Winter Review: Hardiness Zones

Winter is the ideal dream time for cold-climate gardeners, but it’s also a perfect time to review good gardening practices and increase your gardening knowledge. For this Winter Review we’ll examine plant hardiness zones.

Plant hardiness zones depict general temperature-range regions. To help avoid gardening disappointments, it’s good to know your specific plant zone. Find the USDA plant hardiness zone/s in your state,  then get a closer look by searching by zip code.

If you live in Connecticut, you should not assume a plant growing in one part of the state or region will work in a more northern or southern landscape in the state. On the USDA map, Connecticut ranges through 1 1/2 zones -from 7a in Long Island Sound coastal areas to 5b in the northwest corner.

USDA Plant Hardiness ct_ri zones

The same search at Plant Maps shows Connecticut zones ranging from 7a along the shoreline to 5a in the northwest corner. On both maps the ‘a’ designates a slightly warmer and the ‘b’ indicates a slightly colder sub-zone within the same numbered zone.

A closer examination of your specific town, may show you living and gardening on the edge. For instance, the town in which I live, garden, and work is divided into zones 6b and 6a on the UDSA map and into zones 6a, 6b, and 5b on Plant Maps Connecticut map. This means a plant that is borderline hardy in my zone 6b garden may struggle in an open, unprotected section of a garden just a few miles to the north.

You could let these designations make your head spin, but don’t. Plant hardiness zones serve as a good general guide when choosing plants most likely to survive in a region, but choosing a plant appropriate for your zone does not guarantee survival.

OLYMPUS DIGITAL CAMERA This tag for Hakonechloa macra ‘Aureola’ informs this Golden Japanese Forest Grass is most likely to thrive in zones 4 to 8 provided it is also planted in soil, light, and moisture conditions that meet its needs.

When shopping for plants at a garden center, in a catalog, or online it is important to pay attention hardiness zone recommendations. Just because one ornamental grass is hardy to your zone does not guarantee all others are. The same is true for many types of shrubs and perennials – not all varieties are equally hardy in the same zones.

If you crave a perennial hardy only to a warmer zone, say a zone 9 plant when you garden in zone 6, accept that this plant is not likely to survive your zone 6 winter. In plant talk this is a tender perennial. If you must have it then plant it knowing it will die over the winter or find out if it can be overwintered in a pot in an unheated garage or basement.

On the other hand, if you crave a perennial hardy to zone 7, but garden in zone 6, the plant might survive when situated in an area protected from harsh winter winds, if given extra layers of mulch, or if planted near hardscape that tends to help maintain slightly warmer soil temperatures.

Taking such steps is taking advantage of microclimates, which will be the topic of my next Winter Review … stay tuned.

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Connecticut’s new planting zones

The USDA just released a new plant hardiness zone map. Plant hardiness zones are listed on plant tags to communicate the regions in which plant should survive without extra winter protection.

Many will find their zone has changed by a half – mine went from zone 6a to zone 6b, slightly warmer.


Will this alter how I garden or what I plant? No. The map does not take elevation and cover-, slope-, or hardscape-related issues into consideration. Only eyes and experience can do this. I know my property. Plants, shrubs and trees listed as borderline in zone 6 may not survive tough winters without coddling (this means extra winter protection and extra work). If I must have a plant that is borderline in zone 6, I do so with the full knowledge that coddling/extra work will be part of my regular gardening routine.

This is exactly what the USDA recommends on their Maps & Gardening page.

The new map considers 30 years of climate data – a longer period than the 15 or so years previously used and used in other plant hardiness zone maps. More about how the new zone map was created is on the USDA’s What’s New page.

The new map is more interactive than previously. After choosing to view by State, Region, or Nationally, and choosing a size (try Standard then move on to larger versions), you can preview, open or save your map choice.  If you start here, and type in your zip code your zone will pop up under your zip.  Or, for a closer-up view, click on the Interactive Map tab and type your zip code. In the interactive map you can activate a satellite image that shows up under the zone colors. If you increase the transparency of the zone color layer it’s easier to locate your property on the satellite image.

This is particularly useful when trying to determine planting zones in areas near zone borders. For instance, my property, listed as zone 6a in the older USDA map and in the Arbor Day Hardiness Zone Map, is now located very close to the edge of zone 6a in south-central Connecticut but is newly listed in zone 6b.

Follow the various links above and play with the features, but don’t forget that we still must garden thoughtfully. Hardiness zones are guides. Gardeners with the time, experience and where-with-all to push a hardiness zone – say by planting a zone 7 perennial in a zone 6 garden – can be quite successful. Gardeners looking for less maintenance and fuss should stick to their zone or acknowledge that a perennial listed hardy in a warmer zone should be planted as an annual or overwintered with protection.

Digiprove sealCopyright secured by Digiprove © 2012 Joene Hendry

Don’t get bit by phenology indicators

You know how those moisture-retaining gels swell when placed in water? That’s what I envision happens to brains of new gardeners as they soak up any and all available how-to gardening information – I was a novice gardener once long, long ago and did just that. It’s important to seek learned, sound advise from gardening friends, trusted nursery staff, local agriculture extension departments, books, blogs, websites, and magazines. Unfortunately not all gardening information is as clear as it should be.

white-lilac-4-5-09_edited Take the recent issue of Horticulture magazine. In the Q & A section and also online, the editors list characteristics of plants that purportedly indicate it’s time to do other gardening tasks. These ‘indicator’ plants – for example lilac shrubs, oak trees, daffodils, dogwood, lily-of-the-valley, and others – can serve as hints for sowing seeds or planting transplants. Many on the list make sense, such as planting cool season crops (peas and lettuce for example) when lilac leaves are about as big as the ears of a mouse or when daffodils bloom.

Such clues come from years of phenological observations. Scientists, gardeners, farmers, and general nature lovers have watched the timing of plants leaf-out, flower, seed ripening, late season leaf color changes, and leaf drop and noted correlations between the growth timing of indicator plants and the progress of the seasons – similar to that done among Project BudBurst scientists and citizen volunteers. Likewise observations can be made for animal habits. These observations provide valuable general information on overall climate change, plant and animal characteristics, and how weather pattern changes impact every living thing.

But general observations do not take local conditions into account … Horticulture neglects to point this out. Some of the correlations on their list make sense – such as using lilacs and daffodil stages as indicators for planting peas and lettuce (cool-season crops). But some of the suggestions could set up novice gardeners for failure. For instance, among the list of  “some common garden plants and what they indicate to you:” Horticulture mentions planting squash and cucumbers when lilac flowers fade, or beans when lilacs are in full bloom.

In my neck of the woods (Connecticut zone 6a) lilac flowers faded last week after blooming 2 to 3 weeks earlier than ‘normal’ which means outdoor soils are still too cool to allow warm-loving squash, cucumber, and bean seeds to thrive. Such seeds, when planted in cool soils, sit in a sort of suspended animation while waiting for soil temperatures to warm. The past week a couple of nights brought below freezing temperatures in most of Connecticut’s zone 6a, and cloudy, wet weather.  ( Connecticut has three cold-hardy zones, 6a , 6b, and 5a.) Soil just doesn’t warm very quickly when temperatures are this low and the sun doesn’t shine.

Lavender Touch Eggplant Two other indicator plant suggestions – to plant tomatoes when lily-of-the-valley blooms, and peppers and eggplant when bearded iris bloom – could entice novices to plant these warm-season crops in the ground too early as well. In my neck of the woods weekend air temperatures were warm, but the soils are still too chilly to encourage tomatoes, peppers, or eggplant to do more than hang dormant until the soil warms. Such exposures often stunt warm-season transplants to a point from which they may never recuperate. And while there are some maneuvers one can do to warm soils up earlier than would otherwise happen, the use of such techniques is not generally common.

I expressed my concern as follows :

Dear Horticulture Editors,

I applaud your Q & A piece (June/July 2010 issue) on phenology – I’ve participated in Project BudBurst for the last two years and highly recommend the project to others. However, some of the indicator plant suggestions you list could lead novice gardeners to failure. Spring blooms have been unusually early in my zone 6a garden this year. Were a novice gardener to plant tomatoes when lily-of-the-valley bloomed, they would have placed warm-loving transplants into the ground when actual nighttime temperatures fell to freezing. Likewise, bearded iris began blooming around May 15, yet soils remain way too chilly for warmth-loving peppers and eggplant.

Since outdoor planting dates can vary significantly according to locale, wouldn’t it have been responsible to advise gardeners to take note of local conditions and seek planting date advice from trusted local sources?

Nationally geared magazines often rely on generalities that suit gardens across the U.S. However, when it comes to spring or fall planting dates nothing beats solid local knowledge and experience. So take note …  when seeking temperature, rainfall, or hardiness questions about plants, do so locally.  It just might save your valuable tomato, pepper, and eggplant transplants from hypothermia, and you from a lot of extra and unnecessary planting and angst.