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  A Publication of UVM Extension's Vermont Vegetable and Berry Program

Growing Good Transplants

by Vern Grubinger
Vegetable and Berry Specialist
University of Vermont Extension

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.

Sanitation. 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.

Pest prevention. 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.

Consider 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.

Watering. 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 disease development.

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 conditions.

Published: April 2014
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