There are a number of factors affecting a plant's hardiness. One is provenance. Where a plant is from generally affects where it will grow. A simplistic way of looking at it is that tropical plants need tropical climates and plants from the north don't do as well in heat. This gets a little messier when you consider other factors like elevation. A plant may be from a fairly warm area (say equivalent to zone 8) but may range to higher elevations (maybe 12,000' up a mountain) where it is much colder. Selections collected from these higher elevations may survive zone 6 or even 5.
Sometimes doing research into a family of plants that is commonly available as non-hardy will provide an obscure species that is hardy to our area. Everybody knows Croton (at least by sight if not by name) with its colorful foliage found for sale as a houseplant at every lowes, home depot, and walmart or grown as a landscape plant in Florida. Croton alabamensis is a species that is hardy to at least zone 6, possibly zone 5. Another case is Cyrtomium fortunei, which can be grown in zone 4 with ease even though other Cyrtomium species require warmer temperatures.
Another big factor affecting hardiness is growing conditions. This may seem obvious, but so very many gardeners miss it. Every plant has different needs. While we can make broad recommendations and put together a garden with plants that have similar needs, those needs still must be met. Woodland plants don't thrive in heavy clay or full sun, and no gardener should expect them to. Spending an extra $5 on a bag of compost, mulch, or fertilizer might mean the difference between success and failure.
Even placement in the garden can make a difference. Sometimes moving a plant as little as 2 feet from its original spot will show vast improvements in plant growth and hardiness. This can be due to many reasons. Soil might be a slighty different pH, there might be more or less soil moisture, light exposure might vary slighty, and the microclimate might be drastically different. Wind and light exposure plays a big role in affecting both winter and summer conditions. You can really capitalize on a spot that's cooler in summer and warmer in winter when it comes to stretching your zone.
Some plants can be potted up and overwintered in the garage if supplied with a little supplemental water in the winter. Other plants can be screened from the wind. Winter mulch can help tremendously protecting plants from freeze/thaw cycles, and you can try building a cage around the plant and filling it with compost to get non-hardy plants through the winter. Or you can try ugly white rose cones, but I find them the least effective method of winter protection. And they're ugly.
I live with zone denial for a number of reasons. I think most new plants are hardier than they are given credit for. There are some exceptions, Coreopsis 'Limerock Ruby' being sold as zone 5 without extensive trialing comes to mind. But I think for the most part most nurseries are responsible enough to err on the side of caution.
I've found a number of new plants to be hardier than previously thought. When I first got into the industry, Agastache rupestris was widely sold as being hardy only to zone 6. I can say with certainty that it is easily hardy to zone 5, and probably to zone 4 if provided the right conditions. Other Agastache varieties have followed suit, with 'Raspberry Summer', 'Summer Love', 'Purple Haze', and 'Black Adder' surviving in my (and many other) gardens beautifully.
I've also been growing Black Mondo Grass, Ophiopogon planiscapes 'Nigrescens' for a few years now and it is starting to spread. This is another zone 6 plant.
Helleborus 'Hot Flash' in its 4th season. Not big and vigorous, but starting to do well. Listed as zone 7 upon release, now listed as zone 6, 5 with protection. I do no protection other than not removing the maple leaves that fall on it.
Sometimes I like to push zone limits just because I think a plant looks cool. It might provide a texture or color not readily available to us in colder zones. Or it might just be my horrible plant addiction. Either way, I planted Cunninghamia lanceolata 'Glauca' in spring of 2008. This is a neat evergreen conifer with broad, wicked-looking but soft, blue needles that is zone 6/7 borderline hardy. I planted it in my south butterfly garden and other than some pretty good winter-burn it has survived pretty well. I have taken pity on it though and have moved it to a pot with some miniature hostas that will be overwintered in the garage.
There is also another form of zone denial that many of us in the industry have to deal with. The "this plant didn't do well for me, so it must not be hardy" denial. This usually comes about from improper cultural practices, and not providing the plants with the proper requirements. And some people just can't grow certain plants. I've killed ridiculously easy to grow species for no apparent reason. At the risk of my gardening reputation, I'll admit to killing Gooseneck Loosestrife. I'm also very good at killing ferns.
The most common reason most plants die in my experience is that their specific soil conditions are not met. Some plants resent being too wet in the winter and will simply rot away to nothing. The new Echinacea hybrids are a good example. Gaillardia is another example that needs good drainage. I've had Gaillardia 'Frenzy' for 3 years now, and it does very well in a fairly well drained clay-loam. Alpine/rock garden plants like Dianthus licinata or Delosperma species can't be too wet in spring or they will get crown rot. Rhododendrons are widely planted here, despite needing a moist but well-drained acidic soil. Most of them fail in our heavy alkaline pH clay. I've seen a number of huge beautiful Rhododendrons (primarily PJM, but others as well) in the north-central sand counties of WI. Some plants need organic soils that are consistently moist, like Lobelia cardinalis and Lobelia siphilitica. Providing the right soil conditions can make all the difference in the world.
Don't be afraid to try new plants, or even new ways to use old plants. Don't be afraid to try a plant again just because it dies once. If you do some research, it's amazing what can be done in our gardens.
