okay, this feature has turned out to be far longer and have lots more images than I originally envisioned. I'm going to cut it into pieces so the individual pages will load a bit better.

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From Steve Wood, the at Senior Gardening

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last updated 2/5/2019