Cooperative Extension Service
College of Agricultural and Environmental Sciences
The University of Georgia
George
Boyhan, Extension Horticulturist-Vegetables
East Georgia Extension Center
Nessmith-Lane Building, 2nd Floor
PO Box 8112
Georgia Southern University
Statesboro, GA 30460
912-681-5639 | 912-681-0376, Fax | 912-682-3481,
mobile
We largely take it for granted, but light is critically important to plant growth and can be limiting in greenhouse production. It can be confusing to understand light and lighting systems because of the different systems of measurement used with light. A common measure of light is the foot-candle. This measure of light is commonly used in the United States and is a measure of visible light. In Europe, they use a measure called a Lux, which is equivalent to 10.76 foot-candles. A more useful measure for plant growth is umol/sq. meter/sec. of photosynthetically active radiation (PAR), which is equivalent to 0.20 foot-candle. To put all this in perspective, the amount of sunlight outdoors at noon during the summer is about 2,000 umol/sq. meter/sec., or about 10,000 foot-candles.
Looking at the graph below, you can see that during November through January, light levels can be less than 65 percent of the peak levels in May. To increase lighting enough to increase crop yield under greenhouse conditions requires about 40-80 umoles/meter/sec for 6-12 hours. This is usually achieved with high pressure sodium or metal halide lamps. A grower should expect to pay between $4.00 and $6.00 per square foot to add supplemental lighting. Growers should make a careful evaluation of their greenhouse operation to ensure the added cost of supplemental lighting is justified -- and, indeed, in the south, it may not be because of our relatively high light levels. For an excellent overview of light and supplemental lighting, visit http://ceinfo.unh.edu/Agric/AGGHFL/OFAlight.pdf
In addition, visit the following web page: http://www.egc.com/illumination.html. This site offers a conversion calculator among various light measurements. This can be helpful since lighting fixtures usually aren't rated in terms of PAR.
| Monthly Averaged Direct Normal Radiation/Page Method (kWh/m2/day) | |||||||||||
| Jan | Feb | Mar | Apr | May | Jun | July | Aug | Sep | Oct | Nov | Dec |
| 2.93 | 3.53 | 4.4 | 5.43 | 5.62 | 5.53 | 5.16 | 4.39 | 4.33 | 4.43 | 3.63 | 3.18 |
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Many growers have already planted their plant bed onions, particularly if they sell salad onions. Plant beds should be planted at the latest end of September, but this is too early for direct seeded onions. Planting direct seeded onions (they won't be replanted) early will usually result in seedstems the following spring. The optimum planting date is October 15 plus or minus a week.
Direct seeding onions is less forgiving than using transplants because you only get one chance to do it right. Fields must be prepared well in advance, and be free of debris with beds smooth and flat. Field moisture is also critical. If the field is too wet, the shoe on the planter will clog and seed will not singulate correctly. If the field is too dry, the planter can ride up on top of the bed and the seed is not planted at the right depth. Use a vacuum planter with encrusted seed for best seed placement. Use a complete preplant fertilizer, which will help the plants get off to a good start.
Finally, weed control is critical for successful direct seeded onions. A good program would start with 4-5 lbs of Dacthal 75WP preplant. This would be followed by 3-4 lbs of Dacthal 75WP at "onion spike" (This is when the plnats have just emerged.). Follow this with 1 quart of Prowl and 3-4 oz Goal 2 XL at the two-leaf stage. Goal can be used at this rate several times up to the six-leaf stage, when Goal can be used at the 6-8 oz rate. At this higher rate, you would expect to see some onion damage, but they should recover without a problem. Dr. Stanley Culpepper, the vegetable weed specialist with UGA, has been working in this area for several years and may have more up-to-date information. I would encourage you to contact him for the latest on direct seeded onion weed control.
Onion season is upon us once again. We plan on putting seed in the ground on September 19 at the Vidalia farm. Things have dried off considerably here in the past few weeks, but, with irrigation, that should not pose a problem for starting the onions. It has helped at the house, though, since the grass has stopped growing. Talk to you next month.