Tree care during extreme heat waves

It’s hot out there! In our service area and around the world we are experiencing unprecedented weather events as a result of climate change, including a recent heat dome with multiple triple-digit days. Here are some helpful tips to help your new trees weather the heat! 

In general trees in our area need water from April to October, depending on weather. As you may know, extended hot, dry weather can place a great deal of stress on trees. Fortunately, just a little extra care will provide your trees the relief they need to minimize drought stress and keep them healthy. Follow these simple steps to ensure your new trees have the support they need:

Established trees: Give them a monthly soak, perhaps an hour on a drip line, or more if the tree appears stressed (e.g., dropping leaves).  

New trees (in the ground 1-3 years):

WATER: During an excessive heat event, we recommend that you double your normal watering routine. This means that your new trees should receive 15 gallons of water at least twice a week throughout the heat wave, meaning a deep watering every three to four days when it’s about 90-degrees or more for an extended period.  

The best time to water is in the early morning or after the sun goes down to minimize water loss to evaporation. Water slowly so that moisture soaks deeply into the soil and doesn’t run away from the root zone. This can be achieved using one of the following methods:

  • A slow-release watering bag
  • A DIY 5-gallon bucket drip system (check it out in the video above)
  • Trickle your hose within the root zone for 15-20 minutes

Be mindful that it is possible to overwater your trees as well (though unlikely during the Portland region’s dry summers, overwatering is more common when folks water trees in winter. Allowing the soil to completely dry out—or allowing it to stay soggy—will result in a tree that is stressed. Deep, infrequent watering is the way to go: Water deeply, then hold off. Healthy roots follow the water downward as it starts to dry and establish sinker roots for increased drought tolerance. Keeping the soil consistently moist through deep and infrequent watering is essential to healthy trees. Wondering if it’s time to water? If you poke a pencil 4-5 inches down, does it come out dry & dusty? If so, water!

MULCH: Organic mulch is a highly effective way to keep the soil from drying out quickly, especially during excessive heat. A thick layer of mulch from wood chips and ground leaves is even better: Wood is essentially carbon and carbon is nature’s sponge,  and leaves are like vitamins that are taken into the roots and are water-soluble. What we offer at the FOT Portland office is ground up branches and leaves combined, just like what’s found on the floor of a healthy forest. 

“I like to remind folks that the forest floor is perfectly mulched and your street tree should be, too!” Andrew Land, Neighborhood Trees Senior Specialist

Mulch around the base of the tree will help to retain soil moisture and regulate soil temperature, which is exactly what roots need to grow. Apply mulch for a newly planted street tree using the 3-3-3 rule: mulch should be at least 3″ deep, 3′ in diameter, and pulled at least  3″ from the base of the trunk.  If your tree is beyond that first year, you can extend that mulch further because our objective is to mulch the tips of the roots as they grow outward from the trunk. Three-inches thick is also important because it’s insulation (imagine the difference between a wind-breaker and a down jacket, for example:  that dead air space matters.)

PRUNING: Generally it is safe to prune most trees in mid-summer, from roughly mid-July to mid-August to be more specific. However, you may want to pause pruning during an extreme heat event, including right before and right after an extreme event, so consult the weather forecast as part of your pruning plan. Check out our pruning page for lots of useful information, including how-tos.

For additional tree care tips, check out our Tree Care Guide. Stay safe, stay cool, and as always, please don’t hesitate to reach out with any questions. Go trees!