Planting Zones - My Thoughts

Florida flame azalea (Rhododendron austrinum) January 27, 2020

This blog is most often writings that document the changes in my yard as I take a typical largely lifeless turf-dominated landscape to one dominated by natives and the life that it provides for. Secondarily, it is a place to express my wider opinions and thoughts on native plant landscaping in general. Having arrived in Florida 33 years ago and charged with developing a program to certify urban landscapes as wildlife habitat, I've had a good many years to observe and form opinions - and like any good observer, some of my opinions have changed over the years as I've gathered data. One issue that I am adamant about is the need to use plants for their ecological roles above any aesthetic roles that they might also have. This is true for native AND non-native plants. I no longer view the world of landscaping as a native vs non-native issue, but an issue of ecological significance vs ecological insignificance. All plants play some kind of role in a landscape, it's just that some have more significant roles than others. As we look to create living landscapes, we need to throw off the limitations on our creativity that others may want to burden us with. In my opinion, one of the biggest impediments to real change is what I call the "zonists."

"Zonists" are those folks who plan their landscapes based almost solely on what "zone" they live in. They start each conversation with a statement that clearly states what zone they live in - like that bit of data has the most relevance to what they should choose to plant; not their site conditions, but their zone. The USDA has posted Plant Hardiness Zones for decades and they have been very useful to gardeners planning their annual gardens. They are quite good at providing information about freeze dates and minimum winter temperatures. I do not find them to be very useful, however, in planning landscapes. As per the USDA itself:

"Hardiness zones are based on the average annual extreme minimum temperature during a 30-year period in the past, not the lowest temperature that has ever occurred in the past or might occur in the future. Gardeners should keep that in mind when selecting plants, especially if they choose to "push" their hardiness zone by growing plants not rated for their zone. In addition, although this edition of the USDA PHZM is drawn in the most detailed scale to date, there might still be microclimates that are too small to show up on the map."

The USDA map of zones was not intended for anything really other than planning for the growing seasons of cold sensitive plants. The maps, by definition, are "Hardiness Zones" - not planting zones. There is a difference. I can use a hardiness map to provide information on winter temperature extremes - as it was designed for, but it won't tell me anything about a northern plant's adaptability to less extreme winter temperatures. It was not meant as that type of guide. It tells me where I can plant tomatoes and when, for example, and it tells me how far north I can plant a cold-sensitive tree or shrub, but it doesn't tell me anything about how far south a native Florida flame azalea can be planted and still thrive. What it needs to thrive is not related to its cold sensitivity; it's related to its other habitat needs. The question then relates to "can I provide those habitat conditions in my landscape?"
Today, this issue appeared again as I posted the above photo of one of my flame azaleas. Various folks questioned it being planted "out of its zone." It is true that I have planted it out of its vouchered range; it is not true that I've planted it out of its "zone." They are, of course, two very different things.
A native plant's known geographic range is often a good indicator of its habitat needs, but those needs often do not translate well when the plant is grown in a landscape. My flame azalea does not occur south of its geographic range for a good number of ecological reasons. Most likely, it cannot effectively reproduce as it moves too far south. Not because pollinators are absent - they aren't, and not because the seed produced is nonviable - it isn't. Regardless of why, I've grown Florida flame azaleas quite effectively for many years in west-central Florida landscapes. A landscape is a different being as I have the ability to manage it. That doesn't mean that I have to treat this plant any differently than I would a plant commonly found in my zone. It just means that I can manage it if I have to. In a landscape, I can mix plants from various hardiness zones and have them thrive IF I'm mixing plants with similar growing-condition needs. If all of my plants have similar needs for soil type, sun, moisture, etc., they can exist together in a community I've created and not require supplemental inputs such as additional watering, fertilizing, and the use of pesticides. They can survive together "naturally" even when they wouldn't be found naturally living together. That is the difference between natural ecosystems and human-created native plant landscapes.
There will always be discussion among native plant people about the propriety of taking native plants out of their natural range. Folks question whether they are truly native if they are not naturally found in that location. Frankly, I don't have much use for this question when its asked of artificial landscapes designed outside of natural areas. I don't see why it matters. Restoring natural areas requires us to restore their ecological function as well as their structure, but creating life in a landscape only requires us to maximize each patch of ground's ability to provide habitat for the greatest number of species possible. Those of us working in developed areas have an even greater urgency to make a difference in the relatively tiny plots of land given us to manage. We are not restoring ecosystems, but restoring habitat and doing that for as many species as possible. We therefore, have to take a different approach than those tasked with managing natural lands. We are managing the most unnatural lands imaginable and our plant palettes cannot be restricted if we are going to be as effective as possible. What is the point of artificially limiting our effectiveness?
Plant zones only tell us about winter cold. Natural geographic occurrence data only tell us a part of the story. All of us know that, even in a specific zone, a plant's distribution is often spotty and confined to very specific growing conditions. If I can meet those conditions outside of it's natural range, then it should be a plant I could consider for my home landscape.
Very few native plants will "escape" and disrupt natural areas if planted outside their natural range. They would have already done so all on their own. It is possible that local ecotypes of some rare variety of a native plant could be disrupted by planting a different ecotype in areas where they are likely to cross, but it also is true that adding to a gene pool - especially one that is isolated and cutoff from other populations due to development, sometimes benefit from the introduction of new genes to the gene pool.
My yard is surrounded by urban desert. Many of my neighbors have even given up on turf grass and replaced it with gravel. There is virtually no urban "forest" here as a canopy. I have no idea where the closest pine tree might be - the native tree that should be most abundant in this area and likely was pre-development.  Most of the mature trees are invasive non-natives. The understory, so to speak, is composed of invasive lawn weeds and non-native grasses that are mowed routinely. My native azaleas and the other native north Florida plants I've added to my woodland are not going to disrupt this "ecosystem."  Over the next few years, I believe they will add to the diversity of life able to exist here. I can see that happening already.