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Caring For Seedlings
by Shepherd Ogden
It's overcast and light drizzle falls. Frost is done for this season, and temperatures are in the 60s. The soil is dark and crumbly, firm but not packed. The trays of seedlings have been hardened-off. You take one of those stocky plants, knock the plant gently from its pot, and admire the fibrous root ball. You sweep aside a handful of soil, set the plant in the moist earth, and with both hands, pull a bit of soil back around the base of the plant, pressing down firmly. Ahh, to dream.
More often, the sun is merciless, the wind and rain icy cold. Pests and diseases lie waiting. The plants themselves are stressed, too big for their pots, long and spindly. Fortunately, there are things you can do to ensure that your seedlings have a real shot at success, even when conditions are not ideal.
The Indoor Season
Each seed is made up of a tiny genetic model of its parent and a supply of food. This package is dried for storage, but as soon as conditions and genetics permit, it springs back to life. After a few hours (or days, depending on the species), the first seedling root, called the radicle, emerges. As the radicle grows, the seed's starch and sugar reserves are sent to the growing tip, and tiny feeder roots begin to spread throughout the soil. Soon the plant breaks ground and finds the last growth factor it needs: light. A new plant is on its way.
If you didn't add fertilizer to the seed-starting mix, you'll need to start both watering and fertilizing after the seedling develops its first true leaves. From this point on, the new plant is 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 thrive.
Seedlings, such as this white kale, need plenty of light and the right amount of moisture and nutrients. (Photo: Karan Davis Cutler)
Since both heat and light fuel plant growth, the relationship between the two is critical. A common mistake among home gardeners is to keep plants at too high a temperature for the amount of light they receive. What often happens is that the gardener tries to compensate for slow growth with more fertilizer and higher temperatures. The result is limp, leggy seedlings that are hard-put to cope with outdoor conditions when planting time arrives.
On cloudy days, the experienced gardener lowers the temperature to compensate for the lower light levels. While every plant has a temperature range it likes best, within that range, the cooler you keep the temperature, the better off the plant will be. Don't take things too far, though. A combination of low temperature, low light, and overwatering is ideal for the development of the damping-off fungus (for more information, see "Damping-off"). For most common flower and vegetable seedlings, temperatures between 60° and 70°F are about right.
Seedlings also need plenty of light. 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. For the best growth, use fluorescent lights—set four or five inches above the plants' foliage -- and keep them on for at least 14 hours a day.
Fertilizer and water also need to be kept in proper proportion. Plants need nutrients to grow, and without enough moisture they will not only be unable to take up those nutrients, they will wilt and die. However, it is important to remember that too much water washes away the nutrients in the tray or pot, and the plants will starve. The conventional wisdom holds that you should feed young plants every seven to 10 days, but this is not accurate. Rather, feeding should correspond to the number of waterings. One solution is to fertilize each time you water, at 1/4 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 over- or under-fertilization. Leaves that curl under are a sign of overfeeding, whereas discoloration is usually a sign of underfeeding. If the plant is pale, it is likely deficient in nitrogen. Leaves with purplish undersides indicate a shortage of phosphorus; leaves with bronze edges a shortage of potassium. Since seaweed and fish fertilizers contain balanced amounts of all these nutrients, the solution is simply to increase or decrease the strength or frequency of feedings.
Moving Outdoors
It's not enough to raise healthy seedlings if they are so pampered that they can't survive the sun, wind, rain, and seesaw effect of day and night temperature changes. I can get brisk sunny days in the 60s in spring, only to have the temperature fall into the 30s after sunset. But frosty nights aren't the only enemy of tender transplants. Wind can be just as hard on young plants, snapping off brittle stems or flattening them to the ground where they can fall prey to diseases and insects. Even the sun on which they are so dependent can be dangerous: plants grown indoors develop extra photosynthetic cells in the leaves. A sudden increase in light can cause them to overload, then shrivel and fall.
All of these problems are solved by hardening-off, or acclimating, seedlings. At first, move your plants 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, a low, bottomless structure that looks like a miniature greenhouse, with a transparent top that can be opened for ventilation. Keep the frame closed (or open slightly) for the first few days, then open it completely for longer and longer periods each day until your plants are ready to be planted in the garden.
Preparing the Garden
To get the garden ready for transplants, I use a spading fork to turn and open the soil. (I also use a board to stand on, which reduces soil compaction.) Next, I rake the surface to remove any rocks or stubble and break up any clods. If I'm working in a flower r, setting out a single plant or a couple of plants, the process is the same. I turn and open the soil with a spading fork, making sure to remove any debris or clods that might interfere with the transplants' root development.
If your soil is well prepared—rich in organic matter—making a transplant hole is no more than a poke of the finger. For larger plants, a quick scoop with the hand is all it takes. If your garden soil is rich and healthy, no additional fertilizer is necessary at planting time; if your soil isn't in top-notch shape and does not have sufficient nutrients, you may want to add compost to each planting hole or row.
If your plants show symptoms of nutrient deficiency (see above), side-dress or apply alongside them an organically stable fertilizer that they can take in immediately, such as fish emulsion.
Transplanting
The process of setting plants outdoors is fairly straightforward. In the vegetable garden, I like to plant in beds, setting the seedlings equally distant from one another in a hexagonal pattern. Once the plants grow to full size, their leaves will cover the entire surface of the bed. This arrangement reduces the plants' competition with weeds and makes the best use of the space available. If you're setting out perennial transplants—species that will remain in the same spot for many years -- be sure that you've loosened the soil to a good depth and width and enrich it with organic matter, such as compost.
When I work with transplants, I always hold them by the root ball or by the leaves, never the stem. Healthy tops can grow new roots, and a good root system can replace damaged or missing leaves, but the stem is the bridge between the two. Hurt it and the plant has no way to move its nutrients and water.
I loosen the roots a bit if they're especially tangled, then place them firmly into a planting hole at the same level, or very slightly deeper, than they were in the container. A few species—tomatoes are one example—can be planted much deeper than they were growing: The buried stem will sprout new roots.
If plants have thick, fleshy roots, spread the roots and place them in a hole that is large enough so they don't need to be doubled over. Sift soil in around the roots to make sure that no air is trapped underneath, and water well. If the top of the plant is large, stake it so the wind can't pull at the roots before they reset their anchor.
Fibrous-rooted plants that have filled a pot and made a tight ball of soil and roots can't really be loosened. I've found that kneading a fibrous root ball rips out a small percentage of the tiny root hairs and gets them to start growing again. It's like shaking a friend to wake him or her from a bad dream: You can't be too gentle or you won't get anywhere.
Sun
Garden plants live on sun. But too much is almost as bad for plants as it is for people. When you move plants outside, they get double or triple the light they're used to. That's why hardening-off transplants is so important: it gets them used to all the conditions of the open garden.
Wait for an overcast day to set your seedlings in the garden; if you have to transplant on a sunny day, wait until late afternoon, when the strength of the sun has mellowed. That gives the plants overnight to adjust. If the following day is going to be sunny too, give your plants some shade until they get established.
One way to provide shade is to put the seedlings in a box or basket, but make sure there are plenty of holes to let in light and air. Even a couple of evergreen boughs will provide welcome respite from the sun. You can shade large numbers of plants with salvaged window screens propped up on blocks over the row or bed. A high-tech and more expensive alternative is to put up tunnels using polypropylene shade cloth supported by wire hoops.
Water
Whether it's sunny or cloudy, water in transplants well. If you can get a hose into the garden there is a great way to get plants off to a good start: Take a hose with a trigger-grip nozzle down the row, aim the nozzle at each spot where a plant will be set, and pull the trigger for a second. The blast of water will create a small hole full of soil soup into which the waiting plant can be dropped. Pull dry soil from the surrounding bed loosely over the moist area, leaving the transplant at the same depth it had been in its container.
You can also mulch transplants to help lower the evaporation rate of water from the soil and temper its climate. In warm or hot regions, use a thick cover of organic matter that is free of weed seeds; in cool climates, clear or black plastic is preferable because it helps retain moisture while letting the sun warm the soil. Water thoroughly or wait for rain before applying plastic mulches or you'll have a mini-desert under the mulch.
During a drought, water your plants in the evening so that evaporation won't steal the water before it has a chance to soak into the soil. Second best is very early morning, before sunrise. If you don't have a mulch, cultivate lightly as soon as the surface of the soil is dry. This breaks the capillary action of the soil and prevents moisture from wicking to the surface, where it can be drawn off by the wind.
Wind
Wind can be even more stressful than sun, as wind drastically increases the flow of water through the plant. The roots draw water from the soil, which travels up through the stem to the leaves, where most of it transpires into the air. This movement of water is essential to the plant, because the pressure of its passage helps hold the leaves upright and open to the sun and because, without enough water, photosynthesis -- the chemical conversion on which the plant is entirely dependent for its growth—grinds to a halt. When a seedling is set out in the garden, the tops are usually capable of moving a lot more water than the roots can supply. That's why you want to be careful not to disturb the roots -- every little bit of extra water capacity helps.
If your newly set plants start to wilt and watering doesn't help, you can cut off up to a third of the top of the plant. That puts the plant back into balance by reducing the amount of leaf surface that is demanding water. The leaves will grow back, and the plant will be better off without the wilted leaves, which offer a spot for diseases to get established. As long as the plant's roots are healthy and the stem undamaged, the plant will recover quickly, and new leaves will grow.
The same shade covers that protect the plants from the sun will also keep the wind at bay; if the plant is too tall for a basket or a tunnel, put up a wind screen.
I once had to move a whole bed of large foxglove plants. I kept a shovelful of soil with each plant and after putting them in their new home, watered them in well. Then I put up a wind screen on the north and west sides of the bed. It was a relatively simple affair—commercial shade cloth stapled to 6-foot bean poles—but it kept the plants from being exposed to the full force of the wind. My wind screen worked beautifully—of more than a hundred plants, I didn't lose a single one.
Cold
Frost is the major concern of most gardeners when they set out their plants each spring. The last-frost date for any area is an average of many years, but you can't go by only the calendar when it comes to frost. Experienced gardeners use local signs—the blooming of certain plants or the appearance of particular migratory birds—to determine when the time is right to plant each crop.
The same devices that shelter your plants from too much sun and wind will give some protection from frost, with one difference. The more porous the cover—which is important for both sun and wind protection because of the need for ventilation—the less effective it will be against frost. The traditional fall frost protectors, such as blankets and sheets, don't work well for seedlings because their weight can damage the seedlings. Metal and glass, which conduct cold, are less effective than wood, plastic, and fiber covers.
There are many kinds of row covers and cloches available for frost protection, but if they are transparent they are likely to overheat on warm, sunny days. The best, in my opinion, are the spun-bonded floating row covers. These translucent white covers are so light they don't need any supports to keep from weighing down the plants, yet they provide 3° to 5°F of frost protection. In my garden, they have protected squash seedlings down to 28°F. Since they are relatively porous, they allow rain to pass through.
Pests
Even if your transplants survive the vicissitudes of the weather, there are still creatures that would love to make a meal of them Fortunately, there are effective ways to deal with these nightmares.
Take cabbage root maggots, for example. The pesticide typically recommended is Diazinon, but I won't use it. In the first few years of market gardening, I tossed a handful of wood ash around the base of each transplant. It worked but was too time consuming. Paper collars worked, too, but without the benefits of the potassium in the wood ash.
Then I read that the cabbage fly lays its eggs only until May 15 in my part of the world. Sure enough, timing the crop so that I didn't transplant until after the fly was finished cut losses and our labor. Like timing for last frost, this is not a solid date, and asking experienced gardeners in your own area will give you an idea of how changing your garden schedule might make pest control a lot less trouble.
I also discovered that insects couldn't get under floating row covers if I sealed the edges with soil. Now, when I transplant, I cover the plants immediately with a floating row cover. The milky white color and grainy texture of the covers gives seedlings in the cabbage or mustard family respite from sun and wind, protects them from frost, and is 100 percent effective against cabbage root maggots. Better still, row covers protect against a host of other pests, making pesticides unnecessary.
Diseases
Diseases have diverse causes and symptoms. Many are already in your garden soil; others are brought in on seeds and transplants. The best cure is prevention: Buy seeds from reputable dealers or save your own, plant disease-resistant cultivars, and examine transplants carefully.
Healthy, vigorous transplants are less likely to be infected than spindly, weak ones. Make sure your plants aren't stressed—that they have the right amounts of light, water, and nutrients—by protecting them from temperature extremes and making sure they have adequate air circulation. Practice good sanitation in the garden: don't work around plants when they are wet, remove and burn any dead or diseased plants, and practice crop rotation.
Shepherd Ogden, who gardens in southern Vermont, is the founder of The Cook's Garden, a seed company specializing in vegetables, herbs, and flowers for the kitchen garden. A former market gardener, he is a frequent contributor to BBG handbooks and is the author of several garden books, including Step by Step Organic Vegetable Gardening (HarperCollins, 1992).