Starting your seedlings indoors, or purchasing greenhouse-grown transplants from your local garden store, can give you a head start that will add weeks to your growing season. This practice can make or break a crop if you live where summers are short, or in a climate where midsummer heat leads to a melt-down of cool-natured plants.
Growing your own seedlings indoors is easy and fun because you can try vegetable varieties rarely offered at garden centers, and start them under pest-free conditions (see Seed Starting Basics). But take some time to “harden off” transplants before setting them out in your garden.
1. Harden Off Plants
Plants adjust to small, gradual changes in their environment better than sudden shifts, which is what the hardening off process is all about. Just as you get burned when you bare your untanned skin to the sun, leaves of plants started indoors will develop pale, sunburned patches if they are suddenly exposed to too much sun. The story has a happier ending when you introduce seedlings to bright light and sheltered breezes gradually, over a period of one to three weeks. In response to more abundant light, the leaves and stems bulk up on chloro-plasts — the mighty green organelles that transform light and carbon dioxide into energy for the plant. Much of that energy is sent to the main stem, which suddenly needs to get tough enough to twist and bend without breaking when blasted by wind. You may not see much new growth while plants are hardening off. But don’t worry; they are busy growing and rearranging exactly the kinds of cells they will need to prosper once they are set free in the garden.
This period of adjustment can take place on your deck or patio, within a protected enclosure out in the garden, or a little of both. For example, you might begin by setting your seedlings outside in a sheltered spot for two hours one day, and four hours the next, gradually increasing light and air movement levels until the babes have made it through several days and nights in the outdoor world.
If you are away from home all day, put the plants in a location that will receive early morning sunlight and be shaded as the sun’s position changes late in the morning. Watch your seedlings closely for signs of drying out, and be ready to shift them to larger containers if you see roots sneaking out of the drainage holes. If frost is predicted, take the plants inside.
2. Prepare to Transplant
While your seedlings are hardening off, prepare the planting space by amending the soil with compost and mixing in an appropriate amount of your favorite organic fertilizer. (For one strategy, see A Better Way to Fertilize Your Garden.) Both additions energize the soil’s food web, so that seedlings slipped into planting holes will be met by a welcoming committee of root-friendly fungi, bacteria and water-soluble nutrients. Some plants form stronger alliances with soilborne microorganisms than others, but soil scientists think that all plants take up nutrients and resist diseases better when they have strong relationships with life-forms that flourish in the rhizosphere — the fertile real estate where root and soil come together. Mixing at least a handful of compost into the bottom of planting holes used for vegetable seedlings helps these partnerships fall into place quickly.
When the plants have been hardening off for at least a week, and the soil is ready to receive them, check your weather forecast. Soil temperatures should be to the plants’ liking, and it’s helpful to have some cloud cover or light rain during the first few days after transplanting. Delay transplanting if a heat wave is on the way, or if the soil is clammy and cold. Meanwhile, make sure the patients are well hydrated and amply fed going into the transplanting operation. Drench them with a weak solution of organic fertilizer the day before transplanting and again just before you remove them from their pots.
3. Handle With Care
Nature occasionally transplants a broken stem to a moist spot where it takes root and grows, but she never lifts, handles and replants six-week-old seedlings! To make this bizarre adventure more acceptable to plants, push them out of their containers from the bottom rather than pulling them out by their stems. In most cases, you can place one hand over the container with the main stem between your fingers, tip it over and shake or tap to loosen the root ball. If necessary, pry roots loose using a knife. Handle the freed plant by its root ball (a large spoon is great for this), and avoid touching the main stem. As you set the plant in its permanent home, use the lowest leaves as handles. Should they break off, it’s no big deal.
As a general rule, it’s best to keep as much soil packed around the root ball as possible. Exceptions are seedlings whose roots have grown into a solid mass. To encourage these frustrated roots to grow outward into surrounding soil, you should use a fork or your fingers to tease out a few outer roots. Finish the transplanting process by drenching the soil with water — an important step that eliminates air pockets around the roots and helps beneficial soilborne microorganisms move into the plant’s rhizosphere.
4. Cover Plants
Nicely hardened-off seedlings transplanted in perfect weather (mild, cloudy and still) can fend for themselves, but great transplanting weather seldom lasts long enough. To be safe, temporarily shield transplants from sun and wind by covering them for at least two days after transplanting. Increase to four or five days if the weather is very sunny, windy or cold, or if the plants’ roots were damaged as you set them out. For example, if you found three plants rather than one in a purchased pot of basil, and you separated them as you set them out, covering the plants for four days or more after transplanting can make the difference between success and failure.
You can use upturned flowerpots, cardboard boxes or buckets as plant covers. In hot weather, opt for a piece of lightweight cloth held aloft with stakes. Remove the covers after a few days, and then pat yourself on the back for a job well done. After all, one of the huge differences between people and plants is that we are built to move around, while a plant’s idea of a great life is to stay in one place. For plants, good transplanting practices soften the transition from life confined in a pot to life in a garden. For people, they open the door to earlier harvests and a more successful garden.
The Seaweed Solution
Many gardeners and market growers use kelp-based fertilizers to help transplants get off to a fast start. Most of these products are made from Ascophyllum nodosum — a brown, ribbon-like seaweed, often seen attached to a floating bladder — that grows in cold ocean water. Kelp contains growth hormones that encourage root formation, as well as 60 or more micronutrients. In addition to applying kelp mixed with water to the roots of transplants as they are set out, many gardeners report that drenching plant leaves with seaweed (or combination seaweed/fish emulsion sprays) every three weeks results in healthier, more vigorous plants. Some products are not organic due to how they are manufactured, but the Organic Materials Review Institute lists more than 30 kelp-based products approved for use by organic growers.
Contributing editor and gardening expert Barbara Pleasant is passionate about using “found items,” such as milk jugs and cardboard boxes, in her vegetable gardens to protect tender new transplants.