How To Deal With Heat Islands


What is a heat island? If you’re from New England and you want to escape the cold of winter, does Puerto Rico count? Not really. No matter the client or the region of the country, all commercial and residential properties have heat islands: narrow strips of turf, the unsuitable area between the sidewalk and street, the foundation planting with reflected heat off the house, the tree pit in the parking lot of the shopping center, etc.

Heat islands are undesirable for several reasons:

• The volume of soil available for root growth is inadequate.
• They tend to be surrounded by concrete or asphalt.
• The plants are growing in soils that lack nutrients and are compacted.
• Salts for ice removal and other materials are commonly deposited on them.

If you’re working on a new property, endeavor to prevent them. If you’ve inherited the responsibility of maintenance of an existing landscape, do your best to make a change to limit their effect on the function and appeal of the site.

So, what can you do about them? Unfortunately, when encountering a heat island, some landscapers simply tear out the old plants and stick new ones in, charge the client for time and materials and move on. The best approach is a step-by-step procedure: carefully analyze the site; consider pertinent contributing factors; and make changes to create a more suitable growing zone for plant materials.

narrow spaces
Try to avoid narrow “leftover” landscape spaces. Photo: John Fech

Site assessment

At first glance, this might seem like a waste of time. Think of it like preparing a wall for painting. To be successful, you need to scrape off old, cracked or peeling paint; fill in voids in the surface; sand the rough surfaces; and apply a primer coat before you apply the top coat and actually paint it. The same is true for fixing a bad heat island. The site assessment is the prep work.

Start by documenting the good and the bad; what’s working and what’s not. Obtain a base map or plat and make notes on it — the scorched leaves on the sycamore, the brown stems on the yew, the turf that’s thin or dead along the edges from too much salt applied to clear ice. Create another document with a simple list of landscape problems. Place them in priority order, and immediately after the statement of the problem, place value judgment on the various factors and identify reasons for success and reasons for failure of the existing planting. For example, “scorched leaves on the sycamore” would be followed by “due to reflected heat and sunlight off the building,” and “the existing stress could lead to other problems such as canker and anthracnose.”

Together, these two items can be helpful in communicating the issues to the property owner. They might just be the difference between you and another grounds manager in getting the bid. They definitely let the owner know that you’ve given some thoughtful consideration to their property.

Amend soil, fix drainage issues

If we were concerned with hairstyling, carpet installation or furniture repair, there would be multiple factors to examine, but at least they’d be visible. One of the most difficult challenges in landscaping is that half of the plant is under the ground, essentially invisible. A thorough site assessment will provide information about the soil, but it’s really tough to see if the roots are tangled, rotten or growing well.

A good step toward discovering what the rootzone is like is to remove the existing soil, even though it’s time-consuming and labor intensive. Carefully remove any plants that are mostly healthy; toss the rest. Playing nursemaid with sickly plants is a waste of time.

After the soil is excavated, consider possible changes to the original design to improve drainage and expansion of plant roots. Raised beds, berms, gentle slopes and drain tiles are modifications to facilitate movement of excessive moisture away from the roots.

Finally, the soil itself should be considered. In heat island situations, the best approach is to replace the existing fill soil with a sand and compost mixture. The International Society of Arboriculture has highlighted various ratio combinations of these components. As with many sports turf field mixes, the most successful in 70 percent sand 30 percent compost or 80 percent sand 20 percent compost. These formulas provide air to the roots, resist compaction and allow excessive water to drain.

Consider root restrictions

Usually smaller than ideal, the rootzone is critical to the success of a heat island planting. Root restrictions are yet another reason to start with a thorough site assessment. Not only is the space for growing roots usually smaller than ideal; it’s usually much smaller.

Acknowledgement of the limitation becomes important when it comes time to choose plant material to replace damaged or dead plants. In general, the woodier a plant is, the more rootzone space is required. If a large shrub or ornamental grass will serve the same function and aesthetic appeal as a tree, they might be a better choice. In general, change from large trees to small trees, shrubs to ornamental grasses in a downsizing theme. Communicate the effects of the limitation of space to the client both in terms of size of the plants that will grow well in the heat island, and the size of the accompanying root system.

sun to shade
This space changes from full sun to mostly shade in a short distance. Photo: John Fech

Choose the best turf or ornamental plants

As hinted at in the previous steps, it’s best to see the situation for what it is. In many heat island plantings gone astray, plant material has been chosen without regard to size. Likewise, with turf, proximity to a street or sidewalk routinely treated with salt and sand for ice mitigation or the heat from asphalt paving was not taken into account in the plant selection phase.

When choosing plants, research your options with the unique conditions in mind. Salt tolerance, heat tolerance, drought tolerance, air pollution resistance and size are all important factors to be considered in addition to the traditional selection criteria of flower color, disease resistance and winter appeal.

Plan for irrigation

Soon after college graduation, a former classmate of mine took a job that required him to water heat islands. I visited him on one such occasion, and we both laughed at the crude method of hand-watering that was his only option. He wasted a lot of water and time. Twenty-plus years later, the technology for watering limited space landscape plantings is much better, and should be taken advantage of to make good use of a limited resource.

In the first year after planting, close attention must be paid to making sure that the roots are kept moist, but not soggy. This is best accomplished through monitoring and the use of good irrigation equipment. Monitoring can be as simple as probing the soil with a sharpened piece of rebar or a headless golf club. If the probe slides into the soil easily, the roots are likely to be moist enough to promote good root growth. If not, irrigation is probably needed. Follow up the probing test by digging a narrow, 2 to 3-foot hole and feeling the soil to determine how far recent rains or irrigation applications have infiltrated. This should be done at least monthly during the first year.

Inform your client that some sort of drip, soaker or micro-spray irrigation will be required to assist the establishment of the new plantings. Inclusion of this type of equipment adds cost to the overall project; it also adds a big measure of insurance to the success of the planting. Be sure to add a line item for monitoring of the equipment during the growing season into your bid, and then inspect the equipment to make sure that it is working properly.

layered landscape
If space allows, incorporate layering into the design. Photo: John Fech


After the heat island has been installed or renovated, consider it a big investment of time and money that must be well cared for. In addition to monitoring the water and oxygen content of the soil, make sure the mulch is in place at an adequate depth. Many heat island sites lose mulch readily to foot traffic and windy conditions.

Test the soil to determine its capacity to provide for the nutritional needs of the plants. Because it has been modified to a sand/compost mix, it may be lacking in certain nutrients. If so, these can usually be added without difficulty.

Routinely inspect the plants for diseases and insect infestation. Proper selection, irrigation, site assessment and care will limit pests, but because a heat island is stressful by definition, they are always a possibility.

Just as with traditional landscapes, heat islands can be as healthy or as sickly as the follow-up care guides them to be. The single best tip for success is to be aware of the special needs of a heat island and act accordingly.