Growing high quality transplants helps get vegetable crops
off to a strong start in the field, and it’s essential to customer
satisfaction if you are selling transplants. At the recent New England
Vegetable and Fruit conference I picked up many of the following tips
on transplant production from presentations by Jan van der Heide of
Bejo Seeds, David Hambleton of Sisters Hill Farm and Meg McGrath of
Cornell, among others.
Transplant goals. A good transplant is
sturdy, free of diseases and insects, not root-bound, properly
hardened-off, and the right size and maturity for setting out.
Achieving these characteristics requires attention to detail when it
comes to the quality of your potting mix, sanitation, pest management,
germination, plug size, watering, greenhouse conditions, and how you
harden off the plants.
Potting mix. Problems with
poor transplant performance such as poor color and uneven growth can
often be related to improper soil pH, inadequate fertility or high
salts in the potting mix. When buying a mix, ask your supplier for a
copy of its lab analysis; if making your own mix, send a sample to a
lab well in advance of seeding. It’s a good idea to maintain annual
records of the analyses of your mixes so you can compare a ‘problem
mix’ to those that performed well. The saturated media extract test is
typically used to analyze soilless potting mixes, and it is performed
by many university and commercial labs. Conventional commercial mixes
are usually very consistent but organic mixes are more likely to vary
from batch to batch since they contain compost which also varies. For
more information on making and testing organic potting soils, see this
Farming article from last year: http://www.farmingmagazine.com/article-7889.aspx.
All trays, pots and bench surfaces should be thoroughly cleaned prior
to use to prevent diseases caused by pathogens such as Pythium and
Rhizoctonia which can survive in root debris or soil particles.
Removing soil and debris is also important prior to using a sanitizer
otherwise its effectiveness may be reduced. If a crop had a disease
problem last season, avoid re-using those containers. Start with clean
seed; consider having it heat-treated to control diseases like Septoria
leaf spot and bacterial canker on tomato, Alternaria and black rot on
crucifers, and bacterial leaf spot on pepper. Sanitation continues to
be needed after the crop is growing: have employees clean off their
shoes before entering greenhouses, install hooks or wire systems to
keep the hose ends up off the ground, and place purchased transplants
in ‘quarantine’ to look for pest problems--before bringing them into a
greenhouse that contains un-infested plants.
Establish a system for carefully inspecting, monitoring and recording
pest issues on transplants. At least weekly, scout plants at ten
different locations in every 1,000 square feet of greenhouse. Look
underneath the leaves, in leaf axils, and pull up a few roots to check
whether they appear healthy. Carry a hand lens to help with spotting
small pests, like thrips. Check and replace yellow sticky cards each
week, too. Use at least one, but preferably several cards per 1,000
square feet, placed in a grid pattern though the greenhouse. Put some
cards just above the plant canopy to detect thrips and whiteflies and
put others on the rims of flats or pots to detect fungus gnats.
using biofungicides to prevent damping off and other diseases. These
can be applied in the potting mix or as drenches to trays. They
include: Bio-Tam, RootShield, Serenade Soil, Actinovate, Double Nickel
55 and Regalia.
Germination. Seed germination and plant growth
have different environmental requirements. Germination often
requires higher temperatures but it doesn’t need sunlight. If using
germination chamber without lights be sure to remove flats when the
first seedlings emerge so they won’t stretch and become leggy. Seed
germination can be inhibited by high fertility levels and associated
soluble salts, even though higher fertility is needed to ‘grow on’
plants once they use up the available nutrients. In general, vegetable
crops don’t need fertilization until after the expansion of the first
true leaf. Therefore, a low-fertility /low salts mix is generally
better for getting transplants started but as they grow the plants must
either be re-potted into a richer mix or fertilized if they are to be
held for more than a few weeks after emergence.
Plug size. If
you are using relatively small plugs be aware of their limitations.
Although they allow you to produce more plants in less space than
larger plugs, the transplants will be generally ready earlier and be
smaller than those grown in larger cells. Small plugs may need to be
watered multiple times each day, and they will become root-bound
earlier. Transplant quality will decline faster than with larger cell
sizes, if you can’t sell the plants or set them in the field on time.
In general larger cells are better for vegetable transplants held more
than 5 weeks, like peppers and tomatoes, while smaller cells may be
better choice for crops held for less than that. With transplants held
for the short-term, if roots do not completely fill large cells damage
can occur when pulling the plants as soil falls away and exposes roots.
Manage the temperature, quality and quantity of water applied to your
transplants. Cold water can shock the plants, so temper irrigation
water by using an above-ground tank inside the greenhouse, or with a
mixing valve that adds heated water to cold irrigation water. When
watering in the afternoon, remember that the hoses may get hot, so run
some water through before scalding your plants. It’s a good idea to
test your irrigation water pH and bicarbonate content; inject acid if
needed to balance alkaline water.
It can be a challenge to
uniformly apply enough water to transplants, but not too much. Teach
the people doing your watering how to assess the irrigation needs of
the plants by lifting up trays and pots, pulling out plants to feel the
bottom of the cells, and considering the coming weather. You may want
to post the forecast so employees can see if it’s going to be cloudy or
sunny over the next few days. Multiple passes can be helpful when
watering to assure even coverage. Start with spot watering in the
driest areas, and then make a pass over each bench moving first in one
direction, then in the perpendicular direction. Water early enough in
the day to allow the foliage to dry off before nighttime, to prevent
Greenhouse conditions. Good airflow
and temperature control are important to transplant production. Use
horizontal airflow fans to keep air moving; that helps avoid damping
off and also prevents temperature gradients. Passive ventilation can be
enhanced by large openings in the house, so roll the sides all the way
up when weather allows, and consider making vents at the tops of
greenhouse end walls to help hot, humid air exit the house. Many
greenhouses still use cheap, relatively inaccurate mechanical
thermostats to run heating and ventilation. Consider replacing these
with better mechanical thermostats such as those made by Dramm, which
are accurate to one degree, or with an electronic step-controller. This
will save energy and also do a better job providing your plants with
optimal growing conditions. Having therm alarms in place may save you a
lot of money in lost transplants if the heating or cooling system
fails. Alarms may be hard-wired or wireless to your house, or they may
use text messaging to your cell phone.
Hardening plants. The
ideal growing conditions in a greenhouse make transplants relatively
tender and susceptible to stress injury when placed in the field,
especially if the weather is harsh. To minimize this stress, also
called transplant shock, transplants should be hardened off by reducing
irrigation and if possible by slightly reducing greenhouse temperature.
Transplants can also be hardened by moving them outside the greenhouse
for several hours a day, a process that is facilitated by wheeled
benches and hard level ground. Using nutrient stress to harden
vegetable transplants is not recommended. Hardening reduces plant
growth rate, thickens the cuticle, increases dry matter content and
increases the amount of anthocyanins (pink pigments) in the shoot.
These changes can help transplants withstand challenging field