Spring 2010 has been favorable for growing cool season vegetables seeded in late March or early April. Temperatures in April were near normal and not warm in Denver. Watering was necessary to overcome the many dry weeks of the month with the April 21 to 25 rain/snow upslope that left 2 inches of precipitation being the exception.
The first two weeks of May were cool (5 degrees below average) and dry (0.75 inches below normal) until the May 11-12 storm left over an inch of moisture. Fertilization and consistent watering of greens and developing vegetables were needed to maintain vigorous growth and quality.
The goal now should be to harvest spring crops as they reach best quality and mature them before the onset of hot temperatures. Whether higher temperatures arrive as early as late May or in June, bolting (seedstalk development) of spring greens and deterioration of quality will quickly follow. The semi-hardy vegetables can be planted in early to mid May including beets, carrots, parsley, parsnips, cauliflower and Swiss chard.
Plans should be made and seed and transplants purchased to follow spring crops with warm season vegetables. The first to go in are the tender vegetables including beans, corn, cucumbers, New Zealand spinach and summer squash. Transplants of cucumbers and summer squash should be delayed until the time for planting the very tender vegetables the end of May. Soil germination temperatures and daytime air temperatures should be consistently above 60 degrees F for this group. The soil temperature on May 10 was 53 degrees F (Ft. Collins data).
The very tender vegetables include melons, pumpkin, winter squash, eggplant, pepper, tomato and lima bean. This group is intolerant of frost as well as cool day and nighttime air temperatures below 55 degrees F. A daytime temperature of 60 degree F or higher is needed for growth. They should be planted when weather has become summer-like and the cool breezes of spring are past – at least two weeks after the last frost date (May 10 is average last frost date in Denver). They can be planted earlier with water walls or similar devices to provide nighttime heat and frost protection. Transplants are available at the Plant-A-Palooza plant sale May 15 .