I haven't noticed earlier last frost dates, but I have noticed more irregular spring thaws and temperature swings. I view climate change as a long term thing, so I'll probably see shorter winters at some point in my life time, but I don't necessarily expect that in a shorter period of time that I will be changing my gardening practices. There's always been a range of what falls within "normal" and so I am still seeing a range of when my last frost date is, from about the second week of May until the end of May. Last year it was something like May 29th, about as late as I've ever experienced.

USDA changed my zone from 5b to 6a in 2012, but I have always planted tomatoes and other tender plants after Memorial Day (around June 1) and I still do. Maybe I could do it earlier, but I have a long enough growing season doing it that way. I haven't kept any records, but last frost doesn't seem to be any earlier here.

Susan

The last frost date is usually a 90% last frost date. So 9 years out of 10, the last frost will be earlier than then. It's that tenth year you have to watch out for.

and yes, it wasn't that many years ago that we had our last frost in June.

My zone was changed from 5 to 6, probably in 2012? But, I can't imagine it being 6. I plant for 5. I wonder about the data they used for that change. Does anybody know when the previous planting zones were established? I know the 70's was a cool decade. If they were established soon after the 70's I could see my current zone getting warmer, even though it doesn't seem like it.

Since USDA maps aren't copyrighted, they don't have copyright dates. The first map, I think came out in the early 70's. The second map, the cold map, came out in the mid-80's?, then the current map seems to be a warm map. I did a lot of poking around into the data sets used for the current map, and had a few issues with them. The biggest one was that 'valid' weather data doesn't change instruments or sites. However, that doesn't take into account if the area around the site has drastically changed. So to exaggerate just a bit, several of the local sites I'm familiar with were in cornfields 20 years ago, but now are in the middle of a WalMart parking lot.

The reality is that I tend to trust the first map the most. It's the one with the longest data set, and has yet to offend my sensibilities for places I'm familiar with.

Very interesting observations mad_gallica. Thanks! Makes me feel a bit more sane about questioning that zone 6 classicification for my area!

Here's the 2012 map. One can zoom in and the map is still clear.

https://www.loc.gov/item/2013585127/

Here's the 1960 map

https://static1.squarespace.com/static/54edef61e4b0d7bc3127841c/54f9f3f4e4b0a5b9abed668d/54f9f410e4b0a5b9abed817f/1425667088112/1000w/

They updated the map in 1990 since plants grown in the 1940's through 1960's were getting damaged/dying in the 1970's and 1980's. See below. So it appears the current map is likely similar to the climate back in the 40's through 60's.

I think you also have to pay attention to your own micro climate. We're in a little valley but apparently low enough and without nearby trees to hold warmth that we get hit by frost a couple of weeks before our neighbors. I used to notice this at a vegetable garden near the center of town that was same level as the nearby river. His garden would be one of the first hit by frost in the fall. I know that we get a temperature dip right around September 18, so I watch the predictions and have used a portable low tunnel to protect a late planting of bush beans. If I can protect them for one or two nights, they have a couple more weeks of growing season. This is the difference between blooming and young beans. I have seen raised beds in the White Mountains that people edge with rocks to help provide some warmth.

I've still be using Memorial Day/end of May as a planting date for tomatoes and peppers.

I recall being fooled (or being a fool) when they changed the traditional Memorial Day from the last day in May to about one week earlier. I was unaware of the date change and planted things as usual on Memorial Day. Except it was not *really* Memorial Day and all the plants I put out were tinged by the frost that struck a day or so later. After that experience I switched planting day to June 1st and there it shall stay.

Since that time, out of curiosity, I have monitored the week leading up to June 1st each year, and there has not been a single frost during that week. I'm sticking to June 1st however. Better safe than sorry.

I think NHBabs has a good idea here: climate is not something to take into account over 5-10 years but something to consider over larger periods. How your planting site is protected can alter your 'zone' drastically. Do you have a stand of trees, or an outbuilding, etc, blocking the cold northerly winds in the winter that provide intense chill? Do you have any radiating bodies that provide additional warmth in the summer months?

I use the zone system as a general guide. USDA says I'm zone 6b. But I can keep figs alive in my backyard--a plant whose hardiness is typically 7+ ! Get into the garden when your garden signals for you to do so. It's going to be *around* your zone's dates but the chances of it being right on the nose are slim!