How to Propagate Plants
When seeds are unable to reproduce naturally, propagating helps you make more plants.
2011, Dorling Kindersley Limited
Keep a supply of plant propagation materials on hand for use when needed. Label plants as they are sown to avoid confusion later on when seedlings appear.
Most plants reproduce by seed, but this can be hampered by several factors. For example, adverse weather conditions can reduce insect populations and pollination. To counter such problems, plants have evolved backup methods. Perennials often form large clumps made up of many small plantlets that can survive on their own. Other plants can regenerate from a section of root or stem separated from the main plant, and the flexible stems of climbers and some shrubs take root if they rest on the soil. These traits are exploited by gardeners to make new plants.
A few basic items are essential for successful propagation. You will need a pencil or waterproof pen and labels for identifying plants; pruners and a sharp knife for taking cuttings; plastic bags, and elastic bands to secure the bags over pots; a small sieve for dusting soil over seeds; a block or piece of board for firming soil before sowing; and a dibber or pencil to make planting holes and lift seedlings.
Trays, Modules, and Pots
Seed trays are useful for raising large quantities of seeds. The usual size is about 14 x 9 in (35 x 23 cm); half-size trays are also available. Module or multicelled trays have a number of internal divisions, each treated as a separate small pot. These eliminate the need to prick out young seedlings and are useful for minimizing root disturbance or growing several different plant varieties together. Root-trainers are deep modules with ridged sides to help long roots grow straight.
For single or small numbers of seeds or cuttings, 3–31⁄2-in (8–9 cm) pots are the most suitable size; for larger quantities, 5–6-in (13–15-cm) pots are more economical. Plastic pots are easier to use than clay ones, and retain moisture for longer; either type must have drainage holes at the bottom. Biodegradable pots eliminate root disturbance when seedlings are transplanted to larger pots or outside into the ground. As your seedlings grow, they will have to be moved outside to harden off.
Keeping Seedlings Covered
Most seeds and cuttings need protection from drying out. Small sheets of glass or clear plastic lids are ideal for seed trays. Enclose individual pots in clear polythene bags held clear of leaves by canes or hoops of wire. Propagators consist of a tray and clear plastic cover, and are available in various sizes. They keep seeds and cuttings warm and the air inside humid. Heated propagators, especially those with an adjustable thermostat, can maintain higher temperatures to speed up rooting.
For propagation outdoors, glass or plastic cloches offer a little protection from low temperatures and bad weather, and are most effective if you can regulate their ventilation and open them easily for watering.
Choosing a Planting Medium
You will need a suitable potting or rooting mix in which to sow seeds or grow plants. Some plant types need special mixes: alpines, for example, grow best in very sandy or gritty conditions, while acid-loving plants such as heathers and pieris must have a lime-free (“ericaceous”) potting mix.
- Seed or cutting mixes, are ideal; most potting mix is too rich for seedlings. Soil- or loam-based types dry out slowly. Soilless mixes are light and clean, but can be hard to remoisten once dry.
- Horticultural grit on top of soil helps water drain away from stems; in the base of a pot it prevents waterlogging. When sowing water-sensitive plants or rooting cuttings in a cool season, add coarse sand for drainage.
- Vermiculite is a sterile, lightweight material that holds moisture and improves aeration. Choose a grade that is intended for propagation. Perlite is similar—both are added to potting mix or used on their own for rooting cuttings.