When will the cold nights end?

Although this title seems like a song lyric, for the warmest of the warm season vegetables it's true. Tomatoes, peppers, eggplant, winter squash, pumpkins, watermelon and cantaloupe have more requirements to think about than not being able to take frost.

This group of vegetables grows well when daytime temperatures are over 60 degrees AND nights are over 50 degrees F.  It's this night temperature part that's problematic in high altitude climates like the Front Range.  The May 20 low was 38 degrees F and nights are predicted to remain in the forties for at least the next week. It likely will be June until they warm sufficiently for planting outdoors without additional temperature support.

That nighttime temperature support can come from water walls or perhaps a warm microclimate location with lots of stone or masonry to absorb and radiate heat. Otherwise the best approach may be to get on the shuttle - moving transplants out by day and indoors at night. This helps harden off plants for eventual transplanting anyway. Hardening off is toughening them up to lower outdoor humidity, wind and higher light intensities especially UV light not present indoors.

For what seems to be everybodys' favorite vegetable, tomatoes, it may be tempting to plant outdoors in a year like this when the apparent last frost was April 16. Stunted growth and susceptibility to pests such as flea beetle results. Plants can normally outgrow this pest when they are established quickly and get up and growing but are severely set back when stunted by cold nights and eaten by pests.

Waiting to transplant can often result in better growth and a harvest that arrives just as early as plants transplanted before the arrival of warm nights.

Photo Credit: Tomato that was transplanted when night temperatures were warm easily outgrows early flea beetle injury (see pin holes on lower leaves) Carl Wilson