What do usda zones mean

Zone 10 sees some of the hottest temperatures in the U. Tropical plants flourish in Zone 11, which covers Hawaii. There is no frost whatsoever, and native plants thrive throughout the whole year. Save Pin FB More. Even if a plant is rated for your hardiness zone, taking proper care of it will improve its chances of surviving in your climate.

Credit: Marty Baldwin. Related Items Japanese prostrate willow growing on ground. Credit: Laurie Black. Credit: Robert Cardillo. Credit: William N. Credit: Denny Schrock. Credit: Bill Stites. Credit: Adam Albright. Variegated English Holly. The zones range from 1, which includes the chilliest regions of the United States like Fairbanks, Alaska, to 13, which encompasses warm areas that rarely receive frost like Puerto Rico.

Each numbered zone has a difference of about 10 degrees Fahrenheit from one to the next. The zones are further divided into subzones designated by letters a or b, which indicates a difference of about 5 degrees Fahrenheit and adds further nuance and accuracy between specific regional areas. Hardiness zones, Garland explained, are most useful for determining what perennial plants will survive year-round in your area, though they are also useful for determining how well and how long annual plants will thrive.

The online version has a feature where you can search hardiness zone maps by zip code or state. Seed packet maps are often based off of this information as well. The updated maps not only reflect changing climatic conditions — the latest edition is generally on 5 degree Fahrenheit subzone warmer than the previous one — but also more sophisticated methods for recording weather conditions with a greater number of weather stations and consideration for different geographical factors like elevation and proximity to water.

According to the map, Bangor is in zone 5a, with average minimum temperatures ranging between and degrees Fahrenheit. The northernmost regions of Maine go as low as 3b and there are areas along the southern coast that classified as zone 6a. The map does not account for the nuances of factors like the beneficial effect of a snow cover over perennial plants, freeze-thaw cycles or soil drainage during cold periods. West of the th meridian, the USDA zone map is less useful. Each zone can be further split into A and B, which indicate slight variances even within these zones.

For the sake of outdoor gardeners and landscapers, plants have been split into a range of survivable zones. A hardier plant will fit into a broad range of numbers, whereas a picky plant might only survive in one zone. With annuals the time of planting can often be adjusted to allow growth beyond their normal geographical range.

The main factors determining average minimum temperature are altitude, latitude and proximity to the coast. There are problems with maps of this type: the spread of weather stations is insufficient to give clear zones and too many places with different climates are lumped together. Only Australian stations have records of more than ten years one station per 98, hectares!

Local factors such as aspect, altitude, proximity to the sea also complicate the matter. For example, Mt Isa has three climatic stations with more than a ten year record. One is in Zone 4a, one in Zone 4b and the other is in Zone 5a. Likewise Sydney residents can choose between Zones 3a to 4b. Most other cities have similar problems. Different locations in the same city are suitable for different plants making it hard to draw a meaningful map. There may even be a case for publishing a list of weather stations and their zone Classification to allow best use of local conditions.

The map is thus only useful as a broad guide. It needs interpretation that takes into account factors other than low temperature, as well as local knowledge. An alternative system for describing plant hardiness is to use indicator plants the USDA also publishes a list of these to go with their map.