The seedlings that you have tended so carefully in the warm, humid greenhouse will soon be planted out in the field, where they will be exposed to wild temperature swings, wind, rain, and brilliant sunshine. It’s a tough transition for the plants, one that can set them back severely unless you take the time to harden them off.
Hardening off means introducing young plants gradually to the harsh realities that await them outside. Gardening books always recommend that you move them outside for a few hours, increasing the length of time every day for a week or more.
But what market gardener has time for that? Most people growing on a commercial scale are moving thousands of plants through the greenhouse, and can’t keep moving them in and out every day. So most growers create a separate hardening off area where plants are moved about a week before they are scheduled to go into the field.
John Biernbaum at Michigan State University, says it’s important to give plants more light, a lower temperature and less water before setting them out in the field. “There also is an advantage to exposure to the wind,” he said. “At the Student Organic Farm [SOF], we are moving them from the heated greenhouse at the front of the horticulture farm, about 2000’ to the actual SOF where they will either be outside on pallets, or inside in an unheated greenhouse."
“Another possible transplant topic that farmers might not be thinking about relates to fertility. We often have to hold back on fertility in case the weather turns and going to the field is delayed. Too high a fertility and plants are hard to hold. If plants are on the stalled or hungry side, an application of liquid nitrogen—say from fish emulsion, or compost tea, or another liquid fertilizer, applied two to three days prior to going to the field, or even the day of going to the field, likely will help get the plants moving again and improve the rate of establishment.”
Steve Moore, Center for Environmental Farming Systems at North Carolina A&T University, uses portable tunnels that are 5 by 16 feet, with four to five hoops on 2x4 skids. Plastic is used over them with pallets under the flats for a quick hardening-off area.
“Light sprays of kelp strengthen the plants at this point of transition (a chemical/cellular issue of cold stress plant physiology),” Steve said. “Here at CEFS, Bryan Green, the farmer/educator, uses the canopy he takes to market with shade over the top and weed fabric underneath."
“One thing I found important over the years was to use my spreadsheet planning guide to know how many flats I will need to harden off and at what times. It is suboptimal to have lots of flats and to not have enough space—and take uncomfortable chances with transplants’ well-being.”
In the north, growers often have to prepare for freezing temperatures during the hardening-off period. Vern Grubinger of the University of Vermont says many growers add side hoops to their greenhouses that serve as cold frames where they can transition plants. By opening the sidewall of the heated greenhouse, this lean-to cold frame can get heat if needed.
Peter Seely of Spring Dale Farm in Wisconsin says that once flats go outside, they go on benches under the frame of a low hoophouse over which plastic can be drawn in case of freezing nights or hard rain. Peter also has made several “stretchers” on which two people can carry seven flats from the greenhouse to the hardening-off hoophouse.
“In the greenhouse we are just putting up now, we will have the far half of the greenhouse open to the elements (though again, coverable if need be), and so the flats need only to be moved from one side of the greenhouse to the other,” Peter said. “A trolley system, like the one from Growing Systems in Milwaukee (414-263-3131), works great for moving flats.”
Brett Grohsgal of Even Star Organic Farm in Maryland, also moves his plants outside to harden off in locations around the greenhouses, where microclimates of wind and sun determine their placement. With 27,000 transplants each spring, that’s a big job.
“We use carts whenever possible, but the value of each flat is high enough that the labor is worth not begrudging carrying the flats two by two—especially in the context of extremely high prices for conventional plug flats or organic ones grown by others. Once out, they stay out, except for the 5% of the time when we miscalculated the forecast and there comes a violent storm or freeze. Also, we often use market tents/canopies as temporary, part-shade modifiers to ease the hardening off. These are not usually in use in peak transplant season (April to early May)."
Brian and Anita Poeppel of Broad Branch Farm in Illinois used to start their seeds under lights in the basement, but last year they bought a 12 x 24 foot transplant greenhouse from Farm Tek, which requires a small propane heater for growing seedlings. “2005 was the first year we used the small hoophouse and we loved it,” Anita said. “The transplants took off when we put them in the ground. We definitely had stronger healthier transplants.”