Chapter 19

Plant Propagation from Seed

Transplanting Seedlings

Plugs are usually transplantable 3 to 6 weeks after sowing, and a general rule of thumb is when leaves begin to touch one another, the plants are ready to transplant. This usually occurs when the first true leaves appear above or between the cotyledon leaves—the cotyledons or seed leaves are the first leaves the seedling produces (See Figures 19.17). Do not let plants get hard and stunted or too tall and leggy, as this can reduce transplant success.

Steps in Transplanting Seedlings

As you prepare, make holes for the transplants in the medium filling the new container. Gently loosen the germination medium near the roots of the seedling being transplanted. Handle seedlings by the leaves only—even slight pressure on the stem can injure or kill a seedling. Lift the seedling from the germination medium where it has started to grow; allow any medium that is attached to the roots to remain. Place the seedling into the pre-dibbled hole. Dibble boards are often used to punch holes for an entire flat at once (See Figure 19.18). Gently push the root system into the hole. Plant at the same depth as the seedling was growing during germination. Do not bury the seedling deeper or raise it higher.

Plant Growth Regulators

Several bedding plants may begin to stretch, developing long, thin, spindly stems a few weeks after transplanting, especially under low-light conditions in early spring. To keep them shorter, sturdier, and greener some growers apply a growth retardant. Growth retardants decrease the length of the stem (shorten the internode between leaves), thus developing a shorter, sturdier plant.

Containers for Transplanting

A wide variety of containers are available for transplanting seedlings. Containers should be economical, durable, and space efficient. After a container is selected, it can be expensive and time consuming to change to another type. Most greenhouse operations grow a wide variety of species and therefore several different containers are required. Container choice for a particular plant species depends on root system morphology, target plant criteria, and the growing conditions.

Plastic Pots

Plastic containers, round and square, have numerous advantages: they are nonporous, reusable, lightweight, and use little storage space because they will nest. Some types are fragile, however, and require careful handling, although other types, made from polyethylene, are flexible and quite sturdy.

Fiber Pots

Containers of various sizes, round or square, are pressed into shape from peat plus wood fiber, with fertilizer added (See Figure 19.20). Dry, they will keep indefinitely. Since these pots are biodegradable, they are set in the soil along with the plants. Peat pots find their best use where plants are to be held for a relatively short time and then put in a larger container or in the field.

Paper Pots

Paper pots or paper tube pots are more popular with seed plug and cutting propagation of ornamentals, vegetable, and forestry species. They allow for greater mechanization with pot-filling machines, automatic seeders, and wire benches that allow air pruning of the root system. Typically, paper pots consist of a series of interconnected paper cells arranged in a honeycomb pattern that can be separated before outplanting.

Peat, Fiber, Expanded Foam, and Rockwool Blocks

Blocks of solid material, sometimes with a pre-punched hole (See Figure 19.22), have become popular as a germinating medium for seeds and as a rooting medium for cuttings, especially for such plants as chrysanthemums and poinsettias. Sometimes fertilizers are incorporated into the material.

Polyethylene Bags

Polyethylene bags are widely used for growing rooted cuttings or seedling liners to a salable size. They are considerably less expensive than rigid plastic containers and seem to be satisfactory (See Figure 19.23), but some types deteriorate rapidly. They are usually black, but some are black on the inside and light-colored on the outside.

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