http://gardenrant.com/2016/03/...

Environment Canada has recently redone our hardiness estimations.
They take other factors such as altitude , river valleys etc. into account too.
We were zone 3b, but are now 4a on the Canadian system.

I don't really get the a and b - but I apparently gained a half zone as well (8a to 8b). For simplicity, I'm sticking with plain 8.

I'm a little confused, because when the new USDA map came out my zone was changed from 4b to 5a, which I assumed was the result of taking climate change into account; but the article seems to be saying that the new map was minimizing the climate change effects. My personal thought had been it was an effort to get people to buy more plants or something... because zone 5 plants are very iffy where I am, unless planted in a micro climate such as near the house foundation.

I have given up on zones long ago. Trial and error is what I have used for the last 15 yrs.

All my zone 6 are not created equal. Temp alone does not help but the amount of snow cover that could insulate vs a zone 6 that get less snow cover. Without the snow to insulate some plants the cold get to the plant.

A zone 6 like mine with more rain then freeze instead of snow and then cold kill even zone 5 plants that get snow. So instead of cold killing the plant but the block of ice that the rain caused before the cold came in along.

Very interesting article.

I agree on trial and error for zones. I have some Agastache that I grow from seed that are listed as zone 7. Most (not all) of them come back for me. At least for the three years counting now. I also bought a coreopsis, I think it is Garnet, and it was listed as zone 7. It has done fine for me for two winters now. Pink Muhly grass also does fine for me. It just does not get as big as it does for those further south.

Yet I have other stuff that is listed as zone 5 and I cannot get them to overwinter ever! Guara and Salvia greggii come to mind. And Caryopteris. But there are others too.

I try to always look at a plant's demise as an opportunity to try something new in its place. Heaven knows I ALWAYS have something new to slot in due to my PBA (plant buying addiction)!

I was puzzled as to why the 1990 map was used in the "rant".

Within any zone are vast variances in soil types, drainage, etc...

I'm amazed that anyone gets much use out of the zones. They say how cold an "average" winter got, over the last 20-30 years.

Do we buy expensive plants based on their ability to survive HALF of our winters?

I might care if I knew what temperature only one winter in the last 20 years went down to.

We could look at the "rating" for each zone, like "8b gets down to 15-20F in half of all winters". I assume then that a plant rated for "Zone 8b" would be likely to have problems WHEN the air temp actually goes below 15-20F ... ignoring factors like how rapid the drop was, whether plants had had time to vernalize, how much wind, snow cover, exposure, slope, etc.

Or do plant recommendations for zone hardiness try to take the 50% into account and allow another 10 or 20 degrees to cover slightly unusual winters? I don't know that, but "trial and error" sound good to me.

I agree -- I'd be happier to have the zone based on the coldest winter that can be expected... I'd still try to grow things that I shouldn't, but at least it would be a choice based on better information. (well, actually, I pretty much have that information in my head now, after gardening up here for almost 40 years, and in the same spot for 25... so I guess I only have myself to blame when it doesn't work out )

Sandy I have a rule. If I find it cheap I will buy it and give it a chance. If I am not sure I do not pay full price. The one plant that almost had me living in a dog house and drinking rain water was Heucheras.

When they first came out I was paying 20.00 and they either died before summer was over or dead a door knob by the end of winter. I then read they liked morning sun. Got the sane dead results, Then I read they needed good drainage, I tried that again ended up with dead plants. After hundreds of dollars of dead plants I discovered the zone 4 - infinity is just not true.

This was from a local nursery.......
"Unfortunately many of the varieties that have been introduced in recent years grow well under nursery production conditions but quickly fade away under garden conditions that involve stress

Anywhere there's frost at all, hardiness isn't about air temp once there's been a frost, but about soil temp & whether or not the ground freezes, depth & length of freeze, all combined with drainage & fertility.

Then there are cheats like putting a big pile of leaves, pine needles, or straw over marginally hardy plants (to keep the soil warmer,) &/or putting questionables next to hardscape features like a patio or path, large rock, or adjacent to a south or west facing basement wall, strings of lights...

Well, yes... but when the frost goes 6 feet down in the ground (as it did here in the winter of 2013-2014), we are not in zone 5 anymore, Toto!

Before moving to AL, I lived/gardened in Z5, central OH for decades & have seen zone-cheating work there too, with Cannas that survived in a neighbors' yard for over a decade by the time I moved, & 4'0'clocks (Mirabilis jalapa) in mine, also there for about 10 yrs (and now moved to AL.) There's a neighborhood in Columbus called German Village that has mostly brick buildings & a lot of brick courtyards where Z7 plants often survive. That's a cheat of 1.5-2 zones.

New Orleans is in the same zone I am, but I've seen MANY plants in the FQ that are NOT hardy here (without a brick neighborhood to surround them.)

I buy according to the temps I know we have had and it is capable of here. That is negative 10. technically the zone I am in is not that low, but I don't trust my money on it. Map says zone 6, but the weather here is actually zone 5. So when buying plants I look for hardiness down to zone 5. Otherwise you will end up replacing it sooner or later. I do plant semi tender bulbs sometimes, just plant deeper than it recommends and they usually come back, although not as gloriously as in warmer zones. But then again that odd year will strike and they will die. So I plan them to last 2 or 3 years or whatever.