Too Much of a Good Thing? Sun and heat-scorched plants

By Bruce Bennett

Who would have thought the Pacific Northwest could get too much sunshine? I mean 108 degrees in SEATTLE? And, yet, June and July proved the truth of the matter. Although the heat dome that caused this aberrant weather has been noted as a “once in 1,000 years event,” I’ve also heard the same said about 500-year floods that have appeared in our region with alarming frequency over the past decade.

Leaf scorched rhodie photo by Bruce Bennett

So, what actions can you plan to deal with the future repeating episodes of plant sunburn, scorch or sunscald (three names for the same problem) and what do you say to people who visit the clinics that will open in August?

Even plants that are well-adapted to our climate can be scorched. While plants have a great resiliency to the changing weather that comes with each season, the abrupt change to record heat waves does not provide them with enough time or water to deal with the heat. 

In fact, some plants such as Hostas and Hydrangeas, which enjoy shade, can scorch simply from unexpected high temperatures. Now that you are left with several sunburned, brown or bronze leaves, crispy leaf edges and possibly more, what’s a homeowner or even a Master Gardener to do? Have a few minutes for some suggestions …

Picea needle scorch. Photo by Aruna Ravi

It’s natural for home owners to want their gardens to look great year-round and we do know to irrigate more when we hear hot weather is eminent. 

However, some people may not realize that scorching is not necessarily due to the lack of water but, rather, the inability of a plant’s roots to take-up enough water to replace what is being lost through the leaves. 

It’s normal for plants to respond to high temperatures by letting some of the foliage die back so water uptake by the roots equals the amount of water loss from the leaves. 

While it is important to keep the ground around the plant evenly moist, adding more water to ground that is already moist increases the possibility for root and fungal diseases. So, what else can gardeners do to help untidy, but stressed-out, perennials, shrubs and trees? Here are a few of my suggestions:

As was already mentioned, irrigate only if the soil is dry (this will also save you from a larger public utilities bill). Stick your finger an inch or two into the soil. If it’s dry that far down, irrigate. The rule-of-thumb is to provide your plants with an inch of water once a week, not 15-minutes every other day..

To retain more moisture in the ground, mulch plants with wood chips or shredded bark to decrease water evaporation from the soil (and greatly reduce the number of weeds you need to pull). Mulch should be 2-3 inches deep and not placed against perennial plant crowns or shrub and tree trunks.

Sun Sail. Photo:Pixabay

Erect screens, lattice or shade sails on the south and west side of sunburn-susceptible perennials, shrubs and trees as temporary or permanent sun barriers. 

You could use simple greenhouse shade cloth or a beach umbrella. If you want to add a bit more decoration and a pop of color to the yard, consider painted lattice or sun shade sails. Sizes, shapes and colors for these last two are pretty much unlimited.

Where possible, leave scorched leaves and needles attached to their owners. The aesthetics may not be the best, but, these brown remnants may be protecting new buds and green foliage beneath them. 

If the damaged plant is in a high visibility area, improve the looks of things with minimal pruning using hand pruners, rather than hedge trimmers. I have also read of some enterprising gardeners using a can of left-over green spray paint to color things up a bit.

If and when fertilization comes to mind this summer, unless for annuals, don’t do it. Fertilizing mixed with high summer heat can cause more scorch damage rather than helping the plant.

Cornus leaf scorch photo by Bruce Bennett

As if the scorching isn’t bad enough, gardeners should be on heightened alert for more insect and disease problems this summer. 

The sun, heat and dying leaves issues have stressed plants. These conditions may have reduced impacted plant health and may make them more susceptible to attacks by opportunistic insects and diseases. 

If you would like more information about sunscald and scorch, let me direct you to two excellent northwest publications: Washington State University’s Hortsense (http://hortsense.cahnrs.wsu.edu/) and Oregon State University’s Pacific Northwest Handbook (https://pnwhandbooks.org). I think you will find both of these publications extremely helpful when working out the challenges in your gardens.

 
Contributing columnist, Bruce Bennett, has been a WSU Master Gardener, landscape designer and lecturer for more than twenty years. He is the managing partner of a Seattle-area garden design firm, local gardening lecturer and is an instructor with WSU Extension’s College of Agriculture, Human and Natural Resources. Contact him with your questions concerning this article at bennettbd33@hotmail.com