for years, global warming has actually bumped up the average last frost dates significantly in the past few decades. Mother's day used to be the rule a couple of generations ago, but not so much any more. Course there are still outliers.:) This article was written over 10 years ago and the 30 year average last frost dates for nearly all of the cities listed was before Mother's Day. https://www.ksl.com/article/24255228/starting-you-spring-garden