Although we’re excited to get pots, planters, and containers growing, certain plant types can be damaged or stunted when exposed to cool weather, even if temperatures don’t dip to the freezing point of 32 degrees. And if it does actually freeze, they’re easily and quickly killed.
Annual plants (those planted for one season) that originated as tropical natives can be harmed at temperatures that drop below about 45 degrees. If these species are planted into outdoor containers in late April and early May, chances of cool-weather injury is great.
Types that resent cool weather include impatiens, coleus, begonias, sweet potato vine and zinnias. Planting of these is best delayed until mid-May when chances of exposure to cool temperatures decrease. Vegetables easily damaged by cool temperatures include tomato, pepper, eggplant, squash, cucumbers and melons.
Symptoms of cool-weather injury include drooping of foliage, softening of stems and stunted, poor growth.
The best planting window for most gardening efforts is the 10 day period between May 15 and May 25 for the Upper Midwest, especially for warm-loving plant types.