You need to remove the period (full stop!) at the end for that URL to work. It really isn't that high rez. anyhow. But also, even more than the USDA hardiness map this AHS heat map is of very limited utility. Number of days over 86F is essentially meaningless for a lot of plants. Right off the bat, it would have made more sense to plot the average summer nighttime dewpoint temperature. At Sonoma Horticultural Nursery in - surprise surprise - Sonoma County CA, they can grow the Himalayan cinnabarina series Rhododendrons. Or the hardy big leafs like R. rex. At Rarefind in NJ, you cannot. Even the ones that would be winter hardy there. The two locations probably have a similar # of days above 86F, but their summer climates are hugely different as are their winter climates. The cool nights and lower dewpoints in Sonoma County make all the difference.

My Mandevilla vine comes back eventually, every spring. It only gets blooming by late summer, hardly worth the wait for a lackluster performance. I decided this year to see if my mandevilla will overwinter. If it doesn't, I did that cuttings that have rooted and have them in the basement. It's just too much trouble bringing plants in and out of the basement and making sure no critters get to them. That was my main issue with brugmansias. Now that I have a couple hardy ones, I can plant them in the yard this spring and let them fend for themselves. Of course, I'll still make cuttings each fall, but they're much easier to handle. Plus, I can trade them or sell them on eBay.

The USDA hardiness maps, in my opinion, do not really work well on a detailed level. Sure, they do accurately describe general patterns of hardiness, but to claim that some parts of Massachusetts and some parts of Mississippi share the same zone for vegetation (zone 7) is ridiculous and is simply a reflection of the way the zone maps are created. These maps focus only on the lowest temperature typically reached in winter. Zone 7 means temperatures generally don't fall below zero. And yes, for Boston and the coastal areas of Massachusetts, that is the case during most winters. Similarly, zone 7 areas in the upper south also experience similar low temperatures. BUT....and this is a big BUT....the duration and frequency of very cold and windy airmasses in winter is MUCH higher in our area than in the upper south. So while southern areas will have one or two mornings during winter with temperatures in the 0 to +10 range during a cold wave (usually with clear skies and calm winds) they warm up rapidily and that is the end of it. We, on the other hand, have repeated incursions of very cold air, often on strong winds, during the winter. And that makes a huge difference, particularly in late winter when the sun angle is high. Just imagine this: Every winter morning could drop to +1 degree F and that would technically be a zone 7 winter! BUt few if any zone 7 plants could withstand that. And this past winter was also, technically, a zone 7 winter because the temperature never dropped below zero in most areas in southern New England. But the persisent wind and cold of February, without snow cover, damaged or killed a lot of plants that are normally reliable in zones 5 or 6. I have even seen dead zone 4 Arborovitae! So, don't believe anything you have been told about zone drift.

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.