My Favorite Book on Cold Hardy Tropicals

Dr. David Francko has written an excellent book about growing warm climate plants in cooler areas titled “Palms Won’t Grow Here and Other Myths: Warm-Climate Plants for Cooler Areas”

This is actually my favorite book on Cold Hardy Tropicals.  I got to meet Dr. Francko a number of years ago when he gave me a tour of the Miami University Campus. I was considering at the time writing my graduate thesis on Cold Hardy Palm trees. The tour was very informative and it was inspiring seeing all the interesting plants growing in southern Ohio.

Getting to the book review itself I purchased this book when it was first published and have referenced it ever since. I think the title says it all for those of us interested in growing interesting tropical type plants. Palms Won’t Grow Here and other myths. Warm Climate Plants for Cooler Areas. 

The book goes through very thoroughly all the aspects of growing exotic plants in cooler climates. Chapter One begins with challenging the myths of “It won’t grow here” There is a primer on subtropical gardening including a short history of the topic including Victorian era England which had a strong interest in the subject. There is also a discussion on the effect that future climate change could have on gardening. This chapter also discusses the conventions that occur in the book.

Chapter 2 is a primer on north by south gardening. There is an explanation of why palms grow in Florida and a discussion of cold hardiness. Plant hardiness maps are discussed including the fact that zone maps change over time. The concept of Microclimates is explained in this chapter. In chapter three Microclimate based landscape design and planning is discussed. Chapter Four is about four season care of warm climate plants. Chapter Five discusses Cold Hardy Palms.  Chapter Six introduces Broadleaved Evergreen Trees and Shrubs.

Chapter Seven discusses warm climate deciduous trees and shrubs such as Crape Myrtle. Chapter Eight goes over other warm climate plants such as Bamboo, Banana’s and Yucca. At the end of the book is an appendix with additional information. The book is generously endowed with high quality color photographs as well as black and white photographs. Anyways I hope you like my review. This is my favorite book on Cold Hardy Tropicals and it doesn’t disappoint.

Here is a link if you wish to purchase this book from Amazon. Please be aware that I  receive a small commission if you make a purchase which helps support the costs of running this website.

USDA Hardiness Zone Map

2012 USDA Hardiness Zone Map_(USA)
2012 USDA Hardiness Zone Map_(USA)

There are several different climate zone map systems devised for the United States. The first and most commonly known climate system is the USDA Hardiness Zone Map. The first map was issued in 1960. It was revised in 1965. The next time a new map came out was in 1990. This map was based on more reporting stations and it and it divided the temperature zones into five-degree a/b zones for greater accuracy. This map identified some areas as colder than the previous map due to some extremely low temperatures in the intervening years. The latest map came out in 2012. This map takes into account the warmer temperatures of the previous 30 years. Many areas moved up by a half zone and urban heat islands are taken into account. Depending on where your weather reporting station is located you may be in a different subzone or even a different zone. The difference between an inner city garden near downtown and the weather station located on a open and windy airport 20 miles out in the country can be quite great.

The zones are color coded to be more visible. One interesting thing to notice is that the zones are not even east to west across the country. There are a number of reasons for this. Higher Elevations tend to have cooler temperatures and also areas near oceans and large bodies of water have more moderate temperatures. An extreme example is  some areas near Seattle Washington have a Zone 9 A Climate. This is the same zone as Jacksonville Florida. Remember though that the USDA Zones record the minimum temperature. Other factors such as moisture, hours of sunlight and summer heat are important as well.

The USDA has a great website http://planthardiness.ars.usda.gov/PHZMWeb/#  which explains the USDA Hardiness Zone Map and how to use it. You can search your zone by zip code. It’s a fun feature to play with. Where I live in Toledo Ohio the city is divided into zones 6A and 6B with areas closer to downtown and Lake Erie 6B and more western areas 6A.

Microclimates

Microclimates are very important to anyone with an interest in growing cold hardy tropical plants. A Microclimate is an area where the climate differs from the surrounding area. Microclimates can be very small such as the south side of a house or larger such as a densely built up city that has a different climate than the nearby countryside. Some Microclimate differences can be dramatic. New York City is a good example of an area with a very pronounced microclimate. New York City on the latest National Arbor Day Foundation Climate Zone Map is shown as Zone 8. The average minimum winter temperature in a zone 8 climate is 20-10 degrees Fahrenheit. This is the same climate zone as Atlanta Georgia.

Many species of subtropical plants can grow in a Zone 8 Climate. These include species of palms, many yuccas, Southern Magnolia’s, Crape Myrtle etc.
In New York City’s case there are several reasons for its warmer climate compared to other cities at a similar latitude such as Columbus Ohio or Indianapolis Indiana. The first is that New York City is at sea level. It will be several degrees warmer on average than a slightly higher city at the same latitude. The second reason is that it is surrounded by water. Water warms slowly and cools slowly compared to land. This means their summers will be slightly cooler and their winters slightly warmer than cities that are inland. The final reason for their warmer microclimates is the urban heat island effect. This happens when built up areas absorb and hold heat better than open countryside or forests. Other locations on the east coast can have warmer microclimates for similar reasons. Philadelphia, Cape Cod, Nantucket Island in Massachusetts and Block Island in Rhode Island all come to mind.

Picture of a Southern Magnolia in front of the Betsy Ross House in Philadelphia.
Picture of a Southern Magnolia in front of the Betsy Ross House in Philadelphia. The open courtyard around the tree is an example of a Microclimate.

You can use Microclimates to your advantage in growing cold hardy tropicals. The first example would be a south facing wall of a house. Another example would be a house on the side of a hill. The person living on the slope has less chance of late frost in the spring or early frost in the fall as opposed to the person down in the valley. This is because cold air is heavier and flows downhill. Other simple features can influence microclimates. An area right by a dryer vent can have soil that never freezes very deep even in the coldest of winters allowing plants such as Canna Lilies to survive much farther north than normal as the roots never freeze out. Growing up we had a large patch of Canna Liles surrounding our dryer vent. They were never dug up and survived the winter just fine. This was in an area of zone 6 winters. Normally Canna’s require zone 7-8 climates to survive the winter unprotected.

Microclimates can be created. Darker surfaces as generally warmer than lighter surfaces. Having black plastic mulch can raise soil temperatures by several degrees and in some cases protect lower growing plants from moderate frosts. Of Course the ultimate in creating your own Microclimates are various forms of winter protection such as cold frames and Greenhouses. I will touch on more of these types of winter protection in a future post.