Transplant production requires five to six weeks, depending on temperature and light conditions during propagation. Ideal seedling size is about 16 cm. Tomato seed germinates best about 25.6°C. Seedling growth is optimal at 18.3°C minimum at night and 26.7°C maximum in the day. Protocols for producing transplants for soil or peat bag culture are similar to those for field crops (Chapter X). Germination rate for greenhouse tomatoes is generally at least 80%, so only one seed need be planted per container. Seedlings for rockwool systems are generally started in a sterile, inert medium, such as rockwool or oasis horticubes and fed nutrient solution. Rockwool blocks are available in several sizes, and are designed so that seeds can be placed directly into the cubes (Fig.7). Flood and drain (ebb and flow) systems can be used to germinate seedlings. In these systems, nutrient solution or water floods a shallow tray containing the sown cubes or pots, providing moisture from the bottom, which spreads through the rootzone by capillary action. The tray is then drained, allowing rootzone aeration. Plants are irrigated several times daily, but usually not at night. Irrigation solution temperature should be at least 18°C. Seedlings to be utilized in perlite bag culture are generally started in mesh bottom pots filled with perlite.
With either a potting mix or rockwool block propagation system, seedlings should be transferred into larger growing blocks (pots) from the original seedling cubes when the first true leaves emerge, then spaced out as they grow. A rule of thumb is that transplant leaves should not be touching. A good transplant is one that is as wide as it is tall. If plants are somewhat "leggy", with long stems, they can be transferred into the larger blocks with their stems bent 180°, so the original cube is upside-down inside the larger block, and the main stem forms a "U" shape, emerging vertically upward from the block. Adventitious roots grow readily from the bent stems. Transplanting into the final growing media should be done before flowering, usually when the plant is 15 cm tall. The final growing media should be properly leached, moistened, and warmed to greenhouse air temperature before planting. Plants should be irrigated with nutrient solution immediately after transplanting.