In Pennsylvania, vegetable crops that usually start as transplants include tomato, pepper, eggplant, broccoli, and cabbage. Squash, cucumbers and pumpkins are also commonly started using greenhouse transplants. Producing quality transplants in a greenhouse requires healthy seed, a suitable potting mix, clean planting trays, fertilizer and water.
Purchasing seed that is as disease-free as possible is an important step in producing transplants that are vigorous and healthy. Many of our vegetable disease problems, including bacterial and viral diseases of tomato and pepper, can start in the greenhouse from infected seed. When infected transplants are planted in the field, these diseases can cause considerable crop loss later in the season. It is recommended to buy disease-indexed seed when available from your seed supplier. In some cases, hot water baths or chlorine rinses can be used to reduce pathogenic bacteria on the surface of seeds such as tomato, pepper, and cabbage. However, hot water treatments must follow strict time and temperature steps or seeds may be injured.
Potting Mixes/Growing Media
A suitable potting mix (also called "growing media") will support developing seedlings by providing air, nutrients, water, and a place for roots to grow. Most potting mixes are soilless to avoid soil-borne diseases and promote good drainage. A mix of peat moss, vermiculite or perlite, and sometimes fertilizers, can provide a suitable environment with sufficient water-holding capacity, nutrient content, and aeration for plant growth and development. Clean, finished compost can also be added to potting mixes. Compost adds organic matter to the mix and supports beneficial microbes that can suppress soil-borne diseases.
Numerous commercial mixes are available for conventional and organic growers. Most of these mixes will produce high quality transplants when used with good management practices. Commercial growing media will have added lime and may or may not have fertilizer added. Certified organic growers can find commercially available potting mixes that are organic. Check labels carefully to make sure you know what the ingredients are in a commercial mix and check to see if it is listed by the Organic Materials Review Institute (OMRI). It should state "OMRI listed" on the packaging.
Making Your Own Mix
Growers sometimes prefer to make their own potting mix. Advantages of making one's own mix may include uniform and consistent composition, flexibility for making special mixes, and possibly lower cost. The ingredients of soilless mixes (peat, vermiculite, perlite, plus lime and fertilizers) are available to purchase separately. Compost can be purchased or produced on-farm. There are many recipes for conventional and organic growing mixes, but a commonly used one is:
- 50-70 percent sphagnum peat
- 25-50 percent vermiculite
- 5 lbs ground or superfine dolomitic lime per cubic yard of mix
- plus added fertilizers
Flats and Trays
Flats and trays used in the production of transplants should be new to avoid pathogens that cause damping-off and other disease problems. If flats and trays are reused, they should be thoroughly cleaned after use and disinfected using a chlorine solution or a Q-salt product (for example, products such as Greenshield, Physan and Prevent). Allow flats and trays to dry completely before reuse.
Most vegetable transplants are grown in plastic trays with individual cells for each plant. Trays vary in size from 32 cells to over 500 cells per standard 12 x 24 inch tray. Larger cell sizes (32, 50 or 72) are best for vine crops (squash, melons, cucumbers and pumpkins). Tomatoes, peppers and eggplants do well in 72 and 128 cell trays. Small cells (128, 200, 288) are suitable for onions and lettuce.
Water and Fertilizer
Throughout seedling growth, keep the potting mix moist but not continually wet. Water less in cloudy weather, and try to water in the morning so that plant surfaces dry out before the end of the day. If you are growing plants in media without added fertilizer, you will need to water with added liquid fertilizer once the seedlings have emerged. Plants grown in media with added fertilizers will require liquid fertilizer starting 3-4 weeks after emergence. Use a fertilizer that is formulated specifically for greenhouse transplants: it should be completely soluble in water, lower in P (phosphorus) than N (nitrogen) and K (potassium). Follow fertilizer recommendations for application rate (concentration) and frequency carefully. Too little fertilizer will slow plant growth; too much fertilizer can cause excessive growth or burning of the plants.
Mid-Atlantic Commercial Vegetable Production Recommendations , DuPont, T. 2012.
Potting Media and Plant Propagation , Penn State Extension Start Farming.