Sun Scorch
The recent, unprecedented heat wave here in the Pacific Northwest may have abated, but the damage to our landscapes remains: large brown or black areas on leaves, major leaf drop, and shriveled plants are the signs of sunscorch. One of our staff horticulturists sat down to talk about the damage with her former professor Don Marshall who recently retired from Lake Washington Institute of Technology after running the horticulture program there for 40 years.
Susan:
Don, have you ever seen this severe weather here before? The damage is so widespread.
Don:
Never in the Pacific Northwest. It was unbelievable, but short. I’ve experienced this kind of heat in the Northeast, but the humidity was much higher so the plants had access to more moisture.
Susan:
Explain what happens when a plant is burned by extreme heat.
Don:
While I am not a scientist, my background in botany leads me to believe that the cells in the leaves are dehydrating and dying; but it’s important to understand that it’s not so much a leaf problem as it is a soil moisture problem. A combination of the intensity of the sun, and the plant not able to draw up enough water to counter the rate of transpiration (evaporation of water from the leaves) causes leaf cells to die. As a result, large brown patches appear on the leaves – the leaf won’t recover.
Susan:
I soaked my garden ahead of the heat, but I still have some damage to my Rogersia, Viburnum davidii and Thujopsis dolobrata (Deerhorn Cedar). What plants have you seen that suffered?
Don:
I’m seeing smaller fruit, and flowers aren’t producing as much nectar. I am a bee keeper, and my hives are producing a lot less honey. We had a pretty dry spring, so plants were already feeling the stress of that before the heat wave even happened. My Abies grandis (grand fir) are also damaged – but honestly, they’re are in an area that I don’t water. I am also hearing that hemlocks are suffering greatly. They are drought sensitive plants, and this heat is coming on top of eight years of drought. Many won’t recover, and it’s so sad.
Susan:
I have had so many customers asking what they should do for their plants now.
Don:
Just keep watering and see what happens. Deep watering is so important.
Susan: Yes, I’ve been telling customers to take a hori hori or shovel, work it back and forth in the soil and check how far down the water is penetrating, and that they should shoot for 18 inches.
Don:
Well, 18 inches is a lot. Especially if they have sandy soil, or even clay. But 12 is a good goal, especially if the ground is mulched. Listen, mulching is so so important. A good layer of arborist chips or compost on top of the soil protects roots and holds moisture. It keeps the soil from heating up, and is really crucial for shallow rooted plants like rhododendrons.
Susan:
So many rhodies were damaged, and we have a lot of customers asking if they should prune them back or remove the leaves. I think they should just leave them alone, especially if there is still some good tissue left on the leaf.
Don:
I agree – half a leaf photosynthesizing is better than no leaf. The dead leaves will naturally drop. I think we should just keep up with the watering and wait and see. The good news is that the new leaves the plant produces will be better adapted to the heat and drought, likely with a double epidermal layer.
Susan:
Really? Plants are amazing! Let’s talk watering. I remember you telling us that if we learned nothing else, we were going to learn how to water. Honestly, I rolled my eyes at that, but it is so true.
Don: (laughing)
Watering is a science and an art! Overhead watering just evaporates, so drip irrigation is the very best way to water plants. If you don’t have drip, put the hose next to the tree and let the water trickle out for about a half an hour. One thing that I am doing with all of my fruit trees is to put the hose on them for 30 minutes, move to another tree, and then do the whole thing over again three times. That really gets the water down there.
Susan:
How about with shallow rooted plants like Rhodies?
Don:
Remember water moves horizontally as well.
Susan:
How about perennials? Should we just cut them back?
Don:
Well, it’s still early so sure! There’s plenty of growing time left. Why not start over! Lots of summer left.
Susan:
It feels like August….so I forget how much growing season we have! Speaking of August, I expect that plants will do another round of leaf drop.
Don:
That is likely, especially if we have more drought. But I don’t think it is anything to worry about if people keep up with the watering.
Susan:
Thank you, Don. And thank you for all you’ve taught me. Lake Washington’s horticulture program was the best year of my life! It was so fun.
Don:
Good to talk with you! I think the big takeaways here are the importance of mulch and correct watering.
Susan:
and patience.
Don:
Yes, always patience.