Grow Shade for Heatwaves–a review of the hardiest trees in our garden

Whew! I feel like May was a sprint to get all the raised beds prepared and the veggies planted out, sometimes repeatedly, as the deer kept jumping the fences and eating the tomatoes and other veg starts down to sad little nubs. Luckily we seeded more than we thought we needed and those ended up being replacement plants. We netted the beds again. Fingers crossed.

We’ve also been planting replacement trees where some old ones came down. As the days are growing warmer, this meant working up quite a sweat and then searching out shade to rest in. It’s amazing how much better even a bit of shade can feel on a hot day. Even hotter weather followed, and word of dangerous heatwaves happening in parts of the U.S. West here and Mexico and other places is in the news. And that inspired this post: to honor the shade and other sustaining benefits trees give us. I also wanted to share which trees have done well in our long-term garden here. Maybe it will be useful to someone.

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When our garden took shape — Our five-acre property was a horse and sheep pasture when we acquired it a couple of decades ago. It was mostly bare land with a smattering of trees–a magnificent native Valley Oak Tree, two large Arroyo Willows, a poplar, a couple of declining old plum trees, and a lovely Japanese maple. To us gardeners, the remaining open ground was akin to a blank canvas calling us to fill it.

Over the years, some trees we grew did well and others did not and I tried to keep those recorded in my garden journal. Trees here must survive sandy, poor soil, dry summers and sometimes extreme drought with only minimal drip irrigation. Wet winters follow, with temperature ranges from an all-time low of 15°F (-9°C) that we experienced in a cold snap one year, to ever-increasing high temp’s like the 114°F (45°C) we had a couple years ago. Gophers are also rampant in our sandy soil and eat the roots of many plants unless protected with gopher wire baskets around their root ball. Then there are deer and other pests and diseases to consider.

So, any plant, any tree that survives or even thrives through all that gets a gold star in my book. Hence, I thought it might be useful to mention some of those A-list trees here on the garden blog.

If you are interested in planting a tree(s), please continue your own research for your area and be careful to plant an appropriate size tree in an appropriate spot. The Sunset Western Garden Book, mentioned below and in my Resources page, has expert advice on selecting and siting an appropriate tree for the Western U.S., valuable growing details, and explains how to plant them.

Trees–mini cooling centers in your garden!

I’m so glad we managed to establish some trees in the early days of our garden. Back then it was cooler in the main–the afternoons would become foggy around 3pm. But these days, everything has trended hotter, no foggy afternoons anymore, and any shady spots are crucial as little cooling centers for us when we work in the garden on hot days. It also seems like we have wind for longer periods of the year now. Both dry out the soil. Growing trees can help. You probably already know all the benefits of trees, but I’m feeling especially grateful to them and all they do, so here’s a little recap:

• Trees can reduce the radiant heat absorbed by surfaces like the ground, or your roof– cooling the air temperature 5-10°F or even more.
• This in turn helps reduce the burden on power plants during heatwaves and saves us consumers money on home energy costs.
• The shading of asphalt in urban areas reduces damage from extreme heat, saving repair costs.
• Trees help effectively manage stormwater, groundwater recharge, flood control and reduce evaporation from winds.
• “A well-managed urban forest saves more water than it consumes!” (Canopy.org)
• Trees sequester carbon in their plant material and in the soil, helping to stop climate change.
• Native trees support local wildlife.
• Trees help clean our water and help reduce noise pollution.
• They protect our skin from ultra violet radiation that can cause skin and eye damage.
• Trees make a community more livable. A view of trees helps with stress and makes people drive slower.

Ever wondered? Should you water trees during a drought? Should you plant trees during a drought? (Yes. Read those links for the reason.)

Sources: California ReLeaf; CA Urban Forests Council; Canopy; Arbor Day Foundation; The Nature Conservancy; CA Native Plant Society.

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Choosing Trees

For myself, when I am deciding on a new tree to plant, I keep in mind my goals and considerations (see below) including our microclimate and soil type.

I peruse the tree section in the above-mentioned Sunset Western Garden Book. For plant hardiness zones, I prefer to use their Sunset Planting Zone map which is much more detailed than the general USDA Plant Hardiness map.

I might wander through my shelves to skim books like Plants and Landscapes for Summer Dry Climates of the SF Bay Region, or books on California native plants.

I might make some notes, then do a little online research, looking up ideas and more information on sites like the California Native Plant Society, or my local Master Gardeners site for any tree suggestions they may have. Or, if I want to get some ideas about the latest landscaping varieties, I might do a search on the Monrovia site, which has a good search function. I also try to sleuth any potential drawbacks by searching online for “problems with (my selected tree type)” to see if there are issues that people have talked about that aren’t mentioned elsewhere. Local municipalities sometimes have a vetted list of tree suggestions for towns and cities. Visiting public gardens or having a walk in your neighborhood and noticing which trees are thriving could also be useful. Or ask any long-time gardeners near you! 🙂

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My priorities in choosing a tree

• Shade during the hot summers
• Food production or other secondary usefulness
• Wildlife habitat for small creatures like butterflies, pollinators, birds, bees, beneficial insects, frogs and dragonflies. Native plants are ideal for this.
• Privacy
• Aesthetics
• Drought, Pest and Disease Resistance–although I doubt any tree is resistant to all the following, these are considerations I look at in choosing a tree. Resistance to: drought, gophers, deer, Mediterranean Oak Borer, Sudden Oak Death and other Phytophthora., Bacterial Canker, Verticillium Wilt, Fireblight. I try to hedge my bets by planting a variety, so that if hit from one type of pest or disease, others may still stand, especially as we get more affected by the vicissitudes of climate change.
• Climate resiliency–Read more about Climate Forward Trees from the Master Gardeners for the city of Santa Rosa, CA to give you an idea of how we need to think ahead. I hope there will be more studies and suggestions like this for more areas.

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The Most Successful Trees In Our Garden

This list is always getting edited, as some selections fall off the hardy list and others get added. These are not all the trees growing here, just some good performers to date. It is beyond my scope to include complete info about these trees. I’m only adding a few of my own observations or a note. Again, please do more research for yourself if planting any of these. And remember that some of these would be too large for smaller spaces.

I also want to quickly mention that I like to interplant some smaller fruit trees within the boundaries of the veggie garden. They provide some spots of shade there for tired and hot gardeners (us), are easier to keep an eye on, and probably receive a marginal amount of residual irrigation from the raised veg beds to get them through droughts. Just take care not to block sun–especially the low winter sun from the south–from your veggie beds. And not too close to the veg beds or the encroaching roots could be an issue. I wouldn’t do this with, say, a large mulberry tree (too big plus vigorous roots) or a walnut (too big plus allelopathic to nearby plants). Besides spots of shade, I think it just looks prettier in the veg garden to have some small fruit trees and a sprinkling of shrubs and perennials all inter-mixed. Keeps pollinators nearby and probably offers some shady habitat for frogs and beneficial insects.

Here’s the list of our best-performing trees to date:

Albizia julibrissin aka Mimosa–a beautiful shade tree that attracts pollinators, birds and butterflies. Though it does better with regular water, ours does fine with limited drip irrigation and can handle high summer temperatures. (See Sunset Western Garden Book, hereafter referenced as SWGB). We use these in the larger, open spaces. Note: Be cautious as this tree might be invasive in some locations. Ours are along the drive and in the field–not surrounded by irrigated lawn or garden. We have only noticed the occasional seedling in our gravel driveway. I have not noticed any seedlings in the non-irrigated field to date, but I can well imagine they could be invasive in perennially moist areas.

Arbutus unedo aka Strawberry Tree and Arbutus ‘Marina’–These trees may come off my A-list soon, as some may be getting a disease, sadly. But we’ve used these trees extensively–especially ‘Marina’–for large evergreen hedges, having chosen it especially for purported gopher and drought resistance. It is pretty, attracts hummingbirds, butterflies, bees and birds. I hope ours pull through whatever is affecting some of them. Hopefully it was just stress from the extreme drought we just recently went through. I will update this if I find out.

Asian Pears–a favorite fruit of mine and one of our most successful fruit trees. Remarkably hardy for a fruit tree and provides a nice spot of shade in the garden. A beautiful tree in all seasons and I would grow it even without the fruit. Note: yellow jacket wasps are attracted to the fruit, same as apples.

Feijoa sellowiana / Pineapple guava–evergreen tree or shrub that takes well to pruning. Edible flower petals and fruits–I recently watched what looked like some purple finches enjoying some of the petals, which apparently pollinates the flowers! (CA Rare Fruit Growers). Drought tolerant but does better with a bit of irrigation. Rather slow growing. Looks really pretty limbed up in the landscaped garden.

Hazelnuts–we started in the early years with the European species, Corylus avellana, and those are doing well. I love the leaf shape, the bark and the shade they provide. More recently we added some of the native Corylus cornuta californica to a hedgerow. Hopefully those will also do well.

Japanese maples–when we moved in here, I was surprised to see a mature Japanese maple thriving here with no irrigation, planted by the former owners. It is on the northeast side of the house and therefore gets some shade and wind protection. So, that gave me encouragement to try others here in some of the shade/part-shade places that are low-lying and therefore probably have a higher water table in the soil. (But new plantings are of course on drip irrigation.) Japanese maples are said to be resistant to oak root fungus.

Leptospermum scoparium / New Zealand Tea Trees–low-water, evergreen, manageable as a pruned hedge or limbed up as a small tree, not eaten by deer, loved by honeybees. White, pink or red-flowering cultivars. One of my favorites. I see that there is a concern about the potential invasiveness of the Australian tea tree / Leptospermum laevigatum. Note: I have never seen a seedling sprouting from our New Zealand tea trees.

Mulberry–another favorite food-producing tree–although the birds aren’t big on sharing the harvest. 😉 Not for places where littering fruit or vigorous roots will be a problem. We planted ours in a low spot where the water table tends to be high. An upright growth with a good shade canopy.

Oaks–my favorite is the Valley Oak, with its majestic large canopy and angular branching patterns. The year-round wildlife that this tree provides for is wonderful to observe. It is said to be resistant to Sudden Oak Death, but not the Mediterranean Oak Borer. A very large tree.

Olives–a lovely, but tough evergreen tree, even if you don’t want the fruit (in that case choose a non-fruiting variety). It is drought tolerant and does well in places like ours with hot dry summers. Use yeast bait traps for olive fruit fly!

Parrotia persica aka Persian Ironwood–gorgeous golden fall color, an upright growth with dense leaves. Seems utterly tough at our place.

Persimmons–sometimes you get lucky and a fruit tree turns out to be one of the most hardy, successful trees in your garden! That is the case with persimmons for us. A lovely, deciduous fruiting tree with a good canopy of summer shade that also sports good fall color. I love the fruit, but even if I didn’t, I’d grow them for the flickers and other birds who enjoy them. The branches can snap easily, especially as they are sat on by large birds harvesting the heavy fruit. Prune to help prevent this.

Plums–Though we only have one of these–the Santa Rosa plum tree, developed by Luther Burbank of Santa Rosa, California–it has performed wonderfully well over the years. We have the weeping variety, which I love for its beautiful branch pattern that I rarely prune. The fruits are very tasty. It fruits well and also makes a nice shade canopy for a smaller tree. I should grow more of these.

Pomegranates–One of the fruit trees listed on the Climate Ready Tree list for Santa Rosa. They’ve been slow to establish for us and have definitely needed drip irrigation to get them going, but they can handle the heat, and it is said that they can take drought. Resistant to oak root fungus. (SWGB) Gorgeous blossoms, yummy fruit, yellow fall color. A smallish tree. Has some thorns.

Styphnolobium japonicum (formerly Sophora japonica) / Japanese pagoda tree or Chinese scholar tree–“Once established, it is resistant to cold, heat, drought and air pollution and is tolerant of limited root areas, compacted soil and hot, dry conditions.” (LA Daily News). Resistant to oak root fungus (SWGB). I love this tree! It’s growing straight and tall and creates great shade. The lacy-looking leaves are so pretty as I look up through the canopy. White, fragrant, pea-like flowers. So far it’s been remarkably low maintenance for us. Probably too big for smaller spaces.

Vitex agnus-castus–tolerates heat, is deer resistant, and sports lovely lavender-blue flowers (see the book, Plants and Landscapes for Summer-Dry Climates). Attracts butterflies, bees and hummingbirds. Resistant to oak root fungus and drought (SWGB). Ours seems very hardy in terms of low water.

Walnuts–We have one English walnut tree that we planted in the early days and it has done well, makes a good shade canopy and produces lots of nuts. Alas, the nuts mostly feed the squirrel who is smarter than we are. 😉 We have planted more of these (you know, for more squirrels) and I hope to include at least one of the native black walnuts in our large hedgerow. Walnut trees are big, messy and may not play well with other plants in the garden, as they can produce juglone which is allelopathic. But apparently the English walnut produces less than the black walnut.

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Trees I’m still trying, I’m done with, or honorable mentions

I continue to try to grow more California natives like toyon and coffeeberry, but as yet have never managed to get those established. There are two new big leaf maples that I have high hopes for, planted on the edges of winter bog areas. One flannel bush tree is doing well and some ceanothus and elderberry. I keep trying. At least the valley oaks do really well.

Trident maple–I fell in love with this tree several years ago and grew some for shade near the house. They were truly beautiful in form and I loved the charming shape of the leaves and the gorgeous fall color. Alas, at our place they attracted epic hordes of box elder bugs. The bugs were basically dripping off the trees in disgusting numbers, and starting to invade the house. I hoped they would level off after awhile. They didn’t. The birds didn’t like the bugs. Eventually we removed the trees. Sad.

Liquidamber–a lovely specimen grows up the road from us but the one we planted in the early days seems to get repeatedly gnawed by gophers or something.

Figs–Alas, gophers love these. Any that we still have are growing in protected planters now and pruned as multi-stemmed shrubs. Sometimes the gophers eat through the oak planters once the gopher wire under them erodes. Sigh.

Other fruit trees–cherries, apples, nectarines, jujubes, pawpaws, prunes and flowering crabapples–we also grow these, but they can be variable in their success rate for us. The cherries especially succumb to what I think is bacterial canker. To try to prevent this, we do winter dormant spraying with copper and horticultural oil and delay pruning until summer when the weather is dry. The nectarines do better than the cherry trees, and are worth growing for their beautiful spring bloom alone and it’s now hot enough here to grow some. Since we did lose a nectarine tree, they are not yet on my A-list. The apples also are variable in their success. The ones that have done the best and also fruited the most are the ones I’ve espaliered in the vegetable garden. Perhaps I keep a better eye on them there? Note: the espalier form far outpaces the fruiting capacity of all the other apples in tree form. The jujubes and paw paws have failed to do much of anything in all the years we’ve tried (and keep trying) to grow them. I’m now trying a goji berry too. We’ll see how that goes.

Smoke trees–very pretty but so slow growing in our spot. Same with ginkgo. We have them but they are not depended on for shade (in our lifetimes anyway).

Citrus–too slow-growing and temperature-sensitive in our spot to be used as a one of our go-to tree choices. We grow them on the hill away from frost pockets, and in raised, gopher-protected beds.

Fringe tree–We are growing only one and it is very pretty and could be a good shade tree, but ours is only moderately happy. Perhaps it would thrive more in another spot?

Conifers–given increased wildfire concerns, we are very reticent about conifers (and eucalyptus) these days. Junipers, pines, spruces, and firs are resinous and highly flammable. (See this PDF on Home Landscaping for Fire by UCANR) We have a few coast redwoods and incense cedars (which do really well), and a small number of leftover conifers, but there are no plans for more.

Paulownia tomentosa / Empress Tree–there was much hype about this tree back in the day and I do enjoy their purple flowers, fuzzy leaves and shady canopies (but not the sticky seedpods). There was a mid-size one at our former home (where there was clay soil and fewer gophers) and so when we moved here, I planted several different cultivars with high hopes. Alas, they did not thrive at all. I don’t know if it was the gophers or the sandy soil or something else. I cut the piddly things down after some years and they did not grow back.

CA buckeye–a native tree that does really well here with pretty white flowers. Alas, I only found out recently that the tree’s blossoms are poisonous to honeybees. I know that European honeybees are not native here but I do love having them around. Now I’m hesitant about buckeyes.

As a side note, the non-native tree I have seen to be most invasive in our spot is what I think is the acacia baileyana. I have not planted it–it was growing in a nearby property already. It is very pretty but it spreads quickly into thickets, and tall heavy trunks fall frequently, crushing anything below. Quickly evades management.

Lastly, many vines on arbors can provide excellent shade, of course, and even be drought tolerant (witness the rosa banksiae in Arizona) but I’ll save those for another day!

I’ll try to remember to update this list over time.

Update Aug 2024: I just noticed that one of my favorite UK garden presenters, Arit Anderson, has co-written a new book, The Essential Tree Selection Guide: For Climate Resilience, Carbon Storage, Species Diversity and Other Ecosystem Benefits. I’ve ordered it and can’t wait to read it. I’m presuming the tree recommendations are for UK gardens so those of you in the UK may enjoy it if you haven’t heard of it already. I always appreciate Ms. Anderson’s knowledge on ecological gardening, especially peat-free gardening.

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Meanwhile, since I wrote last, the pipevine swallowtail butterflies have been out in numbers and now the aristolochia californica vines are covered in hungry caterpillars. The old garden roses put on a great show with their May blooms this year, after our wet winter. Some are still blooming and scenting the air.

That’s it from me. Which trees have done best in your garden over the years? Are you shifting what you grow due to climate change? Even if you are not local, it is always interesting for me to learn what gardeners are growing in other areas of the world. Thanks for reading and commenting! Good luck with all your gardens!

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