News & Events

Summer Hardiness–which plants survive the heat?

Yucca 'Color Guard', Vernonia lettermanii and Crambe maritima have performed despite the heat and drought .

Yucca ‘Color Guard’, Vernonia lettermanii and Crambe maritima have performed despite the heat and drought .

Well, we know we’re not alone, but here in southern New England, we’ve had an exceptionally hot dry summer. The amount of precipitation in our area has varied due to isolated showers, but I would guess here at Avant Gardens we have totaled less than 1 inch during the past 75 days. Lack of rainfall plus high humidity, coupled with daily temperatures in the high 80s and 90‘s can have an effect on plants. (Many plants begin to suffer physiological damage when temperatures remain above 86F or 30C)


This brings up the classification of plants rated for Summer Hardiness. The American Horticultural Society (AHS) has created a Summer Heat Zone map of the US.  Regions having less than 1 day with temperatures above 86F (30C)  are classified as zone 1, and on the other end of the spectrum, the areas having the most days with high temperatures are classified as zone 12. This may correlate with the familiar USDA Winter Hardiness Zone Map (which rates average lowest temperatures) but in some cases it does not. Folks in the southern US have learned that Summer Heat Zone Hardiness is definitely a criteria when selecting plants. The above map will need regular adjustments now that global warming is causing extreme temperatures worldwide, and northern gardeners will have to pay heed to which plants will survive/perform with higher summer temperatures.

We’ll be ruling out more plants that do well in our gardens in the future, I’m afraid, as climate change continues to affect what we can and cannot happily grow.

Have you come to the conclusion that some plants, which once thrived in your gardens,  no longer will? Which plants have you found best withstand our ever warmer drier summers?


  1. We just bought a succulent and the hosta of the year at your beautiful nursery. The succulent will be fine as it will soon become a house plant. The hosta will be planted in the shade. We have a total outdoor watering ban in our town in MA, so I’m grateful for our rain barrels which fill up quickly, even in a slight shower. I also save gray water for outdoor plants.
    I do think NE gardeners will be re-thinking their garden plans now.

  2. This was so helpful to read. I hadn’t thought of this, but of course the temperature has affected the garden, not just lack of rain.
    I’ve been thinking that my cotoneaster had borers (but couldn’t find any) or Fireblight, but no real sign of it. My plants are just turning slightly brown. The lightbulb went off after reading your post.
    Thank you!

  3. I totally agree! I have been so proud of my NE plants to be pressing on through this difficult gardening season. The Astilbes – I am not sure if they will make a come-back, but I said that last year too, they were smaller this year. Thankfully I have coneflowers throughout the garden and if I remember they were originally a prairie flower (??) so they are such good soldiers…flowering on and on. This was a timely post thank you so much. Your Sedums are thriving and your Baptisia was fabulous this year – in its first full season – thank you! I am concerned about my shrubs – Red Twig Dogwood, Viburnum…they are pretty sad looking. Yes, it may take some NE re-thinking! I love and appreciate Avant Gardens – my fav place!

  4. I’ve lost 3 rhodies I bought mid season. That has never happened before. Yes they were potbound but I know how to untangle and open up a rootball, plant at a proper depth and water properly. I think that in their already stressed condition combined with relentless high heat and sun, they could not take up moisture and respond to normal care. Lesson Learned

    • Marie, Did you buy the Rhodies from a local source? Rhododendrons are shallow root rooted, and so are subject to problems during extreme drought. They like a soil that is well drained but with even moisture.

  5. What a hot one this was. Your analysis, Katherine, really reminded me that so much time over 85 degrees along with little rain has a huge impact. The old trees scare me since they are not replaceable in my lifetime! I look forward to your guidance on which plants might tolerate the changes we sadly face. Today’s rain is a real celebration. PS My prior experience frying a Rhodie in 2003 was that it came back to life with pruning severely. This year the same plant has suffered and will need a pruning again.

  6. I live and garden in Washington, DC. We are still seeing <90 degree days in September. Summer temps should definitely be a consideration here. I have found that epimedium, bergenia, wood aster and my Chinese abelia have thrived. My brunnera all collapsed.

    • Thanks for sharing what has held up in DC with even more heat, Tracy. It’s good information. We are cooling off a bit now that it is mid September, but are still very thirsty for rainfall.

  7. Hard to know until next spring. But this fall I’ve been thankful for my rain barrels. I’m using them to deeply water my trees & shrubs, especially the evergreens. After every rain, I’ve been giving them up to 20 gal. of rainwater at a time.


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