How to plan your garden with global warming in mind

Shrinking ice caps. Growing deserts. The globe is getting warmer; climate zones are changing. Does that mean Marin gardeners should plant cacti now rather than apple trees?

“Between now and the 2050s, for the most part, we see the USDA hardiness zones shifting by only half a zone in California. But, these zones are based on only the coldest temperature you’re going to hit. They don’t account for things like heatwaves, is it wetter or drier? They’re just telling you if your plant is going to die over the winter,” Lauren E. Parker says.

Parker is a postdoctoral fellow with the USDA California Climate Hub and John Muir Institute of the Environment at University of California, Davis, and acting coordinator for the USDA California Climate Hub. Her focus is on issues surrounding perennial agriculture in a changing climate.

“In Marin, if we’re thinking 20 to 30 years in advance, we’re maybe not so concerned about shifting crop systems,” Parker says. “We’re more concerned about planting different varieties that would be tolerant to more variable and warmer conditions. Heat and drought tolerance will be increasingly important.”

Parker points to two science- and research-based websites for climate change predictions: the Climate Toolbox (climatetoolbox.org) created by University of California, Merced, and Cal-Adapt (cal-adapt.org) developed by University of California, Berkeley’s Geospatial Innovation Facility.

With these websites, gardeners can see predictions narrowed down to a home address; predicted minimum and maximum temperatures; heatwave days; chill hours; dry spells; first and last freeze days; and more.

Both sites use computer models based on Representative Concentration Pathways (RCPs), which encapsulate different possible future greenhouse gas and aerosol emissions scenarios to calculate probable future conditions. RCP 4.5 models a future in which there has been an attempt to reduce greenhouse gas emissions. RCP 8.5 is a “business as usual” scenario.

When I looked at Marin County on Climate Toolbox using RCP 4.5, I could see the predicted minimum temperatures for winter might increase by 2 degrees in 2010 to 2039. With RCP 8.5, the increase is 3 degrees. Other seasons have similar increases.

The good news is that warmer temperatures could lead to more growing days during the year and less frost. Warmer temperatures increase evaporative demands, though, so there would be more water needs.

The bad news is that fewer chill days affect apple, cherry and pear trees, which want 1,100 to 1,500 hours under 45 degrees. Climate Toolbox predicts the mean number of chill hours (32 to 45 degrees) for 2010 to 2039 in Marin might be as few as 516 using RCP 4.5. For stone fruit trees, a lack of winter chill results in delayed foliation, reduced fruit set and poor fruit quality. Cherries are especially harmed by a warm November to February.

So, I might plant a persimmon tree (100 chill hours) rather than a cherry tree.

Parker notes the biggest challenges, though, will not be the increase in average background temperatures. It will be the increasing intensity, frequency and duration of extreme events like heatwaves, drought and flooding.

The Cal-Adapt website predicts the probable number of Marin heatwave days (above 94.4 degrees) will increase up to 13 in the future (years 2035 to 2064) given RCP 4.5. With high emissions, heatwave days could hit 16. That’s something to consider when choosing ornamentals.

For edibles, depending on a plant’s growing season, gardeners might consider planting and harvesting early to avoid late summer heatwaves. The downside, though, is a possible phenology mismatch between when things bloom and when pollinators are ready for that.

“I’d be surprised if gardeners haven’t already noticed shifts in phenology,” Parker says. “Earlier ripening and harvest, earlier bloom times and warmer summers causing crops to mature faster. A lot of these changes will play out in a slow process. One day you’ll wake up and say, ‘It just wasn’t like this 15 years ago.’”

For information on plants appropriate for Earth-friendly Marin gardens, go to marinmg.ucanr.edu.

Sponsored by UC Cooperative Extension, the University of California Marin Master Gardeners provide science- and research-based information for home gardeners. Email questions to helpdesk@marinmg.org. Attach photos for inquiries about plant pests or diseases. The office is closed for drop-in visits.