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Should you wait to plant after Mother's Day?

The Mother's Day planting rule works about 21% of the time in Denver.

DENVER — The warm weather lately might have some thinking about planting a vegetable garden or putting in some May flowers, but will the weather stay nice?

The average final freeze in Denver is May 5. But that’s not the best date to use when deciding if it’s safe to plant sensitive veggies and flowers, because that really means about half of the freezes happen after that date.

That’s where the Mother’s Day Rule comes in. On the Front Range we often say wait until after Mother’s Day to plant.

Last year, we got bit by that rule – there was a freeze on May 20-21. But how often does that really happen?

Well, Mother’s Day is not always on the same date, so let’s use this year’s date May 14 for a case study.

Over the last 10 years, there has been a freeze on or after May 14 three times – 30% of the time.

But that's a short window of data, so over the last 75 years at the Denver Central Park weather station, there’s been 16 freezes on or after May 14 – about 21% of the time.

That’s a frost about once every four to five years. Tomatoes probably don’t like those odds.

So, what if you wait until the third week of May. Then it’s only dropped to freezing seven times in 75 years – that’s just 9%. A number that tomatoes can live with. 

Of course, one of those seven times was just last year, but the data shows that waiting just one more week to May 21 reduces you freeze chances significantly.

The latest freeze in Denver history was June 8 which happened in 2007. But that was at Denver International Airport where the temps are much colder. The latest freeze at Central Park was June 2, 1951.

It's a similar story for most of the Front Range below 6,500 feet. 

Ft. Collins:
15 freezes in 75 years on or after May 14. That's 20%
5 freezes in 75 years on or after May 21. That's 7

Greeley:
7 freezes in 55 years on or after May 14. That's 13%
2 freezes in 55 years on or after May 21. That's 4%

Boulder:
16 freezes in 75 years on or after May 14. That's 21%
7 freezes in 75 years on or after May 21. That's 9%

Centennial: 
9 freezes in 24 years on or after May 14. That's 38%
4 freezes in 24 years on or after May 21. That's 17%

Evergreen:
15 freezes in 61 years on or after May 14. That's 25%
6 freezes in 61 years on or after May 21. That's 10%

Sterling:
14 freezes in 48 years on or after May 14. That's 29%
6 freezes in 48 years on or after May 21. That's 13%

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