It is Transplant time !

Ajay Nair

Department of Horticulture 

Iowa State University

Transplant production plays a key role in determining a successful vegetable production system. Growing good 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 and relative humidity, proper irrigation and fertilization, and pest and disease monitoring and management, growers can produce high quality transplants that contribute towards higher yields and productivity.

Why grow transplants?

Benefits of using transplants are many: early start, uniform growth, healthy root system, and generally free from pest and diseases. One of the major advantages is that the grower has a better control over growing conditions so that the plants are ready when needed, eliminating problems associated with unavailability of transplants during high market demand, importing of diseases with purchased transplants, and planting delays due to inclement weather conditions. 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 seeds, and earlier fruit maturity than direct seeded plants.

Greenhouse sanitation

The first and the foremost thing is to prepare the greenhouse for transplant production. Practice good sanitation. Transplant production greenhouses should be free of plant debris, accumulated soils, and weeds which may harbor insect pests and diseases. Greenhouse 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 greenhouse 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. Sanitation is also applicable to personnel entering and handling plants in the greenhouse. This will help eliminate spread of pest and diseases due to human contamination (dirty hands, handling plants after smoking tobacco, and soil and dirt carried in shoes).

 Ventilation and heating

Greenhouses should ensure proper ventilation, air flow, and heating for healthy plant growth. 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 greenhouse can maintain low relative humidity thereby eliminating common diseases such as botrytis gray mold and late blight. Growers should make sure that there heaters and furnaces are properly working and maintaining desired air temperatures. 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 crop requirement. Most plant species grow very little at temperatures less than 45 to 50 degrees F (warm season vegetables like solanaceous crops) while cool season vegetables like lettuce, spinach, cabbage, broccoli, etc will grow under temperatures not suitable for warm season vegetables.

Flats and plugs

Multi-cell plastic trays are widely in use for transplant production. These trays are available in wide range of cell sizes. Cell sizes could range anywhere from 50-800. Selection of tray size 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 have larger root system like asparagus. Larger cell sizes have the advantage of holding more media, thus moisture and nutrients, but compromise on greenhouse space. For smaller cycle crops such as leafy greens a smaller cell is appropriate as the root growth seldom fills the cell space. Most growers reuse transplant trays and containers for economic and environmental reasons. Trays and containers should be sanitized with sanitizing solutions after each use to prevent the spread of diseases. 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.

Seed quality

Seed quality is an important parameter that is critical for successful transplant and crop production. High quality seeds 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 older seeds, perform germination tests preferably a month before the actual start date of transplant production. When purchasing new seeds, growers should purchase seeds from reputed and trusted seed firms and select cultivars that are adapted to ones area and growing conditions.

Growing media

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.

Irrigation management

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. Plant water uptake is influenced by cloudiness, sunlight, and temperature thus water sparingly on cloudy days.

Nutrition management

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 500 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 produced as a result of low light conditions coupled with high fertilizer rates, and/or over watering.

Greenhouse lighting

In the northern climate zones like the Midwest, short day lengths 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. Research is ongoing in various US universities including Michigan State on standardizing and optimizing the use of LED (Light Emitting Diodes) lights for use under greenhouse conditions.     

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 (http://attra.ncat.org/attra-pub/PDF/greenhouseipm.pdf). Three insect pests that present the largest routine problems in the greenhouses are white fly, thrips, and fungus gnats. Disease organisms that cause root rots are also a grave concern as initial phases of seed germination and the early stages of seedling growth are highly susceptible. 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 insecticides and fungicides should be used according to labeled rates and timing.

Hardening off

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 degree 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.