Feb 10, 2020
Southern greenhouse growers hit the mark

Georgia grows lots of transplants and ships them all over the country, but growing transplants on that scale requires the full system of production practices.

Priming seeds is a good start toward a faster and more even germination, and greater uniformity of growth. “Once transplants get behind, they often never catch up,” said Timothy Coolong, University of Georgia (UGA) Extension specialist and associate professor in the Horticulture Department.

Coolong was a speaker at the recent Great Lakes Fruit, Vegetable and Farm Market EXPO in Grand Rapids, Michigan.

Priming seeds also widens the range of acceptable germination temperatures and is simply more convenient. “In terms of production costs, the cost of primed seed is not that significant,” Coolong said.

Germination rooms are another useful tool – especially for crops that are difficult to germinate. “They’re very, very useful when growing seedless watermelons,” Coolong said.

Seed trays are put in the germination rooms for two to three days at uniform moistures and temperatures with temperatures of up to 85° F. “You can’t keep large greenhouses that warm,” Coolong said. Germination rooms also work when cooler temperatures are required for crops like fall brassicas which don’t germinate well in midsummer heat.

Black rot of cabbages and brassicas, and gummy stem blight and bacterial fruit blotch of watermelon are the major seed-borne pathogens in Georgia. “If you test positive for bacterial fruit blotch, the whole greenhouse is thrown out,” Coolong said.

Seed companies do a good job of testing seed but the system isn’t perfect. Sample size is an issue – typically tests are only robust enough to determine if one in 10,000 seeds is infected. Testing also only identifies pathogens on the outside of the seed.

Hot water treatment of seed is effective on bacterial, seed-borne diseases. “Large growers may even ship their seed out for hot water treatment,” Coolong said.

Smaller growers can do their own treatment – if they have the equipment and skills to precisely control water temperature and treatment time. For peppers, the treatment is 30 minutes in 125˚ F water. For tomatoes, it’s 25 minutes in 122˚ F water. Failure to treat at the right temperature for the right amount of time can really hurt seed quality.

The Ohio State University Extension Factsheet “Hot Water and Chlorine Treatment of Vegetable Seeds to Eradicate Bacterial Plant Pathogens” is an excellent guide on hot water treatment and is available at: https://u.osu.edu/vegetablediseasefacts/management/hot-water-seed-treatment.

Hot water treatment only kills bacteria– and some fungi – on the seed’s surface.

The T-rail system is the most commonly used support system for seedling trays in Georgia. “The T-rail system drives the seedling tray choice and growers tend to use foam trays in that system,” Coolong said.

The foam (polystyrene) trays are lighter than hardened plastic trays and less likely to damage equipment. Plastic seedling trays are more durable, easier to clean and it’s also easier to pull seedlings out of plastic trays.

Germination rooms are a useful tool – especially for crops that are difficult to germinate.

When using foam trays, some growers may dip their trays in copper to root prune seedlings and make pulling the plants easier. Copper dipping also prevents root cages.

Root cages are when the roots grow around in a circle around the outer edges of a cell. This condition may be caused by overwatering. Root cages make it harder to pull the seedlings with wellformed root balls from the trays.

Seedlings are also harder to remove from foam cells if the roots have grown into the cell walls. Plastic trays don’t have this problem. In addition, a perlite layer on the top of the media in all types of trays keeps the growing medium from drying out.

Cell size is an important consideration. Larger cells hold more media and can retain more water and nutrients. Transplants in larger cells need less frequent watering and fertilizing, and normally produce stockier and earlier plants. “A bigger cell means shorter plants, which generally improves growth going to the field,” Coolong said.

Smaller cells are generally used for crops such as cabbage, broccoli, cauliflower, collard, kale and lettuce. Larger cells are generally used for tomato, pepper, watermelon, muskmelon, cucumber and squash.

For tomatoes, it’s common to see 128 cells per tray in Georgia although some growers may use 72. For watermelons, it’s 128 or 200 cells per tray. “Growers like 128 but there’s more cost,” Coolong said. “Greenhouse space is at a premium so many growers use 200.” For peppers, it’s common to see 200 to 242 cells per tray.

Water quality is an issue in Georgia. There’s a lot of limestone in parts of the state and that gives irrigation water a high alkalinity.

Alkalinity measures the buffering capacity of water and is the water’s capacity to resist changes in pH. High alkalinity is the same as adding limestone and will cause a rise in pH over time. Alkalinity is a better measure of how much acid needs to be added than pH.

“A lot of our growers use sulfuric acid because it’s cheap and readily available,” Coolong said. “Organic growers use citric acid which is more expensive.”

Water with a smaller degree of alkalinity may be effectively modified with acidifying fertilizers alone. However, only ammonium-based nitrogen has an acidifying effect; nitrate-based nitrogen does not.

Application rates for water-soluble fertilizers applied to greenhouse transplants through irrigation are usually specified in ppm (parts per million). It’s important to use the right conversion tables for accurate application since you may have to convert per acre spray rates to greenhouse rates. “Trying to measure it out on a concentration basis generally works out better,” Coolong said.

One publication in wide use is UGA’s “Conversion Tables, Formulas and Suggested Guidelines for Horticultural Use,” available at: https://hort.caes.uga.edu/extension/publications-and-factsheets.html.

The bulletin also has guidance on how much acid to add to correct different levels of alkalinity. The calculations will vary with the type of injector being used. The Electrical Conductivity (EC) counts the salt ions but the ions could be from different nutrients.

“A lot of people get EC meters, but do keep in mind EC is only an indicator,” Coolong said. “You could have the same EC but vastly different nutrient contents.”

Growth regulators are one way to manage transplant size but again accurate applications are a must. “If you don’t have the ability to put out measurements accurately in the ppm, I’d suggest you not use growth regulators,” Coolong said. “You must use them very accurately.”

— Dean Peterson, VGN correspondent


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