Landscape designers have a new concern, as do gardening gurus and nurserymen: understanding the heat tolerance of our native and cultivated plants. This past summer was a pressure cooker, not only for drought but also for a week of intense temperatures. This affected people, plants and animals. Plants help to cool the planet, yet in an ever-changing climate, what plant goes where? A landscape designer with a strong knowledge of plant and tree temperature tolerance can help assure you of a successful outdoor habitat.
Some of the old rules apply when specifying and selecting plants for drought and heat, but not all of them, such as “the larger the leaf, the less sun and heat a plant can tolerate”. We still need to pay attention to the micro-climate from the urban heat of asphalt and concrete to the surrounding tree canopy. Intense heat can be hard on many plants even when adequately watered. Also, overly wet roots can be damaging for many plants especially conifers-when heat is intense their water uptake often shuts down.
Fleshy leaves, such as sedums and agaves tend to store more water in their cells. A plant such as a large leaved Rhododendron or a Camellia japonica may have thick leaves but they are not fleshy—they took a real hit last summer, especially if their micro-climate was baking. They also appreciate water. For trees, deep water is the best way to prepare for hot weather, especially during drought. Yet, depending on their micro-climate, even though deciduous tree leaves can burn in extreme heat and leave an undesirable specimen– rest assured, if they are well established, most of them should leaf out in the spring, that is, if they were watered deeply.
In general, heat tolerant plants thriving in a sunny location have very small leaves or are fleshy, but not always. Some are very drought tolerant and others are not. Established plants fair better. Also, many of these plants are hummingbird and bee magnets.
Heat Tolerant Plants and Trees:
Blue fescue grass-Festucca glauca (water a bit)
California lilac-Ceanothus varieties, both large and groundcover type, native and non-native.
Coyote bush-Baccharis-native to southwestern Oregon
Douglas Fir-Pseudotsuga menziesii
Dogwood tree-Cornus tree varieties (water deep)
Epilobium (Zauchineria)- california fuchsia varieties, native and non-native
Heath-Erica (also known as heather-water a bit)
Heather-Calluna (water a bit)
Hens and Chicks-Sempervirens
Huckleberry, evergreen-Vaccinium ovatum (native and in the shade)
Lavender cotton-Santolina varieties
Japanese false Camellia-Stewartia (water deep)
Kaminski milkwort-Polygala c. ‘Kaminski’ (water a bit)
Manzanita-Arctostaphylos varieties, both large and the ground cover kinnickinnick, native and non-native. These appreciate good drainage, planted in a quarter-ten gravel/soil mix.
Oak-Quercus Northwest natives (water deep)
Oregon grape (The tall one)-Mahonia (native and in the shade)
Pine-Pinus, especially Northwest native pines such as lodgepole (contorta) and Ponderosa
Red hot poker-Kniphophia varieties
Rosmary-Rosemarinus varieties, large and small
Russian sage-Perovskia varieties
Smoke tree-Cotinus varieties
Snowberry-Syphorocarpus alba (native and in the shade)
Strawberry tree-Arbutus unedo varieties
Sunrose-Helianthemum (water a bit)
Juniper-Juniperus varieties, all trees and shrubs
- In addition, roses and peonies seem to be an exception to the rule—they survive and bloom even with burnt leaves. Hydrangeas and deciduous blueberries cooked and many ferns, native and non-native took a hit. Native sword fern, in the shade performed fairly well.