Planting Zones
In order to help gardeners determine which varieties of plants will grow best in local climate conditions, the U.S.D.A. has devised a system of hardiness zones.
Hardiness zones, developed by the U.S.D.A., provide a means of comparing prevailing atmospheric conditions and the durability of various species of plants. These detailed computations and projections unique to a given place describe the climate by quantifying the range of temperatures during the cold months over a period of years. Horticulturists then have a gauge to figure out whether or not a certain kind of plant can survive winter conditions in such a place.
Hardiness zones, along with reliable recommendations of plant species based on appropriate use of the maps generated by detailed climate statistics, guide residential gardeners in selecting all manner of plants that will thrive and prosper in their specific region of the country. From flowers and flowering bushes to shrubs, succulents, and trees, hardiness zones provide gardeners with a valuable tool that aids in selecting, planning, and achieving vibrant and beautiful landscape projects.
Hardiness zones, implemented by the U.S.D.A., use annual climate statistics to quantify the severity of winter in a given region for the purpose of determining which plants would best flourish there.
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Seasonal gardening guide
A monthly game plan for busy gardeners
From landscape planning to ordering and sowing seeds, to the delights of spring’s first tender plants, there’s fun to be had in the garden in every season of the year. Keeping note of the best times to do different tasks in the yard also makes for larger yields in both the flowerbed and vegetable patch, and keeps a gardener’s spirit growing year round.
January
Because temperatures can be the lowest in January, now is the time to ensure that tender plants that have been mulched heavily for winter are still healthy. With Christmas just past, compost the tree and other natural trimmings, such as wreaths and centerpieces. Greenery can be cut up and used as insulation on annual beds or be chipped and added to the compost pile.
February
February is ideal for pruning fruit trees. On a temperate day, remove any dead wood and crossing branches. Concentrate especially on young growth and shoots running vertically to old wood.
This is also a good time to prepare materials for plants sown indoors. Sanitize all pots, tools and racks. Buy necessary materials: soils, new light bulbs for growing fixtures, and seeds. Test older seeds for potency.
March
As soon as the soil is workable, start preparing beds for planting. Apply time-release fertilizers (where necessary) and several inches of good quality compost. Remove early weed sprouts and tidy up raised beds. Since much of the country tends toward soggy weather in March, this is also the time to be vigilant about slug and snail eradication.
April
April is a great month to remove winter mulches and make way for new plants. Discard mulching materials to the compost heap. Check on any perennials divided in the fall. Make sure that their roots have remained well covered and discard materials that did not survive last fall’s transplant. This is also the month to divide perennials. Take pie-shaped wedges from established plants and reuse them in other areas of the garden.
May
It’s time to reintroduce plants to the outdoors that have been wintered inside. Take tropical plants, citrus, and other frost-tender species and leave them outside during warm afternoons. In many climates, this is also the month to start planting the vegetable garden in earnest. May is also the season to prune back early flowering trees and shrubs.
June
Tackle lawn care in June. Remove any clumps of crab grass from the lawn and apply a nitrogen rich fertilizer and weed killer. Also, take out the mower, clean the undercarriage, and sharpen the blades. Keeping the lawn regularly cut, removing no more than 1/3 of its total height, will keep it in top shape all summer. Plant colorful annuals for late summer displays.
July
July is the high season for annual weeds, so it’s more important than ever to stay on top of them. Use manual methods first and chemical methods only when absolutely necessary. Discard any seed-bearing weeds in the refuse or in an autumn burn pile.
Since this is the season of greatest glory in the garden, why not take some time to enjoy all the hard work? Plan a garden party or just enjoy a cool iced tea in your favorite spot in the garden.
August
Concentrate on deadheading during August. Removing faded blooms will elongate the glory of annuals and keep the garden looking tidy through the first frost. Watering intelligently is also important in the late summer. In the morning or early evening, give lawns and beds deep waterings. This will encourage good root development.
September
Transplanting is most successful in the fall. Prepare beds for new divisions by weeding, removing faded annuals, and fertilizing. Dig up perennials for division and separate clumps with a pair of garden forks. Replant both sections immediately and water well. September is also a good time to clear out the vegetable garden and to get it ready for a winter rest.
October
If it hasn’t been done already, October is the month to protect frost-tender perennials and to trim back spent foliage. Spring bulbs should also be put in the ground and fertilized. Planting crocus and daffodils in drifts will make for brilliant spring displays.
November
Take a well-deserved break.
December
It’s time to start plotting all over again. Pour through seed catalogs, using small sticky notes to mark potentials for next year’s garden. Make a drawing of your yard and consider last year’s successes and learning experiences. Make appropriate changes to the plan and order plants and seeds for the spring.
Zone flower planting times guide
The fine art of orchestrating blooms
One of the best tools for determining which plants may thrive in a given climate and when to start different flowering plants is the United States Department of Agriculture’s map of planting zones. This helpful guide illustrates average low temperatures all across the United States. The U.S.D.A. has compiled data for 60 years to make the maps we use today. This depth of data makes the map both reliable and extremely useful. The U.S.D.A planting zones map provides information for large swaths across the country, and even for smaller microclimates, in some cases. These subdivisions act as a starting point for the garden planner and describe planting times for flowers and other garden mainstays.
Timing is everything
Most of the plants that we use as flowering ornamentals in the garden can be broken into three broad categories: bulbs, annuals, and perennials. In the most general sense, each of these categories of flowering plants has a season, which is an optimal time for planting:
- Bulbs that flower from early to late spring should be planted in the fall. Exactly when to plant can be determined in part by one’s planting zone. In general, the colder the climate the earlier the fall planting date. Some bulbs that bloom in the summer need to be planted later. Many of those bulbs and tubers that produce summer’s floral bounty are vulnerable to frost; these plants die when temperatures drop below freezing. In northern and very moist winter climates, these bulbs should be planted after the threat of winter rain and deep frost has passed.
- Annuals, like the frost-tender summer flowering bulbs, can only survive and thrive in the balmy temperatures of summer. While annuals put on their shows from late May to frost in many of the U.S.D.A.’s planting zones, they too have a preference for specific planting times. In any climate, they should be planted as soon as the threat of frost has passed and before hot summer temperatures. Depending on one’s planting zone, this could be February to May. In fact, some warm climates support annual planting all winter long. The later annuals are planted, the less time they have to produce blooms and foliage and the more likely they are to be stressed by punishing summer heat and drought.
- The most forgiving of the flowering plant categories are the perennials. Flowering perennials can be planted in a variety of seasons with good results. The prevailing wisdom is to plant these hardy flowers in the fall when cool weather and moist conditions make for easy root transitions from pot to ground. Despite this, fall “sweet spot” perennials can do just as well when planted in spring, as soon as the soil is workable. Finally, for those of us seduced by the garden center’s high-season displays, perennials can even be planted in the summer. When plating in the summer, the gardener must take extra care to make the plants transition from the pampered nursery conditions to the wild of the garden as smooth as possible. Some ways to ease this difficult transition are to plant in the cooler evening or early morning temperatures, keep the plant mulched and watered, and give the plant the best soil conditions possible.
- No matter which zones a gardener lives in and what type of flowering plants they enjoy, timing in planting can mean the difference between breathtaking success and unfortunate failure. A careful examination of a map of planting zones and an understanding of the conditions in an individual’s area are the best first steps in timing the planting of a bountiful flowering garden.
U.S. plant hardiness zones
Getting in the zone
Hardiness zones help gardeners determine which plants will thrive and prosper in a given climate. A hardiness zone is a geographically defined zone in which a specific type of plant is capable of growing, as determined by temperature hardiness, or its ability to withstand the minimum temperatures of the area. These zones were first implemented by the United States Department of Agriculture (U.S.D.A.), and have subsequently been adapted for use in other countries. The zones are differentiated from one another based on the average of the lowest temperature recorded each winter, termed the "average annual minimum temperature."
In North America, the official hardiness zones map was revised and published by the U.S.D.A. in 1990 and serves as a guideline for selecting annual plant varieties. Each zone in the United States is labeled with a number ranging from one to 11. In 2003, a preliminary draft of a new map was produced by the American Horticultural Society (A.H.S.), using temperature data collected from July 1986 to March 2002, a period of atypically warm winters in the eastern part of the United States. The 2003 map places many areas a zone higher (warmer) than the 1990 map. The proposed map also shows a finer level of detail, for example reflecting urban heat islands by showing the downtown areas of several major cities as a full zone warmer than the outlying areas. As of the current date, the U.S.D.A. still endorses the older version of the map.
Plants vary to an extreme degree in their ability to withstand extremes of climate. The ability of a plant to withstand cold is the most basic and pervasive measure of its hardiness. The selection or breeding of varieties capable of withstanding particular climates can be challenging even to the most experienced agriculturalist or horticulturist. As living organisms, plants can adapt to some extent to changes in climate. Much of the work of nursery growers of plants consists of hardening (or hardening off) their plants to prepare them for likely conditions later in their lifespan.
For best results
The hardiness zones are effective in that, for many situations, extremes of winter cold are the most important factor in deciding whether a plant species can be cultivated outdoors at a particular location. However, some discrepancies exist in the system, requiring aspiring gardeners to approach plant selection cautiously. Hardiness zones generally address issues of cold and conspicuously ignore summer heat levels. Paradoxically, a site that may have the same average lows over the winter but markedly different summer highs will still be accorded the same hardiness zone. Gardeners attempting to grow plants at the extreme ends of the spectrum of hardiness zones in which that plant can survive may find that an unusually cold winter or hot summer threatens the success of new plantings.
In addition, many plants and plant species have special characteristics that may make them harder to grow in certain climates. Many plants will not live through severe winters; others will wither in heat; and many spring-flowering bulbs and trees need a cold period to stimulate their growth cycles. Plants usually survive the cold better in a dry area, where moisture trapped underground cannot compromise root systems. Deciduous plants tolerate more exposure to sun and summer heat. Evergreens prefer a sheltered area with more humidity and cooler summers.
Zoning out
There are also different climates and frost dates within planting zones due to topography, lakes, rivers, canyons, or mountains. These can cause altered airflows that can raise or lower the temperature, changing the zone in a localized area. Densely populated urban areas often experience warmer climates than surrounding areas due to the influence of man-made objects and architecture. The chemical balance and texture of the soil, exposure, altitude, rainfall, humidity, sun light levels, and wind chill factors can also alter the local climate subtly, potentially skewing the predictive accuracy of plant hardiness zones.
International planting zones extend the work of the U.S.D.A. in creating hardiness zones to help gardeners all over the world find the right plants for their own use. Useful for cultivating exotic plants and planning novel species, these systems can help you get the best garden ever.
International Planting Zones
The best-laid plans
International planting zone maps and charts help gardens stay in peak condition all year. With all the considerations that influence a plant’s success (water needs, light requirements, and soil conditions, along with a host of other important issues) it can be difficult to know which plants will grow well in any given area. A good place to start is to know a plant’s winter hardiness. Thankfully, this information has been compiled in easy-to-use maps.
Planting zones, also known as hardiness zones, were first developed by the United States Department of Agriculture in the 1960s. Since the 1960s, the U.S.D.A. has been compiling data and creating maps to aid large-scale agricultural producers and hobbyist gardeners alike. These maps separate the country into many smaller micro-zones based on minimum annual low temperatures. Hardiness maps and their corresponding zones give gardeners information about which plant species are most likely to survive and thrive in their respective areas. Each zone in the United States is labeled with a number ranging from one to 11. Each of these numbers corresponds to a mean annual low temperature. For example, areas in Zone 1 have average low temperatures of about -50 degrees Fahrenheit, while those in zone eleven average lows of 10 degrees.
Taking the show on the road
Since the system used in the United States is founded on such simple principles, other nations around the globe have made similar maps based on their winter low temperatures. Many of these international hardiness zone maps are accessible online and in print through specialty garden retailers and through international garden societies like the United Kingdom’s Commercial Horticultural Association. A word of caution: since maps created in different international locations may not adhere to exactly the same system developed by the U.S.D.A., not all spots in any given zone will have exactly the same mean winter low temperatures.
If a gardener has concerns about the reliability of a zone map’s information or the compatibility of different international hardiness zones, there is a simple way to calculate the correct zone. To approximate a hardiness zone, find out what the average low temperatures are for the area you want to label. Information about average annual lows can be gathered at national weather services, as well as agricultural extension offices. This information translates to a corresponding hardiness zone label of 1 through 10.
Final calculations
Part of the beauty of hardiness zones is in their simplicity, but that lack of complexity does make for some limitations. In the gardener’s vernacular the term hardiness takes into account many things. In addition to cold, hardiness factors include heat, drought, and wind tolerance. While knowing one’s zone illustrates which plants are more likely to survive the punishment of winter, the zones only address the coldest climatic conditions. This means that while a plant may survive a garden’s winter temperature, that same plant may die in that climate’s summer heat. In short, these zones take no account of soil conditions, rainfall or other factors essential to the success of garden plants. That said, hardiness zones are useful when the information they contain is utilized properly.
Power tips
The best way to use planting zones is as a tool to determine a plant’s likelihood of surviving winter low temperatures. This can be especially constructive when dealing with exotic, unfamiliar, or non-native plant materials. For example, a gardener living in Dublin, Ireland (in zone 8) could expect a non-native plant from other zones also labeled 8 to over-winter in his or her climate. This information is increasingly useful as international plant distributors move species around the globe at a record pace and exotic plants find their way into gardens all over the world.