I just had a radon removal system installed. The exterior vent, which expels air 24 hours a day, year-round, is located between two panicle hydrangeas, about a foot above the ground. The air is blown directly toward the ground. Have I just signed their death warrant?
Answer
Radon is a radioactive gas that occurs naturally in the soil, produced by the decay of uranium. Outdoors, it immediately disperses in the air and becomes harmless, but in a home, it can accumulate and reach dangerous concentrations over time. It enters buildings through cracks or unsealed joints in the foundation.
To prevent this accumulation, mitigation systems are installed that draw air from the soil under the concrete slab and vent it outside, as in your case. Once expelled, radon is immediately dissipated and poses no danger to you or your plants. The question then becomes: how can the expelled air affect plants, particularly your panicle hydrangeas? Three factors must be considered: air movement, temperature, and humidity.
Air movement
The airflow from a radon mitigation system is constant but weak, much less powerful than that from an air conditioner or dryer vent. Even weak airflow can slightly affect growth: stems may lean away from the airflow or strengthen to compensate. It can also increase transpiration from exposed leaves, sometimes causing slight localized drying. On the ground, as the air is directed downward, some erosion or drying may occur. However, none of this puts your hydrangeas at risk.
Air temperature
The temperature of the ground beneath a house varies very little throughout the year, generally between 5 and 10°C in southern Quebec. Since the system draws air from this depth, the expelled air is relatively cool in summer and warmer than the ambient air in winter. In cold weather, this relative heat can melt the snow right in front of the outlet, removing some of the natural insulation provided by the snow cover. This can expose shallow roots to freeze-thaw cycles. This phenomenon can disturb some sensitive plants, but rarely panicle hydrangeas, whose roots are deep and well adapted to the cold.
Air humidity
The air coming from the ground is generally more humid than the outside air. The area around the outlet may therefore remain slightly more humid throughout the year. In winter, this can cause condensation, frost, or ice to form on the stems or on the ground, but this has no harmful effect on hydrangeas. In summer, the humidity can prolong the wetness of the foliage, which slightly increases the risk of leaf diseases in susceptible plants. The soil may also remain moist for longer, which is not necessarily a problem for most shrubs.
Hydrangeas and other plants
Your panicle hydrangeas should not be significantly affected. They are very hardy shrubs, resistant to wind, extremely rustic, and have deep roots. Since they lose their leaves in winter, they are not affected by the warmer, more humid air during this season.
Other more fragile plants may react more strongly: tall, delicate species such as delphiniums or cosmos, brittle plants such as certain ferns, perennials with shallow roots exposed to freeze-thaw cycles, evergreen plants sensitive to winter drying, or plants prone to leaf diseases such as roses and lupins. Dry-soil plants, such as lavender or certain sedums, may also be less suited to locations with increased humidity.
Simple solutions
The easiest way to help your hydrangeas adapt is to add a 5 to 10 cm ()2 to 4 inches) layer of organic mulch. This protects the roots from freezing and thawing, stabilizes the soil temperature, reduces evaporation, and keeps the soil evenly moist. You can also simply leave plant debris on the ground in fall and spring, which acts as a natural mulch. It is a good idea to leave about 30 cm of free space in front of the system outlet. The air flow could be deflected slightly further away from the opening, but in your case this would probably be unnecessary and unattractive.
The best approach is simply to observe. Plants are often much more adaptable than we think, and there is every reason to believe that your hydrangeas will acclimatize without difficulty.
For more information on radon, see Radon – A Guide for Canadian Homeowners.
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