Post written by Allen Centennial Garden Horticulturist, Isaac Zaman
In late 2023, the US Department of Agriculture (USDA) released a new zone hardiness map, updating the version they released in 2012 (which was an update from 1990). The new map illuminates much about the recent changes we have felt and seen in our climate this winter and helps to inform us about the choices we are making at Allen Centennial Garden to combat climate change.
One of the highlights of the new map is how detailed it is. The original plant hardiness zone maps were generated with decades of data, the first being released in 1990 with data from about 7,000 weather stations in 1974-1986 and then again in 2012 with updated data. This new map was generated from the past three decades of data showing a trend of warming all across the United States. This data was also bolstered by an astounding 13,000 weather stations – almost double the amount used in the previous map. All of these measurements have led to a clearer understanding of how maximum low temperatures have been changing. The data mostly comes from weather stations. These are spread across the country with higher concentration around larger cities with more resources. This results in zone borders that are clearer and better reflect small differences in certain regions (caused by things like urban heat islands or bodies of water). In addition to being more detailed, the data also reveals a general trend of warmer temperatures, which the map shows as zones tending to be farther north in the 2023 map.
The new USDA zone map is available to everyone and can be a good first tool to use when learning more about your growing space. If you are curious about a specific area’s zone, feel free to hop on the USDA interactive map site to input your zip code and maybe you will find specific data relating to these new sites and maybe learn something about your area that you didn’t know before. Do note that this data is measuring the lowest experienced temperature within an area and can’t account for minute differences you may have in your area such as structures, bodies of water, forests, or topography. This is just to give you a general idea about the wider area surrounding your space, and for most it may not change much at all since the difference between half zones like 5a and 5b are only changes of a couple degrees and most plants that will grow in one will grow in the other.
When we shift our frame of thinking to what this means for places like Allen Centennial Garden, taking a deeper look is quite interesting. Here is the overall Wisconsin plant hardiness map as released in 2023:
At first glance it is hard to tell Allen Centennial Garden is in, but when you use the interactive map you can zoom in on campus, which is right on the edge of zone 5a and 5b. The detail in this section indicates that we are in an area with lots of data gathering sites like those found atop university buildings and dedicated weather stations like the one found at the Eagle Heights Community Garden. However, these data points don’t tell the entire story since microclimates exist within these areas that aren’t reflected because we aren’t measuring down to the square meter. At best, we can make inferences through this map.
The truth is that ACG has always been an area on campus that exhibited strange microclimates due to our topography and our proximity to Lake Mendota.
Although the map doesn’t give us the entire picture about what we can plant in our area, we still use the idea of being hardy to guide the decisions we make about what plants we use in the Garden. A plant being hardy to a specific zone means that it is likely to be able to survive our winter temperatures. The Garden has always been an exception for planting things that aren’t quite hardy even before the differences came out on paper. Previous horticulturists planted trees and perennials that were considered edge-hardy, which means that those plants were unlikely to be able to survive the coldest temperatures our zone was likely to experience (-20°F in the case of zone 5a). However, because of the Garden’s microclimate they were able to survive. Some of these plants included the old Japanese Maple and several perennials in our rock garden. Effects of being edge-hardy still showed in our Japanese Maple, like winter damaged buds, high disease pressure, and stunted growth in certain years. Despite all of that the maple survived for many years more than the ACG Team expected before succumbing to all of the negative factors during the drought we had in 2023. Luckily, our other edge-hardy specimens are still around handling all that our Wisconsin weather has to throw at them.
These microclimates also affect other aspects of our Garden, like the acceleration of seasonal shifts we see in our bulbs and perennials. Plants in the Garden seem to move faster than other plants around campus, with tulips and daffodils blooming two weeks before anywhere else or trees budding and blooming before other members of the same species on the opposite side of town. Some of these anomalies are due to abiotic factors in our space – one example is the Dean’s Residence which affects temperature in the beds since the foundation holds heat and the space itself is heated during the winter which keeps the plants warmer too. It is these observations combined with the inferences from the USDA data that allow us to plan for the future and plant things that will be able to handle the coming changes and stay resilient as we move forward.
New in the garden this past spring are many specimens that follow this strategy of planting for different temperature changes we are likely to see in the future. To list a few, we’ve planted a Northern Catalpa tree that is a cross of a southern variety, a climbing hydrangea meant for zone 6, and several ‘Chicago Hardy’ figs that all have survived a year in our climate already as of 2023. These plants are nestled in areas of the Garden that were suited to them: the hydrangea surrounded by the cover of our English Garden and pergola, the catalpa shrouded with the cover of other giant trees, and the figs cozied between our brick paths and mulched over for winter. Pushing the boundaries can be a fun part of gardening once you get to know your garden, and if you are curious about what you can do in your space, come see us at ACG and ask us about our other fun ways to overwinter plants outside!