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.
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