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A Step-by-Step Guide to Propagation
by Ellen Zachos
Propagation, the practice of making more plants, can be accomplished in several ways. Since unusual plants are sometimes expensive or hard to find, it's handy to know how to start your own. Once you can propagate the plants you want, there's very little to stand in the way of your growing uncommon beauties. The following is an introduction to propagation from seed, from cuttings, and by division.
Seeds
Many of the annuals discussed in this handbook are easy to grow from seed, which is worth trying for several reasons. First, the feeling of satisfaction is enormous. You can look at your garden and say to yourself, "I did this, from start to finish." Second, it's much cheaper to buy seed than to buy small plants. A package of seeds usually costs less than two dollars, even for exotic varieties. And quite a few unusual plants are not easy to find, whereas the seeds tend to be more readily available.
Read the Seed Packet. Most seed packets provide good information, so be sure you read their planting instructions before proceeding.
Moisten the Mix. Next, purchase sterile soilless seed-starting mix and pour it into a large bucket, add water, and stir. The mix should be moist enough to hold a clump when you squeeze it but not so wet that water drips out of the clump when you squeeze. Fill your flat (or large pot) with soil and firm it down. The soil level should reach to half an inch below the edge of the flat.
The basic steps of seed starting, clockwise from top left: Pour soilless mix into a bucket, add water, and stir. The mix should be moist enough to hold in a clump when you squeeze it. Fill a seed-starting flat with the moistened mix, tamp it down, and plant the seeds. Be sure to keep the emerging seedlings moist. To hold in humidity, cover the flat with a see-through lid until the seedlings are so tall that they nearly bump up against it.
Sow the Seeds. Most seeds do not require light to germinate, so they should be covered with a thin layer of the soilless medium. A good general rule is that a seed should be buried as deep as it is large. For example, the very small seeds of Portulaca grandiflora need to be covered by only a very thin layer of the mix. If you're planting morning glory (Ipomoea alba), a seed the size of a kernel of corn, cover with about a quarter inch of mix.
If seeds are large, poke holes in the seed-starting mix with the end of a pencil. Make the holes about half an inch deep and space them two inches apart. Place a seed in each hole and press the mix over the seed with your finger. If the seeds are small, take a ruler and press a long trench, about a quarter-inch deep, into the mix. Scatter the seeds into the trench, then press the mix over the seeds.
If your seeds require light to germinate, like the seeds of stock (Matthiola longipetala) or flowering tobacco (Nicotiana), don't cover the seeds with any mix. Small seeds can be sown in a trench but should not be covered. Large seeds can be pressed into the top of the mix and left uncovered.
If you're sowing seeds in individual pots, you must decide how many seeds to plant per pot. If seeds are large, try one or two per pot; if they're small, try more.
After planting your seeds, mist the soil surface with water to settle the seeds in place. To keep the humidity high, cover the flat or pot with a piece of plastic wrap or glass, or enclose the entire flat in a clear dry-cleaning bag.
You may want to invest in an inexpensive germination/propagation unit, which includes a clear plastic cover for keeping ambient humidity high. Once seeds have sprouted and top growth is approaching the plastic, it's time to remove the dome and think about transplanting. Some propagation setups include an insulated base and heat mat, which is especially handy for starting seeds of plants that require extra warmth and for rooting cuttings.
For the widest selection, it's best to start annuals from seed. Fortunately, they are some of the easiest plants to grow.
Provide Moisture. Your seed packet should tell you how much time the seeds need to germinate. During this time it's important to keep the starting mix moist but not wet; high humidity is also helpful. Check the surface of your mix every day, and if the top feels dry to the touch, water it. Since you don't want to disturb the seeds, be sure to water gently. Try breaking the force of the water by pouring it through your fingers or by using a rose attached to your watering can.
Transplant the Seedlings. The first leaf (or pair of leaves) to emerge is called the cotyledon, which means seed leaf. This is not a true leaf but was once contained inside the seed as a source of nutrition for the young plant. The next leaves to emerge are true leaves. When seedlings have two sets of true leaves, they are ready to be transplanted, either into the garden or into larger pots, where their roots can grow without becoming entangled with those of other plants.
If you're moving plants from a flat into individual pots, be sure not to use pots that are too large. A pot two inches in diameter is large enough for most seedlings. Lift the seedling out of the flat with a spoon, being careful to disturb the roots as little as possible. Hold the seedling by a leaf, never by its stem. If you damage a leaf, the plant can grow another. If you damage the stem, you kill the plant.
Plant Outdoors. Once the risk of frost in your area has passed, it's time to plant your seedlings outside. Transplant seedlings at the same level at which they were originally planted. Do not expose the roots or bury the stem. Remember, your seedlings will grow a lot, so give them room. In most cases, crowded plants don't get the nutrition or air circulation they need and will not grow to their full potential. Water in your new seedlings and give their roots a few weeks to establish themselves before their first feeding.
Cuttings
Propagation by cuttings is another way to increase your plant collection. It's also a good way to keep plants over the winter if you don't have room indoors for the entire plant. Cuttings only work for tender perennials grown as annuals. They will not work for true annuals—these need to be propagated from seed. Tropical perennials like coleus (Solenostemon), Hoya, Mandevilla, and Abutilon are easy to grow from cuttings. You'll need rooting hormone (liquid or powder), a lightweight potting mix, and some small pots (two to four inches in diameter).
Prepare the Potting Mix. Begin by moistening the potting mix and filling your pots as you did for starting seed. Once again, soilless mix should be firmed down and reach to half an inch below the edge of the pot. Taking a pencil, poke one hole into the soil for each cutting you'll start. Poke the hole two to three inches deep.
Prepare the Cutting. Next, take a six-inch cutting from the end of your plant, making the cut just above a node (where the leaves branch out from the stem). Pinch out the growing tip of the plant and remove the bottom two leaves from the stem. Dip the bottom of the cutting into rooting hormone so that it covers the stem up to the node where the leaves were removed. Tap off excess hormone and stick the coated end of the cutting in the potting mix. Firm the mix around the cuttings and water them in.
If you've made cuttings to overwinter some of your favorites, limit their size by growing them in small pots and pinching the growing tips.
Provide Humidity. Cuttings will grow roots along the stem where you've applied the hormone. Because they have no root systems with which to transport water, cuttings need supplemental humidity while they're rooting. By misting the cuttings and enclosing them in clear plastic to create a greenhouse effect, you will improve your chances of success. Placing the cuttings on top of a heat mat can further speed the rooting process.
Transplant the Cutting. When you see new top growth, you'll know roots have established well enough to allow transplanting.
Division
Many perennials are propagated by division, which simply means digging up a large plant and splitting the root system (with top growth attached) into pieces. This technique works for many nonhardy perennials that are grown as annuals. Four-o'-clock (Mirabilis jalapa) is a tender perennial that can be overwintered by digging up and cleaning off the roots after the first frost. Roots should be put in a plastic bag or pot of soil and kept in the dark at temperatures between 35°F and 40°F. Check them periodically to make sure they don't shrivel up or dry out. You may need to water them once or twice during the winter. In spring, roots can be divided by cutting them into pieces with a pruner. Make sure each piece includes at least two nodes. Then pot, water, and move the divisions into the light to restart top growth.
Ellen Zachos is a Harvard graduate and received her Certificate in Horticulture from the New York Botanical Garden. She specializes in tropical plants and has restored several greenhouses in the New York City area, which she now maintains for her clients. Her company, Acme Plant Stuff, installs and maintains commercial and residential interior and exterior gardens in New York City.
Top photos: Neil Soderstrom; bottom photo: David Cavagnaro