THE COASTAL GARDENER:

Does your garden seem to be suffering through the hot summer? Does this summer seem hotter than usual? Does it seem as if no matter how much you water, your plants just keep getting more stressed? If you’re asking these questions, you are not alone.

Let’s break these issues down a bit by first looking at the issue of water stress versus heat stress. Backyard gardeners often misunder- stand plants suffering from the stresses caused by heat, figuring more water will solve the problem.

Even modest gardeners are aware of plant hardiness zones, also known as climate zones or growing zones. It seems even non-gardeners almost instinctively know bananas won’t grow in Minnesota. Why? Because it’s just too cold there; almost everyone knows that. The few who don’t are soon reminded, following the first night of freezing temperatures.

But cold is not the only factor needed to predict how well a plant will perform in your garden. An equally important but far less considered factor when choosing garden plants is their suitability to the heat expected in their new home.

As evidence, gardens in Portland, Ore., and Las Vegas experience about the same minimum temperatures. Therefore, they both share the same hardiness zone. However, the heat of a Las Vegas garden is much higher than that of a Portland garden. Hence, hostas thrive in Portland, not in Las Vegas. Saguaro cactus are effortless in Las Vegas, but impossible in Portland.

It’s not the winter cold that limits these plants — it’s the summer heat.

Cold damage is rather easy for a gardener to understand, and easy to see. The feedback is rather instant — too cold means a dead plant. However, too hot, and the effects are much more subtle. Either way, they are just as limiting.

Since a plant’s reaction to heat is gradual, its impact is misunderstood by gardeners. In a vain attempt to mitigate what is in reality stress from too much heat, a gardener’s usual response is to apply more water. But in this case, more water won’t make things better.

Heat damage may first appear as flower buds that wither and drop, leaves that droop, plants becoming more attractive to insects, plants that stop growing, leaves that shrink and roots that stop growing or even retract. Plant death from heat stress is slow and lingering and often obscured by secondary afflictions, like root rots. Most often, the plant survives; it just looks miserable.

But now there is a second, more significant issue involving the effects of heat on plants. Recently, the online science journal PLoS ONE published the results of a four-year landmark study titled “Climate Change and the Future of California’s Endemic Flora.” The study is the first to examine how climate change will affect California’s plants.

California is home to about 5,500 native plants, including roughly 2,400 that live nowhere else.

The study concludes that as temperatures warm, redwoods along California’s coast may move farther north toward Oregon, and native oaks may disappear from the central part of the state.

As temperatures rise, the study concludes that by the end of the century two-thirds of California’s wild plants will persist in a range reduced by more than 80%.

The effects of rising heat will result in deserts moving west, and pines and fir forests giving way to shrubs and scrub. Garden plants will surely follow a similar path.

Why do we struggle with Japanese maples in summer? Why can’t we grow dogwoods and Aspens? Will our children soon be unable to grow sweet peas, delphiniums or columbines?

In summary, when planning a garden, consider not only a plant’s tolerance to cold, but also its ability to handle heat. Gardens are becoming warmer and this will challenge us. In the meantime, be careful not to misinterpret a plant suffering from heat with one that needs more water.

ASK RON

Question: When is the best time to dig and divide my bearded iris clumps?

Costa Mesa

Answer: Right now. Although bearded iris never go completely dormant in Orange County, they slow down in the hot summer. They should be divided and replanted every two to four years depending upon their vigor. Although you can probably divide them any time, if you do it now, you will have the best chance of not disturbing their bloom cycle for next year.

ASK RON your toughest gardening questions, and the expert nursery staff at Roger’s Gardens will come up with an answer. Please include your name, phone number and city, and limit queries to 30 words or fewer. E-mail [email protected], or write to Plant Talk at Roger’s Gardens, 2301 San Joaquin Hills Road, Corona del Mar, CA 92625.

RON VANDERHOFF is the nursery manager at Roger’s Gardens in Corona del Mar.