Even in Colorado, it's supposed to be safe for plants after Mother's Day.
Not this year.
Gardeners who followed that rule of green thumb are probably regretting it after a chilly, wet spring storm closed out this week, bringing rain and freezing rain to Colorado Springs and snow to higher altitudes in the region.
The slow-moving storm that moved in late Wednesday and was expected to linger into Friday evening sent temperatures plunging overnight, from a high of 75 degrees on Wednesday to a low of 34 overnight Thursday. A frost advisory for El Paso County is in effect until 9 a.m. Friday, while the northern part of the county will be under a freeze warning until 11 a.m. Friday.
Snow isn't common in May, but it's also not unheard of. Only trace amounts of snow had been recorded in years past on May 18. The .01 inch of snow recorded at the Colorado Springs Airport was a record for the date.
The National Weather Service in Pueblo predicted a high of 44 degrees Friday and a low of 32 degrees - far below the day's average high and low of 70 degrees and 44 degrees.
At the demonstration garden in Monument Valley Park, gardening expert Diane Brunjes was working Thursday to guard roughly 2,000 flats of plants against what she expected to be the second hard freeze of the season overnight.
The plants were being set out for the Horticultural Art Society's annual sale and main fundraiser this weekend.
The sale runs from 9 a.m. to 5 p.m. on Friday and Saturday and 9 a.m. to 4 p.m. on Sunday at 222 Mesa Road.
With more thunderstorms in the forecast through Monday, Brunjes recommends that gardeners hold off on planting.
"I'd wait another week as least," Brunjes said. "You don't want to be digging in the soil because it could ruin the soil structure."
For true warm season plants, like tomatoes, she recommends waiting until after Memorial Day.
If plants are already in the ground, Brunjes suggests covering them with plastic, buckets, coolers, cardboard or bedsheets - anything that will help them stave off weather damage.
Plants are especially susceptible to the cold right now because many of them are in bloom. She estimated plants are flowering about 23 days ahead of last season.
"Once we get through this, it's going to be spring and summer again," Brunjes said.
While downtown Colorado Springs and the southern part of the city saw only brief snow showers, there was no mistaking Thursday for spring or summer in northern El Paso County, where 2 to 5½ inches of snow fell, said Randy Gray with the weather service. One Black Forest resident measured 8 inches of heavy, wet snow by mid-afternoon.
By the end of the day Friday, another 4 to 7 inches could accumulate on the north side, Gray said.
On Friday, less than an inch of snow is expected downtown.
Rain or snow - or a uncomfortable mix - is expected to continue into Friday night.
Denver up to the Wyoming border along the Front Range got it much worse than the southern part of the state.
The Colorado National Guard was sent out to Douglas, Larimer and Weld counties Thursday afternoon, and Rocky Mountain National Park says all roads on the park's east side have been closed because of heavy snow. Park spokeswoman Kyle Patterson said at least 2 feet had fallen in some areas by Thursday afternoon.
Much of central and northwestern Colorado was under a winter storm warning until 6 p.m. Friday.
The Associated Press contributed to this report.