Temperatures have been increasing on planet earth since the 19^th century. According to NASA, (https://earthobservatory.nasa.gov/world-of-change/decadaltemp.php) the average global temperature has increased by 2˚F since 1880, with two-thirds of that warming occurring since 1975. The US Environmental Protection Agency (EPA) states that unusually hot days have been an occurrence in the US since the 1970's with unusually hot nights happening at an even faster rate (https://www.epa.gov/climate-indicators/weather-climate). Take a look at the astounding chart, below, from the EPA website showing temperature anomalies in the contiguous US between 1901 and 2015.

EPA Temperature Chart 2016

Why should a gardener care about warming temperatures? While many plants, such as summer vegetables, grow well in hot temperatures, most plants biological systems shut down at temperatures exceeding 96˚F. Many plants suffer at temperatures exceeding 86˚F. Tolerance to hot temperatures varies by plant type, however, when the temperature raises beyond the particular plant's heat tolerance threshold, the plants integral systems stop working. Photosynthesis and respiration provide the energy plants need for growth and development. These chemical processes within the plant affect its cells, tissues and organs, and, along with transpiration and translocation, control the plants vigor, growth, vegetative and reproductive development. Extreme hot temperatures inhibit the plants ability to perform photosynthesis and respiration, and will result in shorter juvenile growth, low vigor, weak, tough vegetative growth, and less resistance to pests. Plants suffering from extreme heat stress die slowly; exterior signs include reduction or elimination of blooms and pale, drooping leaves.

What is a gardener to do about a planet with temperatures on the rise? We can't change the weather, but we do have control over how we garden. The single most important factor gardeners should consider when choosing plants is to select those adapted to the local environment. Native plants are the most obvious choice, as they have acclimated to local conditions over a millennium. Native plants have also evolved with our native insects and animals, so provide food and shelter for them. However, a plant that is indigenous to an area with a Mediterranean climate is also a great option. Areas with Mediterranean climates have long, hot, dry summers and short rainy winters. In addition to most of California, there are four areas around the world with Mediterranean climates: the countries surrounding the Mediterranean Sea, southwest Australia, the western cape province of South Africa, and central Chile.

Follow recommended plant selections for our area from the USDA, American Horticultural Society (AHS), and Sunset Western Garden Book. The USDA provides climate zones based on winter minimum temperatures, which determine a plant's hardiness, but this system ignores summer heat influence. Conversely, the (AHS) calculates heat zones which are derived from the average days above 86˚F, the temperature where many plants start to experience heat stress. Sunset Zones evaluate temperature; latitude; elevation; ocean influence; continental air influence; mountains, hills and valleys; microclimates; growing season; and soil. Most of Solano County falls into Zones 9, 14,15, or 17, with most zones receiving some marine air influence.

Whether or not California is experiencing an official drought, gardening practices that support drought-tolerant plants are responsible and sustainable.

• Using drip irrigation avoids lost water due to evaporation, and directs the water to the desired plant while avoiding the weeds. Established plants should be watered deeply and infrequently.
• Place plants with similar water needs together. This practice, known as “hydro-zoning”, prevents over-watering less thirsty plants.
• Add organic matter to your soil to improve texture and nutrient uptake and reduce root damage.
• Mulch to keep roots cool and avoid water evaporation from the soil.

We can (and do) debate whether the cause of rising temperatures is a human-caused greenhouse effect, or whether this is a naturally occurring cycle. However, the fact that our global temperatures are rising is indisputable. If we do nothing else, we can choose our plants with more intention, garden a bit more sustainably, and recognize that our world is slowing changing.

For more information about sustainable gardening, visit https://anrcatalog.ucanr.edu/pdf/8504.pdf