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Gardening With Heat Zones
November 08, 2010

Thank you all that participated in our "Fall Favorites".

Readers comment to me on favorites all the time and how they enjoy them.

Favorites also give me an opportunity to get to know you just a bit better.

You will have a chance to participate in December as I will do a Christmas Favorites once again.

This gives you some time to dig into your memory archives.

I sound like a broken record, but every year I go into a bit of a funk when we fall back to Standard time.

It takes some getting used to, is all.

It is like losing an extra hour of daylight for me.

(Pictures are from this past week around here.)

The first week of November was a busy one.

After Monday morning's killing frost and freeze, there were many yard chores to finally tackle.

Annuals to cut back and pull out,

Certain perennials to cut back.

A couple of bird baths flipped over for winter (I still have a working, heated bath going).

There were a couple more quarts of tomatoes to can, giving me a good 70+ qts. off 4 plants and several green maters for frying and some to ripen for personal use.

Yes, it was an exceptional year.

There is deep watering the flower beds, and a few other odd jobs.

If I could have killing frosts as late as November 1st every year, I could like that.

So what if I get to tasks later in the season.

I would rather have color and some veggies throughout October than I would a barren wasteland look.

Tuesday was vote day.

Michigan has term limits which means most of the state officials will be new to the job.

We had snow flurries on Friday.

I know, I know ...................... wash my mouth out with soap and send me to my room for using the 'S' word.

Fear not, We are promised a couple of 60 degree days this week.

Yes, weather transition is in full swing.

The 'Honey Do' list continues to grow this time of year and much of it is put on hold.

The kitchen is gutted and remodeling began, only to have the contractors find out one of the counters is cracked and several cabinets are missing.

They did hook up water, however.

Now the rest of the house is in total disarray and kitchen stuff is strewn from stem to stern.

I can't paint or do other things Karen wants me to do........ uh, I mean I want to do right now (darn).

Hopefully things will be put together soon.

Walks with Keet aren't quite the same this Fall.

For the last few Autumns, (from puppy till now) Keet would always attack and kill the fallen leaves (or anything) that the wind would blow around.

This year she doesn't even flinch at the movement.

It makes for easier walk times, but less entertaining for sure.

Oh well.....................

Wild bird populations have changed dramatically the past week or so.

Just like that, Canada geese are fewer in numbers now, yet Mallard ducks are plentiful.

The duck population that comes to feed every evening has grown to a small flock of 22+.

I accommodate and feed them some cracked corn.

The wild turkeys still visit regularly.

I don't see the resident Great Blue Heron as often and the Green Herons have been missing for a few weeks now..

Red-winged blackbirds and Brown-headed cowbirds have left the area and this means less feeding, as these birds would show up in flocks and empty my feeders in no time.

I seem to have fewer House sparrows too and I haven't seen the albino one since I last reported.

The remaining juvenile White-crowned sparrows have moved on as well.

The joy is knowing they will stop by next spring for a few weeks and all of the birds will have a nice white striped cap and a happy song for me.

Dark-eyed juncos are more numerous,

American robins are forming larger, open flocks and remain in great numbers.

Killdeer are still hanging around, as I mostly see and hear them in suburban parking lots at night.

A few Red-breasted nuthatches have appeared, must be an irruptive year (more at a later time).

Along with my regular birds, I'm doing okay.

By now you should have finished repairs on feeders and nest boxes, if not there is still time before it gets to nasty outside.

Have you noticed a small spike in some of your bird feed?

Me too.

It seems that global markets and demands are driving up the price of corn and sunflower seed.

The cost of fuel for transportation is always an issue and this market is always on the lookout for any kind of an excuse to rise the price of fuel and oil.

Cost of transportation is always passed onto the consumer.

Last week I discussed Hardiness zones with you.

This week is a topic you may be less familiar with.

Heat Zones and planting.

You may be wondering why I am talking 'Heat Zones' in November, when the snow begins to fly.

Actually, for any gardener, this is a great time.

I start to think about and make some plans for next years gardens in September.

I want to know what grew and what didn't.

I want to try something different next year.

Will it work?

How hardy is a plant?

Will it tolerate heat and humidity as well as cold of winter?

How about watering or the amount of sunlight required?

These are all very important factors when planning your gardens.


You may have discovered, that too much heat can be as detrimental to certain garden plants as too much cold is for others.

And while most gardeners are familiar with the USDA Plant Hardiness Zone Map, few gardeners are aware of 'Heat Zones'.

Here is what you need to know about 'Heat Zones' and why knowing your zone can affect what you grow.

So what are 'Heat Zones' and how do they work?

'Russel Lupines' are hardy in zones 4-8 and do wonderful in my Michigan Z5 gardens.

Yet, do you think a Hardiness Z7 is the same in Washington or Oregon as it is in Texas or North Carolina in mid summer.

(They all have Z7 hardiness regions.)

Days and weeks or upper 90 degree days and even triple digit temperatures will do a number on Lupines.

Lupines and many other plants that may be cold hardy, simply can't handle the heat and the searing sun directly over head.

What about night time temperatures or humidity and moisture?

Barrel Cactus thrive in the heat of Arizona and New Mexico. yet die off in Florida and Georgia.


Cactus aren't built for the the winter rains and excessive wet times.

So you see, there is more to growing than know 'Hardiness Zones'.

A bit of history:

In 1997, Dr. H. Marc Cathey, President Emeritus of the American Horticultural Society (AHS), teamed up with Meteorological Evaluation Service Co., Inc. to create a new hardiness map for gardening consumers.

The result is a map featuring zones based on "heat days", that is, the average number of days each region experiences temperatures over 86 F (above 86 F cellular damage starts to occur in plants).

Data used to create the map was taken from the archives of the National Climatic Data Center and compiled from thousands of weather stations across the U.S.

The map is divided in 12 Zones with "heat days" ranging from none in Zone 1 (the northernmost zone) to over 210 days in Zone 12 (parts of Florida, Texas, and Hawaii).

How "Heat Days" Affect Your Plants

As I mentioned last week, apple trees need a combination of hot and cold weather to produce fruit, making them a natural for Regions like Washington state, Michigan and others regions where there are more than 600 hours with temperatures below 40 degrees,

Apple trees blossom later than some other types of fruit trees, making them more likely to avoid an April killing frost.

Many spring flowering bulbs need a good 14 to 16 weeks of cold as well or they wont blossom next spring or do poorly.

Yet these same plants require sun and heat to produce flowers and fruit.

Here in Michigan, the past two years are prime examples of how heat can effect our plants and gardens.

2009 was like the summer that wasn't.

Many plants that need the heat were a good month behind and many never matured.

This past summer was a warm one and those same plants and gardens were a good 2 to 3 weeks ahead.

A good 6 week difference from one growing season to another.

Yet for some plants too much heat isn't good.

Depending on the plant, sustained periods of extreme heat can shut down physiological processes.

Symptoms of heat stress can appear suddenly, or plants can linger in distress for years.

Some plants turn a ghostly pale blue-green and appear droopy, while others stop blooming,

Leaves may have a burned edge look or plants may drop their leaves, and in some cases, expire altogether.

If the heat stress doesn't ultimately kill them, it most certainly leaves them vulnerable to attacks from insects and disease.

What the Maps Don't Tell Us;

One of the biggest advantages that the AHS Heat Zone Map has over the USDA Hardiness Zone Map is that it assigns heat zone ratings not just to perennial plants, but also to annuals.

This includes flowers, garden vegetables, and herbs, as well as field crops.

With hardiness zone maps, gardeners could now zero in on a zone lines just about any where in North America to find out which plants they could possibly grow in their own region.

So here is how it works.......... Raleigh, North Carolina, is in the same zone as Reno, Nevada, Dallas/Fort Worth, Texas, and most of Vancouver Island in Canada

Could this be right?

In fact, the answer has to be "No."

You are beginning to see a few of the problems with the USDA winter hardiness map?

Each of the current USDA 20 climatic zones (Zone 1, 2-10a,b, Zone 11) is based on the average minimum winter temperatures.

In Zone 7b, (which includes the regions mentioned above), average winter minimum temperatures should be from 5-10 degrees F.

Keep in mind that there is often a major difference between what will grow in the "a" and "b" regions of each zone.

What the winter hardiness map doesn't tell us is how many times the temperatures dropped that low, and how long these low temperatures lasted.

There are a number of plants that can survive 5 degrees F. for a couple of hours, but could not survive these temperatures for a longer period, or more than once during a winter without protection.

Cold temperatures for one night is not the same as cold temperatures for a period of weeks, even though the same low temperature is reached in both cases.

In many cases, a low temperature of 0 degrees F., may cause cellular damage that will start to heal if the temperature rises rapidly.

If the temperatures remain low for several days, cell damage may continue and result in the death of the plant.

It is truly difficult trying to assign a hardiness zone to all plants, especially when using the minimal 10 USDA Zones.

This is why we find it critical to differentiate between the "a" and "b" zones whenever possible ... we would prefer a "c" and "d" also.

A drawback to growing new and different plants is that there is no information on their hardiness.

Another factor not taken into account by maps is winter acclimation.

A plant growing in our gardens in midsummer can be easily killed by temperatures in the 20 degree F range.

The same plant, if properly acclimated, can withstand temperatures of -20 degrees F.

Amazing what some mulch and care can do to stretch your zone growing.

We see the exact same thing in some late spring frosts.

After a certain number of hours at a specified temperature, each type of plant will switch from a dormant winter mode to a growing spring mode.

It is at this point that winter hardiness is lost.

If a late frost occurs while the plant is still in its dormant mode, there is little, if any, damage.

If the late frost occurs after the plants have switched to active growing mode, even a mature tree may be killed or severely set back.

If we have an abnormally warm fall, many plants that rely on cool temperatures to trigger dormancy can be killed when the temperatures drop suddenly.

Another phenomenon, seen in England and in the cool areas of the West Coast of the US, is the difference in winter hardiness due to a lack of summer heat.

In many plants native to warmer climates, summer heat causes increased sugar production, which allows the plants to survive more stress in the winter.

In areas without summer heat, a particular plant may only be hardy to 20 degrees F, while in an area with hot summers, the same plant may easily be hardy to 0 degrees F.

Like the USDA Hardiness Zone Map, the AHS Heat Zone Map should only be used as a guide.

The map is based solely on temperature and does not take into account unusual weather patterns, areas with sparse populations, humidity, or variances in nighttime temperatures-a significant factor in determining how far south a plant will grow.

The overall accuracy of the map is based on the assumption that plants are getting adequate moisture and does not factor in drought conditions.

In some areas, you may be able to create a microclimate (shade, moisture and so on).

Heat Hardiness Maps:

One of the most frustrating problems for gardeners in the south is summer hardiness.

Reference books and most plant catalogs have completely neglected the effects of heat on plants.

The summer sun can be HOT.

Although the AHS Heat Zone Map is not as widely used as USDA Hardiness Zone Map, it is showing up in more and more catalogs and an increasing number of plant labels-over 15,000 different plants have been assigned heat zones since the map was first released.

If included, the heat zone numbers usually appear directly after the hardiness numbers on the label.

The highest number (hottest zone) is listed first-the exact opposite of how the USDA Hardiness Zones numbers are presented (lowest zone to highest).

Many plants from the north are not able to withstand our hot summers.

A I mentioned above...............

Lupines do well in My Zone 5a gardens and Vancouver (Z7),and thrive throughout most of the temperate regions.

Yet this same species, suffers dearly in Zone7 heat of Texas, Nevada and North Carolina.

Enter the Heat zone Map:

In 1997, the American Horticulture Society published a "heat map", and while a good idea, the map as published serves no practical purpose for gardeners.

The Heat-Zone Map confuses gardeners with two sets of numbers ... i.e., Zones 5-9 and 9-5. One set of numbers is for cold and one for warmth.

There is no reason that one complete map could not serve both needs.

Gardeners in Zone 7 on the West Coast can grow many successfully that can't stand the heat of summer in other Zone 7 locations.

What gives?

The Heat-Zone Map simply shows the number of days above 86 degrees F. for each region of the country.

Heat hardiness is more an issue of night temperatures, humidity, and precipitation during the hot season, not simply the number of days above 86 degrees.

As we mentioned this, another issue enters the picture when we talk about night temperatures as compared with day temperatures.

In many cases, the culprit is not only the high day temperatures that cause plants problems, but also the high night temperatures.

During the day, plants store up energy produced through photosynthesis.

If the nights are cool, the energy goes into growth of the plant.

If the nights are too warm, the energy is burned up by the plant.

Many plants, due to their metabolism derived in a cool night climate, are not hardy in other areas, simply because of their warm night temperatures.

There is still another factor in heat hardiness that we have overlooked ... one of dormancy.

A hosta, for example, will not grow well in parts of Florida, (parts of Zone 9, and 10).

The problem here is that temperatures do not drop low enough in the winter time for the plant to go completely dormant.

Many plants, both perennial and woody must have a specific dormant period in order to start growth again in the spring.

A hosta must be exposed to at least one month of temperatures below 40 degrees F.

If this temperature requirement is not met, the plant will begin to decline in the spring, or in the case of some trees, will never resprout in the spring until the dormancy requirement has been satisfied.

The AHS map is under copyrights so the only way I can share it with you is to send you directly to their website.

Click here for AHS Heat Zone map.


What I hope you will realize is that growing plants can be very complex.

The hardiness zone maps are a great guide, but are only a guide and only when the zones assigned to plants by nurseries are accurate.

Don't be frustrated when a new plant dies, and certainly don't give up trying to grow that particular plant.

After you kill it three times, use the compost you've created to help grow another plant.

If you live in suburbia or a city where asphalt, concrete and buildings heat up and retain some of that heat, your zone may actually be one or two zones higher than your rural friends.

City landscapes may be a good 10 degrees warmer than suburbia and even more so than rural areas, that adds a higher zone to your growing habitats.

Micro climates exist throughout the planet as well.

Trees are good and it is suggested to plant more trees to cool down local temperatures, and it is known that trees absorb carbons and pollutants from the air.

What so called experts fail to tell you is this.......................... trees have to dispose of the carbon somehow and falling leaves and needles are mostly carbon.

Carbon that goes right back into our planet and the air we breathe.

Go figure.....................

I digress.

The AHS was awarded a grant to update the USDA Hardiness Zone Map.

They studied 30 years of weather data and are in the process of updating the zone maps to include mitigating circumstances such as the length of cold spells in the winter, airflow patterns, the effect of large bodies of water like oceans and lakes and heat factors.

The distinction of a and b sub-zones is gone. There will now be 15 zones instead of the current 11.

Realizing the effect that heat and humidity have on plant hardiness, the American Horticultural Society also divided the United States into twelve zones, divided by the average number of days each year that a given region experiences temperatures over 86 degrees.

The zones range from Zone 1 (less than one day above 86 degrees) to Zone 12 (more than 210 days).

Still, there are those years when 'Nature' lets you and me know who really is in control.

Pushing the envelope is more than Zone hardiness, it also means heat and other factors.

Here is a reverse side of things.

To have Spring blooming bulbs like tulips, etc. you must have at least 14 to 16 weeks of temperatures in the 40's or cooler.

Planting an apple tree?

Apples need a good 25 days with temperatures below 40 degrees F. in the winter to encourage them to set fruit.

No Ifs, Ands, or Buts About It.

Next week I will discuss some of these for you as well and I promise to be a shorter letter.

Time seems to whiz by when you're having fun, and it is time for me to fly for now.

Before I go, here is your positive thought for the week.

Remember that if the opportunities for great deeds should never come, the opportunities for good deeds are renewed day by day. The thing for us to long for is the goodness, not the glory.

F.W. Faber

Seek to do good, not to be a glory hound.

I can live with that.

"Little children, let us love, neither in word nor with the tongue, but in deed and truth."

1 John 3:18

"Treat the earth well:

It was not given to you by your parents,

It was loaned to you by your children.

We do not inherit the Earth from our

Ancestors, we borrow it from our Children."

Ancient Indian Proverb

Your friend indeed,

Ron Patterson

PS. If you enjoy these letters, please forward them to friends, family and co-workers.

Better yet, have them sign up so they can receive their own letters.

Gardening For Wildlife.

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