Dr. Ajay Nair
Assistant Professor, Department of Horticulture
Iowa State University
Benefits of starting your vegetables from transplants are many and include: early start, uniform crop growth, and healthy root system. Vegetable transplants provide a head-start to target early-season market, insure a good stand of vegetable plants without the uncertainty of direct seeding or the added cost of field thinning, especially with the ongoing high cost of hybrid seed. However, production of transplants calls for early planning and optimum utilization of available resources. This is critical especially for vegetables like tomato, pepper, and celery that need sufficient growing time to attain adequate size. Some factors that warrant attention are greenhouse sanitation, transplant trays/flats, growing medium, seed quality, light, temperature, irrigation, fertilizers, pest and disease management, and finally hardening.
The first and the foremost thing is to prepare the greenhouse/hoophouse for transplant production. Practice good sanitation. The area should be should be free of plant debris, last year’s residues, accumulated soils, and weeds which may harbor insect pests and diseases. Benches, floors, should be properly disinfested prior to use. Products such as chlorine bleach, Green-Shield, Physan 20, Oxidate (hydrogen dioxide, BioSafe Systems), chlorine dioxide, etc. could be used to clean and sterilize benches. Product labels need to be followed precisely depending upon purpose and mode of use. Disinfecting benches and floors is an effective method to prevent disease causing organisms such as Pythium and Rhizoctonia.
Ventilation and Heating
Good ventilation and air circulation is important as it helps keep the foliage dry and mitigate various bacterial and fungal diseases. Well aerated and ventilated production areas can maintain low relative humidity thereby eliminating common diseases such as botrytis gray mold and late blight. Uniform temperatures are essential for adequate control of plant development and production of uniformly sized transplants. Thermometers should be placed at plant level in several locations to monitor air temperature. Temperature control is critical and should be tailored based on specific crop requirements.
Multi-cell plastic trays are widely used for transplant production. These trays are available in a wide range of cell sizes and number. Cell numbers range anywhere from 50 to 800. Tray selection depends upon plant species, duration of transplant growth, and available greenhouse space. Larger cell sizes can be used for transplants that need longer growth cycle (>5 weeks) and carry large root systems such as asparagus. Larger cell sizes have the advantage of holding more medium, thus moisture and nutrients, but compromise on greenhouse space. For small duration crops such as leafy greens, smaller cell is appropriate as the root growth seldom fills the cell space. Cell sizes also have an impact on spread of diseases, as closely packed cells increase relative humidity and create conducive environment for bacterial and fungal growth. Most growers reuse transplant trays and containers for economic and environmental reasons. Trays and containers should be sanitized after each use/season to prevent the spread of pests and diseases.
Seed quality is critical for successful transplant and crop production. High quality seed have higher germination percentages as compared to old seeds that often have poor germination and reduced vigor, which leads to, lost uniformity, revenue, and productivity. If using old seed, it is important to perform germination tests preferably one month before the actual start date of transplant production. When purchasing new seed, growers should purchase seeds from reputed and trusted seed firms.
Vegetable growers have the option of using soil mixes or artificial mixes when producing vegetable transplants. The best growing medium is the one which has good moisture-holding capacity, good drainage, and high nutrient retention capability. There is a huge selection of various soil-less commercial media available for transplant production. These soil-less mixes are sterile, easier to handle and in addition carry small quantity of start-up fertilizers for initial seedling growth. In case of soil mixes, it should be made sure that the soil is sterilized, light, well drained and free of herbicide and pesticide residues. A soil test could be useful to correct for pH and fertility problems. Organic growers should make sure that the medium used for transplant production is approved by OMRI.
The success of transplant production can be stymied by improper irrigation management. Proper irrigation management is crucial to produce strong, sturdy and healthy transplants that grow well and yield a quality crop. Some things to consider while making irrigation decisions are when, how, and how much to water. In early stages of seed germination it is critical that the media is kept moist at all times. As the seedlings grow roots and get established water the plants only when moisture is needed. Water should be evenly distributed across the flats and uniformly applied without missing corners of flats and benches. Irrigation schedule would depend on plant species, flat or container size, crop growth, and prevailing environmental conditions inside the greenhouse. Over-watering, a very common error, results in restricted root growth and poor quality plants that are susceptible to insect pest and diseases. Over watering often promotes growth of fungus gnats and certain diseases especially damping-off. It is a good practice to water the plants in the morning, to permit foliage and soil surfaces to dry before evening.
Fertigation, or the use of water-soluble fertilizers at the time of each watering, is a common method of fertilizing vegetable transplants. No generalized fertilization regimen is available but a common alternative include a 75 to 100 ppm N solution applied twice weekly, or a 300 to 400 ppm N solution applied once a week. If higher concentrations of fertilizer are used periodically, it is recommended to occasionally flush the root zone in order to mitigate salt accumulation. Adjust fertilization according to temperature and light conditions. Tall and leggy transplants are a result of low light conditions coupled with high fertilizer rates, and/or over watering.
In the northern climate zones like the Midwest, short day lengths and lack of sufficient quantity of light during spring season can adversely affect transplant production and limit plant growth. High-pressure sodium lamps (400 watt) are commonly used for supplemental lighting in greenhouses; however, escalating energy costs make their use cost-prohibitive. If growing transplants in a building/non-greenhouse environment, additional lighting using fluorescent light bulbs should be provided. Place the bulbs 2-3 inch from the top of the plants. The natural light from a window is seldom enough for good, strong seedling growth. Plants stretch and lean towards the light and will not yield sturdy transplants. Fluorescent light fixtures and tubes are the best choice for growing transplants indoors.
Pest and Diseases
Warmer temperatures inside greenhouses promote optimum transplant growth but also create environment for rapid growth and development of pest and disease populations. The best approach to keep greenhouse clean and transplants free of pest and diseases is to develop a management plan based on sanitation, monitoring, and understanding of pest and disease life cycle. A good article to read in this regard is the article on integrated pest management in greenhouses on ATTRA website (https://attra.ncat.org/). The best control for pests and diseases is proper greenhouse sanitation, proper formulation of the growing media, proper irrigation scheduling, fertilization practices, and constant monitoring and removal of infested or infected plants. A number of disease problems can be minimized by keeping the water off the foliage and low level of relative humidity in the greenhouse. Under severe outbreak of pest and diseases selective pesticides should be used according to labeled rates and timing.
Plants should be acclimated to the shock and stress of transplanting into the field by hardening them off. This can be achieved by removing the optimum growing conditions of the greenhouse by reducing the amount of water and/or fertilizer and reducing greenhouse temperature by 5-10 °F. Alternatively transplants can also be hardened by moving out of the greenhouse and placing them in a lath house for a week or two before transplanting. Hardened plants can rapidly recover from transplant shock and better withstand adverse field conditions such as low temperatures and high winds.
Transplant production plays a key role in a successful vegetable production system. Growing high quality plants requires skill, proper care, and knowledge of the fundamentals of crop production. With adequate greenhouse sanitation measures, good quality seed and growing medium, greenhouse environmental conditions such as light, temperature, relative humidity, irrigation, fertilization, pest and disease monitoring and management, growers can produce healthy and disease free transplants that contribute towards higher yield and productivity.