In our USDA Zone 7B
Spring: Onion plants are hardy to 20° so they can be planted early. The earlier the onion is planted, the more time the onion has to make more top and therefore larger onions.
Short and Intermediate can be planted starting in mid Febuary thorough mid March. After the weather starts to heat up, it is really too late get the slip established and have a good tasting onion.
The best varieties for Parker County are: Intermediate Day (needing 12-14 hours of daylight)
Sweet Red:
Adds special color and flavor to salads! This is the variety used by Italian restaurant and great for hamburgers too! Sweet Red bulbs grow well in Parker County, but have limited storage capability.
Cimarron: Popular intermediate variety! This large, yellow, onion possesses the unique combination of earliness, productivity, and quality.
Gardeners can expect Cimarron to mature later in the south and earlier in the north than the designated varieties bred for their areas.
The Cimarron is mildly pungent in flavor with medium storage capability.
Here is a link to the Texas A&M Horticultural Website: http://aggie-horticulture.tamu.edu/archives/parsons/publications/onions/ONIONGRO.html
This link has a good amount of info on onions and care.