Various means of increasing plants can be used successfully by the gardener and can be accomplished without high-tech tools or elaborate equipment. The four main methods of propagation are by seed, division, stem cuttings, and root cuttings.
Seed and Asexual Propagation
Seeds are used for propagation of those true species that come relatively true from seed (for example, hybrid columbines). However, many desirable garden cultivars must be asexually propagated. Four seed germination requirements must be understood to ensure success.
Use Fresh Seed
The primary means of survival for many perennials is through overwintering crowns of rootstocks. Seeds formed in summer and fall must germinate rapidly. If germination is slow, seedlings will be too tender to overwinter. Unfortunately, storing seeds in small foil, paper bags, or your favorite containers of seed companies is not a condition to which they are accustomed. Often, the purchased seed has already gone dormant and, regardless of how nice the germination conditions are, will not germinate.
The following seeds particularly benefit if sown fresh:
Many species do not require any specific pregermination conditions, but some seeds germinate more rapidly and more uniformly with pretreatments.
This is a moist, cold treatment that essentially duplicates winter conditions for seeds. Seeds should be stored in moist sand or sown in a moist seed mix and placed in a refrigerator at 35 to 40°F. Generally, a period of 4 to 6 weeks is necessary for stratification.
Stratification may also occur naturally outdoors under the snow, in an unheated greenhouse, or in a cold frame. In most cases, stratification can begin immediately after the seed has been purchased, but if seed is collected, allow it to ripen at room temperature for 2 to 3 weeks.
Seeds that benefit for stratification are:
- Clematis recta
- Geranium Pratense
- Phlox paniculata
- Rudbeckia laciniata
This involves making a small cut or abrasion in seeds with hard seed coats to help them absorb water more effectively. This can be done with a sharp knife, a nail file, or sandpaper. Soaking in hot water (170 to 210°F) is also effective in softening the seed coat; allow seeds to remain in the soaking water as it cools.
Seeds that benefit from scarification are:
For most seeds, rapid and uniform germination occurs under warm, humid conditions. Constant temperatures of 65 to 75°F enhances germination. Covering the seed tray with a pane of glass or a plastic bag can help keep the temperature and humidity constant; use a fine-mist sprayer to ensure a moist environment for germination. Germination can be done indoors, in a home greenhouse, or in a south-facing cold frame. Humidity is more important than warm temperatures for seed survival; if trays dry out, the seeds will die. If temperatures are too cold, germination is delayed and nonuniform.
These perennials should be sown at cooler temperatures (55 to 65°F): Arabis, Astilbe, Aubrieta, Aurinia, Campanula, Centaurea, Dianthus, Digitalis, Erigeron, Gypsophila, Limonium, Lychnis, Oenothera, Papaver, Penstemon, Viola.
The presence or absence of light is not important for most seeds. Very small seeds should be planted on the surface and barely covered with fine sand or vermiculite. Larger seeds, those easily handled, may be planted 2 to 3 times deeper than their diameter.
Some seeds benefit from germination in the dark, including:
Conditions optimal for germination are detrimental to growth once seedlings have emerged. After the first true leaf is visible, the seedlings should be removed from the warm, high-humidity conditions and placed in cooler, brighter conditions. This is necessary to “harden off” the seedlings and is essential for strong growth. One of the most serious problems is the lack of sufficient light to allow the growth of the seedlings. A cold frame is most useful for seedling development.
Fertilize seedlings with a dilute solution (at half the recommended strength) of potassium nitrate or a complete fertilizer. Transplant to a bigger pot as soon as the seedlings can be handled. Always handle seedlings by the leaves, not the stem. Once 2 or 3 true leaves have formed, fertilization rates can be raised. Plant in the garden, when the roots fill the final container.
This is the simplest but often the most inefficient method of propagation. Plantlets, with some roots attached, are separated from the “mother” plant. In some cases (Aster, Boltonia), literally, hundreds of plantlets may be obtained, but with others (Hemerocallis, Iris), the mother plant yields only 1 or 2 divisions. The division is best accomplished in the spring or early fall. If temperatures are not too hot and water can be provided, the division can be done almost anytime during the growing season. Once plantlets have been divided from the mother plant, plant in the garden immediately. The size of the division depends on location, weather, soil, and time of planting.