April 1, 2008 By Dan Wooley
horticultural specialist Alana Respondek’s initial advice to vegetable
growers grappling with the challenge of raising transplants is always
start with a clean, properly sanitized greenhouse.
AgraPoint International horticultural specialist Alana Respondek’s initial advice to vegetable growers grappling with the challenge of raising transplants is always start with a clean, properly sanitized greenhouse.
There should be no foreign objects on surfaces growers use to grow or pot transplants, and growers have to keep walkways in and around their greenhouses spotless and free of weeds, said Respondek. She emphasized that growers have to avoid contamination of their greenhouse and related equipment.
Water sources should be free of pathogens, she added. Irrigation systems should be checked to ensure water filters are clear and clean and water lines aren’t leaking, she advised, adding that puddles can be breeding grounds for insects and pathogen sites.
As well, growers must create protocols for people visiting the greenhouse with a record of the date of their visits and any prior stops at other farms and greenhouses, suggested Respondek. As additional assurance, growers might want to consider having visitors wear protective footgear and clothing as soon as they reach the greenhouse’s entrance, ensuring that they are not coming into the facility with outside contamination.
Soil mixes should also be contaminant-free. Respondek recommends growers run fertility and sterility tests on any new mixes they plan on bringing into their facility. They also should ensure their homemade mixes are clean and pasteurized.
Rock wool is also increasing in popularity as a growing medium. Respondek suggests vegetable producers should make sure it is sterile from the start and, if they are re-using it, they should sterilize it at the end of the transplant growing season.
Seeds can also be a source of contamination in the greenhouse and should not be ignored. “Tolerance doesn’t always equal resistance,” noted Respondek, explaining that just because seeds are labelled as disease tolerant, doesn’t mean they are completely disease-resistant. “You should not expect a clean crop with just disease-tolerant seeds.”
Germination rates and seeding density should also be calculated when seeds are being purchased. And before re-using seed for another season, Respondek urged growers to run a germination test to determine the continued viability of the seed and its seeding rate. Any seed to be re-used for future seasons should be sealed in an air-tight container and stored in a refrigerator.
Once seeds begin to crack the soil and start emerging, transplant trays should be removed from the germination chamber, said Respondek. All water applied to the seedlings should be heated to between 15ºC and 20ºC to avoid shocking the tender plants.
Roots can also be affected by irrigation. If the roots do not grow all the way to the bottom of the plug and are brown in colour, this means the seedlings are not receiving enough moisture, explained Respondek. “You will run into trouble after transplanting in the field.”
To help build a healthy root system, phosphorous may be required. Respondek said symptoms of phosphorous deficiency include purple discolouration under the leaves and on the stems of transplants.
Growers should be prepared to adjust their fertility rates quickly. While pale leaves can mean a shortage of nitrogen, growers should also be prepared for the opposite, which is usually apparent through lush, dark leaves with white stems.
Temperature management in the greenhouse is very important for seedling health, said Respondek. It can be difficult to raise numerous crops in the same location, especially if they have different temperature requirements for germination and growth.
Respondek suggested that growers should try to keep their greenhouses at an optimum crop growing range by adjusting the ventilation system based on outdoor weather conditions. The greenhouse should be kept cooler on cloudy days and warmer on sunny days to assist photosynthesis.
Lighting in the greenhouse is also important as low light can create weak transplants. Respondek said lighting should be augmented if needed. However, since most greenhouses start operations when daylight increases with more daylight hours, additional lighting may not be cost-effective.
During the finishing and hardening-off of transplants, Respondek suggests reducing water “but don’t let them wither.” Seedlings should be placed outside five to seven days before field planting. Growers should ensure the transplants are not dried-out before field planting. The plants can be watered by dipping the transplant trays in a water tank “but don’t submerge them,” said Respondek.
“Depending on the crop, you might want to use a starting fertilizer,” she added. “Good quality transplants will result in higher yields.”
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