How Hot Can My Garden Get & How Will It Affect My Plants?
Some plants like it hot-(but not all) here is how to measure heat in the garden!
If hardiness zone maps can tell us how cold we can all expect our gardens to get; then how do we measure how hot they can get?
We know by now that the USDA plant hardiness zone map, (new in 2023) tells us the ‘mean extreme cold temperature for our hardiness zone’. Therefore the problem addressed by the USDA map is ‘how cold can I reasonably expect my garden to get?’ And of course, ‘What are the plants that I can safely grow?’
And, the map works pretty well, getting more comprehensive with each edition and it has been universally adopted by plant growers, online dealers, and regional garden centers. Even your seed packages show the appropriate hardiness zones for the plants. Pick up a plant you like, and read the label, it will list the plant hardiness zones that plant testing and trialing has shown as appropriate for the plant.
How We Got Plant Hardiness Zones
The Arnold Arboretum published the first plant hardiness map with eight zones in 1927. They published maps until as late as 1971. In 1960 the USDA began publishing a hardiness zone map and today they are produced about every 10 years and each now uses the last 30 years of each climate zone’s extreme cold annual weather data. What is published for each zone is the average of the data sets.
Today, as of the 2023 map, you can use the interactive, online version to collect valuable and very local information about cold weather in your garden. Here is how to access the map.
This is the piece I wrote after spending some time working with the map.
The Map’s Data Sources
The US National Weather Service has a long and rich history. Colonial leaders, living by agriculture were serious weather observers. Later, in the early 1800’s weather observation station networks began to grow, helped by the communication power of the telegraph and the number of weather stations providing data continues growing today. Here is a recent example:
- The 2012 USDA Map was made using the data from 7973 weather stations.
- The 2023 Map was made using the data from 13625 weather stations.
Likewise, the USDA is a historic American institution. Formed while the US president was Abraham Lincoln it has long experience in providing information for the use of farmers.
The latest version of the online map looks like this and it will have a section on top for you to enter your zip code.
But What Will Tell Us How Hot The Garden Can Get?
But is there a high-temperature equivalent of the plant hardiness zone map? Or is there anything like it so we can measure both of our worst risks, freeze damage, and the opposite danger, heat disaster?
Here Is Something We Can Use To Measure Heat In The Garden
Starting in 1997 the American Horticultural Society developed the National Heat Zone Map. What does it do for us? This map divides the US into 12 heat zones and tells us the number of summer days when the temperature in each zone gets over 86 degrees.
- Zone one includes only places in the far north and expect only one day over 86 degrees.
- Zone twelve has over 212 days in that territory.
Why 86 Degrees Is Important
Why is this important? At 86 degrees F, a plant begins to suffer physiological damage. Both extremes of temperature, heat and cold, can damage or kill our valuable plants. Death from cold is quick, we go out to visit the garden on the next frosty morning and we know instantly we have trouble!
But heat distress is a little more subtle, and assuming the plant has sufficient water, the problems are not always immediately visible but will slowly appear. The plants slowly die, the buds drop off, we see chlorosis, and issues like pale leaves appear.
Signs Of Heat Stress In Plants
- Leaves will appear cupped or rolled. The plant does this to reduce leaf surface and thus limit moisture loss.
- Leaves are wilted. This happens because there is less moisture and this reduces the water pressure in the plant. The longer the leaves are wilted the more chance of permanent damage. A good sign that the damage is from heat is that the leaves wilt in the heat of the day and recover in the evening.
- In plants with large leaves, the outer edges will dry and curl.
- If high heat occurs in places with poor air quality, ozone damage can result. The spotting can look like a plant disease.
- The plant may conserve its resources by dropping buds and fruit. If the heat passes soon enough, it should recover.
- We have all seen bolting, the aggressive production of flowers and seeds during periods of extreme heat. Consider harvesting fruit and vegetables before an imminent heat wave.
- Sunscald, is surface damage to fruit, consider temporary shelter or moving container plants.
Relief For Plants
- Provide water, shade, mulch, and in some cases a morning mist of water to increase humidity.
How Do We Use This Information About Heat In The Garden?
The Maps Are A Guide
First, like the plant hardiness zone map, the heat map makes a great guide for us gardeners in making decisions for growing. But, also like the hardiness map, it is not a rule, it does not cover all the reasons why plants succeed or fail for us. We may over time develop more comprehensive maps, but I don’t think we’ll cover everything.
Plant Selection Issues
The USDA Plant Hardiness Map has a long history, back to 1927, and is well-established. This means that it is very hard, anywhere in the country, to find a plant for sale that does not show its hardiness zones.
Heat Zone information is far less universally available. As a much younger, (dating to 1997) source of information it does not have the universal following of the Plant Hardiness map. To my knowledge, this is the single greatest problem with the heat zone map. (Of course, time can solve this problem.)
- Fewer plant dealers have the heat information available and if they do you will see two zone ranges on the label. For example, a plant will be offered for zones 3-8 and 8-1. The first reading is USDA hardiness zones and the second is the AHS heat zones.
- Because the heat zone map is based on the number of days with heat over 86 degrees we will have a sense of the extent of heat duration.
Heat Zone Map
Heat Zone Maps By State
You can find the heat zone maps by state. Florida is divided into two parts, this is the peninsular with the color codes for days above 86 Degrees.
This will show you the heat zone maps for different states. The index on the left shows the color codes.
What The Heat Zone Map Does For Us
- Using the same data sources as the Plant Hardiness Zone Map the Heat Zone Map will give us a guide to the second important piece of data, ‘Can a specific plant grow in our garden’s environment’?
- Both systems are guides for us, not absolute rules.
- For southern gardeners, this is extremely valuable information.
- For gardeners all over, who are participating in the growing interest in tropical plants this is helpful. Many temperate climate gardeners enjoy tropical plants in the summer garden and later store or use them as house plants. (Elephant Ear Plants)
What The Heat Zone Map Won’t Tell Us
- The number of plants we can all buy with information on both hardiness zones and heat zones has grown but until this is commonplace, the impact of the heat zone map will be limited.
- I do not see the heat zone maps updated, With half of the US moving to a warmer 1/2 zone in the newest USDA map, is this going to be accurate enough?
What Damage To Expect In Times Of Extreme Heat
We have gardened wherever we lived, in many states and different zones. However, we have been Florida gardeners in three different places for some years. Our Florida climate allows a blooming garden for all twelve months, but what blooms and fruits each season is varied. Summer is the challenge for a Florida gardener. The work is harder in the heat and the risk of damage to beautiful plants is the greatest.
Here is what you should expect in any zone when your summer heat becomes excessive.
- First, when the temperature reaches 86 degrees F, the plant’s growth will slow down. This happens because the rate of photosynthesis goes down as the temperature rises beyond 86.
- In contrast, at the same time, the rate of respiration continues and depletes the plant’s food reserves. If this goes on for weeks the plant can die from lack of nutrition alone.
(Photosynthesis is the process by which the plant uses the sun’s energy to create carbohydrates as a food source.) Respiration is the process by which the plant uses carbohydrates to grow.)
- In high temperatures, the plant can lose more water through transpiration than it can absorb through its roots. If high temperatures persist for long enough this can kill the plant.
(Transpiration is the normal process whereby the leaves release water vapor into the atmosphere.)
What You Can Do To Improve Conditions In Times Of Extreme Heat
These steps are standard procedure in our hot and humid summer days in Florida. These are the best steps we can take.
- Water in the morning, you will lose less water through evaporation and the plants will start the day hydrated. It is easier on the gardener too. Try to do the heaviest work before the day becomes oppressive. (The later part of the day is for the pool!)
- Know the native regions of your plants. a Mediterranean herb will prefer dry and sunny areas and does not require fertile soil.
- Test the soil every day, is your finger dry, it’s time to water. Always check the soil before watering.
- Assume that any container plants, in particular hanging baskets, will want water every day.
- Pay attention to any newly planted material. Roots that are still in the root ball dry out quickly.
- Use mulch, it keeps the soil cooler and shades the roots.
- Find some shade if you can. Move container plants to shady spots. We find we can get a longer life span from herbs by doing this.
- Use a shade structure if you can.
- A heat wave is not a time to fertilize and encourage growth which will stress the plant.
- Wait until the weather cools down before doing propagation.
Are There Any Alternative Ways To Measure Heat In The Garden?
- Some growers simply offer you the second number on the hardiness zone range as the heat range. If the plant thrives in hardiness zones 9a-10b, its heat zone is 10b.
- Some growers will offer groups of plants designed for hot climates. This is a list created by Proven Winners.
- Some growers offer their own measure of cold hardiness and heat sensitivity. White Flower Farm in Ct uses the hardiness zone coupled with E and W zones for the warm and humid climates in Florida and the warm and dry climates of California. So, on their site, you will see recommendations like 3-8S/10W. This plant should perform in hardiness zones 3-8 in the south and even farther, to zone 10 in the west. This is their excellent discussion of heat and hardiness zones.
- Other growers will offer plants for certain hardiness zones and light conditions. This is a Bee Balm Plant with its requirements. American Meadows.
- For Gardeners in Western states, the Sunset zone system is an effective way to handle the variations due to elevation and water bodies.
- Talk to your best local experts. Your best local garden center will have the experience you need, as will your local County Extension Service. This is from the University of Arkansas, pick your state, then put in your town.
Summary, Working With Heat In The Garden
The combination of the USDA Plant Hardiness Zone Map to measure potential cold damage and the AHS Heat Zone Map to measure just as deadly heat damage is a valuable set of tools for all of us gardeners. However, until I can reasonably expect my sources of plant material to include both sets of information the Heat Zone Map is going to be less productive.
But, let us not forget that the Heat zone map is a relatively new tool in an environment in which we need to have long sets of data analyzed for us. After all, the groundbreaking Hardiness Zone system began in the 1920s as a black-and-white drawing made by one man at the Arnold Arboretum. Time can solve a lot of problems!
Resources You Can Use
‘How To Acclimate Plants To Indoors For Winter’