To my mind, any vegetable that can be raised from transplant probably should be, especially if your garden is small or your season short. Doing so will greatly improve the productivity of any garden because it allows successions and interplantings to be more closely planned. As soon as one crop is harvested, you’ve got another set of plants ready to make use of the space. Why give each broccoli plant a foot and a half of space when it’s young and only needs a couple of inches?
Not all vegetables transplant well, though. Root crops like carrots, leafy biennial herbs like dill and fennel, and the heading types of Chinese cabbages are all likely to run to seed in response to the stress of transplanting, instead of yielding a crop. Still others grow so quickly when sown directly in the garden that transplanting isn’t really worth the extra effort. Greens like spinach, mustard, arugula, and cress are good examples. In the end, your own garden plan is going to determine which plants you should start indoors.
I think one of the biggest mistakes gardeners make when growing seedlings is starting too soon. I know gardeners in Los Angeles who harvest tomatoes in January, but north of USDA Zone Six you shouldn’t even think of starting tomatoes until February. When we lived in Vermont, we started ours in two batches in mid-March and again the first of April. If the weather broke early, it was worth it; but if spring was slow, the early ones just got tossed out as they were too big when planting season arrived. That’s why it’s a good idea to make two sowings of seed, a week or two apart. If something happens to one set, you’ve got the backup. With two plantings, a week before and a week after the theoretically ideal planting date, you’re ready either way. You can always give away the extra plants if things work out.
Keep in mind that all else being equal, you are better off with young, vigorous plants than older, root-bound ones. The best produce comes from plants that grow quickly, without what the pros call “checks” that is shortages of any nutrient, of water, or of temperatures to their liking. If the plants are a little small, all you’ve lost is a week or two in the garden; that is often made up in the good growing days of early summer. Two weeks in March is only worth a couple of days in May, or so we said in the nursery trade.
Celeriac, leeks, and parsley are among the first plants to be started, a good twelve weeks before their intended transplant date, which is itself two to four weeks before the last frost. Check the USDA Zone Chart for your location and count back from the frost free date. You should probably just wait to direct-seed them in the garden a month before the last frost, and save the seedling space for other crops. Gardeners in the North should also start seedlings of thyme, sage, rosemary, and other perennial herbs at the same time if they want a decent harvest in the first season. This is true whether you grow your plants in a windowsill or a greenhouse.
Tomatoes, peppers, eggplants, and basil need really warm conditions. But once they germinate they grow faster, and so can be started only eight to ten weeks before the frost-free date. If you have a greenhouse, you can cut another week off that, due to the increasingly great difference spring brings to greenhouses versus windowsills. We have grown great tomato transplants in only six weeks; even a month-old plant will make a good sized bush by mid-summer and yield a lot of fruit.
There is a third group of vegetables that most gardeners direct-seed, but that can be started indoors if you have a very short season, or simply can’t wait for spring. All the members of the cabbage family do well grown from transplant, as does lettuce. Start them only six to eight weeks before the last frost, though; because if they get too big in the flat they don’t withstand transplanting as well, and are unlikely to yield a first-class harvest. Melons, cucumbers, summer squash, beans, and even corn and peas can be started in plug flats, peat posts, or large soil blocks a mere two to four weeks before the frost-free date. The plants grow so fast and are so succulent that it’s difficult to hold them in a tray longer than that.
Gardeners use all different kinds of containers to start seedlings: wooden flats, egg boxes, sawed-off milk cartons, recycled garden center six-packs, peat pots, soil blocks, and plug trays. All will work, but whichever you choose remember that the key to growing healthy transplants is consistency: all the plants of a given size and type should get equal treatment, which is difficult using a mishmash of containers in all different sizes and shapes.
One of the most common modular systems is peat pots and strips. Many organic gardeners prefer them simply because they want the benefits of individual containers for each plant, but they don’t like to use plastic. Peat containers have the benefit of consistent sizing, but they are messy to work with, and unless you use the strips, their shape makes them top-heavy once the plants have reached a decent size. Once they tip over they are difficult to keep moist as the water runs off instead of sinking in. You can buy peat pots or pellets mounted in special plastic trays, but they are more expensive that way, and if you’re going to use the plastic at all you might as well go with a completely integrated system. And peat itself is not really a renewable resource.
Perhaps the best system to use, from an organic perspective, is soil blocks formed by compressing a peat-compost mix into cubes of different sizes with a small, hand-held press. These blocks are then placed into special growing trays. All you need for this system is one or more of the presses, or block makers, which cost from $15 to as much as $100 (depending on the size), plus a collection of special three-sided trays. Both are available by mail order. Generally, soil blocks require a bit more practice to make, and skill to maintain, than a plug or peat system, as watering is more critical. Too much water can erode the blocks and make them hard to separate at transplant time; too little water may let them dry out enough so that it is difficult to get them wet again. The time required to make soil blocks also has to be considered.
When we were commercial gardeners we did a detailed analysis of the different systems, and without a doubt the most efficient and least expensive integrated system is plug trays. These are inexpensive plastic inserts for the standard 10 x 20-inch plastic greenhouse tray. They are similar to the six-packs that store-bought transplants are grown in, but are slightly heavier gauge plastic and so, easily reusable. The insert is made up of conical “cells” of different sizes. According to the size of plant that will be grown in them. Commercial growers, with automatic water and fertilizer systems, often start their plants in plug inserts with 288 or even 406 cells per 10 x 20-inch tray. That gives each plant a root mass not much larger than a pencil eraser!
Low-tech market gardeners and amateurs use trays with 162, 98, 72, 50, or as few as 24 cells per tray. This makes it considerably easier to care for the seedlings, because each has a much larger amount of potting soil to maintain itself. The plant plugs produced by this system are easy to transplant, because the conical shape of the cells directs root growth downward; if set out at the right stage of growth the seedlings take right off once set out in the garden. The trays and inserts are not expensive, and with proper storage in the off-season will last a number of years. The plastic used for these trays is also recyclable, though programs may not be in place everywhere to actually do so.
What you put into the container or block is just as important. Straight compost—what my grandfather used—is fine for open flats, as long as it is fully matured and screened. But the texture of straight compost is too dense for plugs and soil blocks. In plugs it tends to pack down and then become hard to water; in blocks it erodes and soon you have a tray full of compost instead of individual blocks. A mix that is at least half peat will allow the blocks to hold together until the seedling’s roots have a change to spread, binding it into a sturdy and stable package. In plug trays and recycled six-packs the peat’s coarseness helps keep the surface from packing down.
Commercial potting mixes are available which are mostly peat, and may be a good choice for the beginner, though you should be sure to look for an untreated, unfertilized mix. Also be sure you are buying a seedling mix. What you want will be brown in color, very light by volume, and sprinkled throughout with white or grayish specks—these are vermiculite or perlite (or both), which are added to the mix to improve its aeration and drainage. A loosely packed bag the size of a feed sack or large bag of birdseed should weigh less than ten pounds. What you want to avoid is the small bags of fine-grained, soot-colored potting soil. They are much heavier and usually don’t have the perlite and vermiculite; these mixes have all of the disadvantages of compost, and none of the benefits.
An additional benefit of the commercial mixes is that they are, for all intents and purposes, sterile. If your compost was not properly made—that is, the pile did not heat up fully—it may contain weed seeds and disease spores from the plants that were composted. In the incubator-like conditions under which most plants are started, that can be fatal. Of course, you can sterilize your compost by heating it (to 160ºF for four hours) in the oven before mixing it with the peat. Whatever way you choose, remember that the first few days of a seedling’s life make a big difference to its eventual success in the garden, and it is less resistant to problems that it might shrug off outdoors.
If you are using a tray system or container to start your seedlings, fill it loosely with potting soil—until it overflows—then scrape off the excess. Don’t pack down the mix, because young plant roots need air. The mix will pack down naturally as it is watered. You should moisten the mix slightly before filling the containers, though, to keep down the dust, and so that the particles will adhere to one another.
The process of making soil blocks is a bit more complex, and requires a bit of practice. The mix should be wet enough to mold well in the block-making press; when you squeeze a handful, some water should drip out between the base of your fingers. First, fill a flat-bottomed tray or tub with the mix, then grasp the block maker down around its base and scrape it across the bottom of the tray or tub that contains the mix, forcing the potting soil into it by pressing against the side of the container; it might take a couple of passes to get enough mix into the press. Once the press is full, release the blocks onto a growing tray by pushing down on the plunger. The ideal growing tray for blocks is smooth-bottomed and three-sided, so that the blocks can be moved (if necessary, but try to avoid it) by sliding them out of the tray rather than picking them up. Continue in this fashion until the growing tray is full, set it aside, and start the next tray. Most blockers make a small dimple in the top of the finished block where the seed should be down.
Outdoors, seeds should be planted three times as deep as they are across, but indoors they need only be half that deep, which means less waiting for them to break ground.Why? Because while depth brings consistent moisture and temperature, seeds also need to be near the surface to get oxygen, and in some cases, light. Indoors you can control temperature, moisture, oxygen, and light, so it’s just a matter of balancing these needs. The first stage of germination sees the seed swell with water, and activate stored enzymes that start digestion of the seed’s food supply. As this process accelerates, oxygen is needed, which is why, although seeds want moist conditions, you shouldn’t drown them. Just soak the medium once after planting, then cover the flat with some sort of moisture barrier until the plants break ground.
After a few hours (or days, depending on the species of plant) the first seedling root emerges tiny feeder roots begin to spread throughout the soil. Soon the plant breaks ground and finds the last vital growth factor it needs: light. If you haven’t added fertilizer to the soil mix, you will need to start both watering and fertilizing. From this point on the seedling becomes dependent on the nutrients its roots can find in the soil, the moisture that makes them available, and the quality of light its leaves receive. Without the right amount of each-in proper proportion to each other and the temperature—no plant will prosper. Optimum temperatures vary from plant to plant, but most vegetables will thrive in temperatures of 65-75ºF. If your house isn’t this warm and you don’t have a greenhouse, you can start your seeds on top of the refrigerator or in the wash room. But keep a close eye on the flats, because the moment they break ground they need all the light they can get.
Since heat and light fuel plant growth, the relationship between the two is critical to growing healthy, vigorous seedlings. A common mistake among gardeners without a greenhouse is to keep plants at too high a temperature for the amount of light they receive. Not only is the light from a south-facing window more short-lived than it might seem (rarely exceeding eight hours a day), but the glass in house windows screens out some parts of the sunlight that plants need. What often happens is that the gardener tries to compensate for slow growth with more fertilizer and higher temperatures; both of these make the problem worse, since they increase the imbalance between light levels ad the other factors necessary for good growth. The result is limp, leggy seedling that are hard put to cope with outdoor conditions when planting time arrives.
Fertilizer and water also need to be kept in proper proportion. To grow, plants need nutrients, and without enough moisture, they’ll not only be unable to take up those nutrients, they’ll wilt and die. But too much water washes away the nutrients in the tray or pot and the plants will starve. The conventional wisdom holds that you should fertilize every certain number of waterings not every certain number of days. That way the amount of fertilizer is based on the amount of water that the plant has taken up, not some abstract calendar date. One compromise solution is to fertilize every time you water, at one-quarter strength. That way you don’t need to keep track of when you last fertilized, and the plants get an even, constant supply of nutrients.
Keep an eye out for signs of under- or over-fertilization. Leaves that curl under are a sign of overfeeding; discoloration, though, is usually a sign of underfeeding. If the plant is pale, that is likely a sign of nitrogen deficiency; leaves with purplish undersides indicate a shortage of phosphorus; leaves with bronze edges show a shortage of potassium. Since seaweed and fish fertilizers contain balanced amounts of all these, the solution is just to increase or decrease the strength or frequency of feedings. The mix we’ve used, both for growing bedding plants to sell, and for our own gardens, works out to a tablespoon each of liquid fish fertilizer (or fish emulsion) and liquid seaweed per gallon of water. This is the full-strength formula and should be diluted 4:1 if you fertilize with every watering.
It’s not enough to raise vigorous, healthy seedlings if they are so pampered that they can’t survive the sun and wind and rain and the seesaw effect of day and night temperature changes. But frosty nights aren’t the only enemy to tender transplants. Wind can be just as hard on young plants raised in the still air of a greenhouse or windowsill, snapping off brittle stems or flattening them to the ground, where they can fall prey to all kinds of fungi and insects. Even the sun on which they are so dependent can be dangerous; plants grown indoors develop extra photosynthetic cells in the leaves, and a sudden increase in available light can cause leaves to literally overload, then shrivel and fall.
All of these problems—changes in temperature, wind, and light—can be solved by hardening off the seedlings. As planting day approaches, help them adjust gradually to outdoor conditions. This will give them a chance to develop their defenses. At first, just move the flats outside for a few hours in the afternoon; then gradually increase the time they spend in the open air, exposed to the sun and wind. Hardening off is one of the best uses of a cold frame: put the plants in the frame after the first few days, and then leave the lid off for longer and longer periods each day until it is no longer needed. This process could last as long as a week, but doesn’t have to if good transplanting weather comes along.