Using the USDA Hardiness Zone Map

I have written an article same time last year describing the effects that freezing temperature has on vegetation, and recent freeze events prompts me to bring up a common tool we use to choose our landscape plants that is related to annual low temperature: United States Department of Agriculture Plant Hardiness Zone Map.

I often have clients refer to the USDA Plant Hardiness Zone Map when thinking of purchasing garden plants. The department developed the map to define the mean minimum temperatures for areas within the continental and island United States, which in turn provides a supplemental resource to help consumers choose appropriate landscape plants for their region. The map is taken from data collected over a 30-year period from participating horticultural, botanical and climatological experts, beginning in 1976 and ending in 2005 for its revised publication in 2012. Data collected to construct the most recent maps uses Geographic Information System (GIS) resources, providing more accurate results for ongoing research. You can view the map online:

Keep in mind that the map should be used as a guide and not as an absolute authority; landscaping choices should be made using the map as well as site-specific conditions and additional research supported information. There are many factors at localized sites that can modify ambient temperature and plant reaction, such as relative humidity, drought, wind protection from buildings and modifying soil temperature fluctuations via mulching. The irony of the map is it provides lower temperature averages rather than information about mean maximum (i.e. heat-related) temperatures. The beauty of the map is that it is easy to understand, and offers the public awareness of localized weather events. The majority of Brazoria County lies in zone 9a of the map, average minimum temps between 20-degrees and 25-degrees; coastal areas such as Freeport lie in zone 9b of the map.

We already had hard freezes within the surrounding counties this past month, with a few more anticipated by end of winter. That means we need to remember the 4 P’s of winterization: pipes, pets, plants and people. A simple and effective way to protect tender plants from freezing events is to trap ambient heat by covering our plants. Make sure to use a cloth covering either alone or as a barrier between an outer layer of plastic. Plastic alone does not insulate the plants, can trap moisture on the leaves which in turn may freeze plant tissue. Drape the covering down to the soil and secure it with landscaping bricks, rocks or boards to help hold in the warmer air. If you have enough time to prepare, you might also try building hoop tunnels covered with landscape fabric; arches constructed with PVC pipe set 3 feet apart along the length of your garden bed and covered with row cover. You can also use tomato cages as a framework when covering for small stature plants. Be ready to pull back row covers during the day to allow for soil warming by the sun. Some of our cold snaps occur on crystal-clear nights with high winds. Make sure to water your garden well in advance of any freeze. Moist soil provides added insulation for the plants and helps keep plants hydrated through drying winds. Water only when temperatures rise above 45-degrees or higher the day before a freeze.

Landscape care tips to remember for the rest of winter: irrigate landscape plants once or twice a month in cold weather and as applicable, turn off your irrigation controller when expecting a freeze, and remember to hold off pruning any of your plants until mid-February (we often say “wait until Valentine’s Day.) We are trying to prevent initiating new tender growth from maintenance pruning until the last anticipated frost date for our area. Last frost date for Angleton and surrounding areas is estimated annually in early February.

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