Planting for soil health

By Lauren Lewis

With the arrival of the (hopefully) rainy season, our thoughts go directly to what the rain can do for our gardens. Besides the obvious benefit of free, un-transported, apolitical water to nourish the plants, the rain also benefits the soil, by catalyzing the decomposition of dead plant material that’s lying around. So much of the soil in a city is compacted beneath buildings and pavement, but the health of our remaining exposed urban soils has a real impact on how our city responds to the winter rains. So with soil health in mind, my next thought is how can our gardening behavior most benefit the soil?

The four elements of soil are minerals, organic matter, water and air. Those last two are counterintuitive because they really have to do more with the spaces in between the minerals and organic matter than can be filled with water or air. The spaces are created mostly by the movements of soil-dwelling insects, worms, microbial species, etc. Those animals move through the soil in search of food, so the presence of organic matter (their food) encourages their presence and facilitates their movements, which in turn creates space in the soil. The spaces created, and also the sponginess of the organic matter itself, makes that soil much more able to soak up rain and prevent it from running off into the streets and drains and into the bay.

Happily, our gardening practices have a big impact on soil health. To increase the amount of organic matter (dead plant and animal parts) in the soil we can choose to leave dried leaves and chopped up plant cuttings hidden around the garden rather than moving them to the green bin. When the rain comes, the moisture helps that organic matter soften and become easily accessible food for the soil animals.


Research has shown that the diversity of our landscape plant choices can also affect soil health. In a study of prairie species, researchers created plots planted with between one and 16 species, and tracked various measures of soil health over many years. The study found that more diverse plots had greater overall plant production, meaning the diverse mix of plants facilitated each other’s growth, and as a result, the soil had greater microbial biomass and fungal presence. The researchers point out that this relationship is likely only relevant in a soil that’s lacking in organic matter to support the soil dwellers, which suggests that when a soil is low on organic matter, diverse plantings can help remedy the problem.

Diverse plantings and hidden piles of organic matter are two of the Small Spot Gardens calling cards. Soil health has always been our guiding goal, since healthy soil grows better plants and makes a small but real impact on our Bay Area environment. It works out so nicely that the soil-supportive practice of diverse plant choices also lets us design gardens with the dense, varied aesthetic we love.

In our Mediterranean climate, the seasons reverse

By Lauren Lewis

The tree losing its leaves is arguably the most recognizable image of autumn. In temperate areas of the world, like North America, the most common reason that trees lose their leaves, a process called abscission, is to protect themselves from cold damage. The plant senses a decrease in daylight hours, and responds by withdrawing nutrients from leaves for storage (the withdrawal of green chlorophyll results in a yellow leaf), creating a layer of barrier cells between stem and leaf, and then letting the leaf detach. Here in SF you’ll see plenty of this going on pretty soon in the gingkos, stone fruits, London planes, etc.


This buckeye was in full bloom in early June in preparation for July leaf-drop.

But in plants native to Mediterranean climates like we have here in coastal California, the reason for leaf abscission isn’t cold, it’s dryness. Mediterranean climates occur in parts of Australia, central Chile, coastal California, South Africa, and of course the land around the Mediterranean Sea. These places have relatively mild temperatures year-round, dry summers, and wet winters. With mild temperatures, plants don’t have to prepare for cold by dropping leaves. Instead, the dangerous time for the plant is the dry summer, and one adaptation to this challenge is to lose leaves and go dormant during the driest months of the year. A great example is the buckeye (Aesculus californica), a staple of California’s native landscapes, whose leaves brown and drop in July. Purple sage (Salvia leucophylla) drops its large, springtime leaves during summer and replaces them with smaller, whiter leaves that reflect light and withstand heat.

This very regional pattern is made even more nuanced in San Francisco and other especially coastal parts of the Bay Area, where summers are characterized by fog and an even smaller temperature range. Plants here get some summer moisture from the fog, so they’re under less pressure to go dormant, and their dormancy can be shorter or less extreme. The first winter rain, which is possible in October, jolts dormant plants back into growth mode.

Climate change is producing changes in dormancy patterns by way of hotter summers and drier winters. A recent study of California perennial grasses noted that non-native annual grasses have recently been out-competing the once-dominant perennial species in California landscapes. The study showed that perennial species with more pronounced summer dormancy characteristics, like earlier reproduction and shallow roots, are similar to annual grasses in those characteristics, and therefore might be more competitive as droughts worsen and dormancy is made more advantageous than before. In other words, recent success of annual grasses suggests that perennial grasses that “mimic” annuals by going dormant in summer are likely to have higher survival as the climate changes. Landscape restoration efforts would therefore do well to promote summer dormant plants.


The garden smells we love are plant protectors

By Lauren Lewis

If you visit the San Francisco Botanical Garden on a hot day like we’ve been having recently, your nose may well have a more interesting experience than it would on a normal foggy day. That’s because most plants are constantly sending out odiferous volatile compounds (essentially chemicals), and warm air allows those volatiles to move around more and intensify. Some are even synthesized specifically to protect a plant from heat damage, so a hot day triggers greater release of those volatiles than a cool day. Our noses know many leaf volatiles, particularly from herbs like sage, rosemary, basil, and we certainly know the volatiles that give flowers their sweet scents, but plant volatiles have complexity that goes far beyond our olfactory system.

img_5022.jpgA simple but accurate way of thinking about plant volatiles is that they help solve a plant’s challenge of being stationary, and therefore limited in its ability to escape dangers or attract assistance. Plants release these self-made chemical compounds into the surrounding environment for a huge number of benefits to the plant. Some volatiles provide direct defense for the plant, like the isoprenes that help an oak quickly return to a normal photosynthetic rate after exposure to high heat. Or the volatiles released by a leaf that is being eaten, that temporarily deter herbivores from continuing to eat the leaf. These are essentially plant-produced pest deterrents.

Other volatiles allow for more complex forms of protection for the plant. Some allow a plant to summon another species: a leaf being chewed by an insect releases a volatile that attracts predatory insects, who arrive to eat the herbivorous insect. A longer-term use of volatiles is a phenomenon called allelopathy, when a plant releases compounds that prevent the growth of other plants around it, therefore reducing competition for resources. A well-known example is the eucalyptus tree; examine the ground in a eucalyptus grove and you’ll notice that not much else is growing there.

Many of these volatiles that plants synthesize to protect themselves also have benefits for humans, which is why the sense of smell is an important element of horticultural therapy, or healing gardens. Researchers have shown benefits from multi-sensory garden therapy for patients with mental illness and dementia. Unfortunately, little work has been done to isolate the effects of scents in the garden, but research has shown benefits from essential oils, which are plant volatiles concentrated into liquid form. One study of two common garden oils, rosemary and lavender, showed that both had positive effects on mood, and that exposure to rosemary (typically thought to be a stimulating oil as compared to sedating lavender) temporarily improved memory. Follow our Instagram this month as we explore various scented volatiles and their possible human health benefits.

The myth of California wilderness

By Lauren Lewis

The concept of “Small Spots in a Big World” is about putting our outdoor spaces in the context of space and time, in order to understand and appreciate them better, and consequently interact with them better. This post focuses on the context of time, on the millennial scale, and the way that plants and people have interacted intimately on the land we still inhabit. I’m taking a tiny dip into the immense story and detail contained in M. Kat Anderson’s Tending the Wild: Native American Knowledge and the Management of California’s Natural Resources. If the big ideas you find here are intriguing, the whole book is worth exploring.img_5026-e1501772019606.jpg

The big ideas in Tending the Wild are big in the sense that they up-end previous ways of thinking, and they have the potential to benefit California’s future immensely, if we choose to use them.

Big idea #1: The first Europeans to explore and settle in California found it to be awe-inspiring in its landscapes and impressively diverse and abundant in plant and animal life. They found what they thought was a wild, natural landscape and a small, inconsequential native human population. But in fact what they were seeing was a landscape that had been intensively managed and changed significantly by the indigenous people.

As Anderson illustrates, the thoughtful tending and use of California’s natural resources (plants in particular) by indigenous people actually “promot[ed] habitat heterogeneity, increas[ed] biodiversity, and maintain[ed] certain vegetation types that would otherwise have undergone successional change” (p.5). Indigenous Californians interacted so closely and purposefully with their natural resources that they changed the environment in ways that supported their lives here. An example is how native people used fire to maintain the coastal prairie environment that we now assume was always San Francisco’s landscape. (The use of fire is actually a key takeaway from the book.) They burned areas of prairie at a much higher frequency that would have occurred only with natural fires (since lightning is so rare here), in large part to maintain grazing land for large animals. The landscape would have been more treed if not for that practice.

Other examples illustrate the way indigenous resource management not only benefited the people involved, but the plants and other animals too. Seed beating was a common practice throughout California, that helped promote ongoing growth of certain plants over others. Seed beating meant hitting the seedy part of a grass or flower with a long-handled basket, to knock the ripe seeds off into another basket. People therefore harvested the ripest seeds for consumption, while also semi-unintentionally scattering some seeds in place and letting the unripe seeds stay on the plant to drop and germinate later. On a large scale, this practice changed plant populations – the tremendous wildflower fields witnessed by the first Europeans were a direct result of intentional propagation.

Big idea #2: We think of “wilderness” as land that is unspoiled by human presence and activity. But our vision of most wilderness in California is actually land that underwent these indigenous management techniques, and countless more, for thousands of years. Restoring our degraded habitats therefore can and should involve indigenous uses of the land. While our huge population certainly prevents us from returning to the full indigenous richness of the past in every corner of the state, the pervasiveness and positive impact of indigenous resource management through California’s history suggests that it should have a place in California’s future.

The first people of our modern day San Francisco managed, controlled, modified and tended plants and ecosystems for their day-to-day survival while we nurture garden plants for less immediately urgent needs. But with our changing climate and modern development causing mass extinctions of flora and fauna, and with chronic illnesses caused by sedentary, electronic-filled lives and poor diets, we may want to see our urban and suburban outdoor spaces differently. We may want to see these small areas as part of a bigger picture and learn how we can tend them carefully and knowledgeably like the first people in this area nurtured and tended their surroundings. Our goals and methods will be different than the people who preceded us, but our gentle care may turn out to be just as critical to our ultimate wellness.


In summer, even the veggies are fruits

By Lauren Lewis

Even in a place like California, with year-round food production and farmers markets, farmers depend on bustling summer markets to see them through the leaner winter months. Some of the difference in market attendance is attributable to weather that keeps people away in the winter, but a lot of the difference is certainly due to the produce selection at a summer market. Apricots, plums, peaches, tomatoes, beans, cucumbers, zucchini, corn, peppers, eggplants… I think I’m not alone in feeling a giddiness when the locally-grown versions of all those guys become available. If you look at that list you’ll notice that everything is a fruit. Even the “vegetables” are fruits, in a botanical sense. Summer = fruit.

Why is this? A fruit is the result of one seed dispersal strategy that plants use: endozoochory, or dispersal by vertebrate animals. Plants surround their seed or seeds with a tasty and energy-rich substance (fruit), animals eat the fruit and then deposit the seeds somewhere else after eating them, encased in fertilizer.


Credit: Gardener’s Supply Company

For maximum reproductive success, plants need to not only attract fruit-eating animals, but also do so at a time when the seeds that get consumed have the highest chance of germinating and creating a new plant. Soil temperature is a huge determinant of seed germination, and the optimal temperature range for germination of all the common summer vegetables is high compared to fall and winter vegetables. Evolutionary logic says: to disperse seeds at the best time for germination, produce fruit at that same time. And so we get our vegetables-that-are-technically-fruits in the summer, when the soil is warmest.

San Francisco is a notoriously challenging place to grow fruiting plants (fruits and vegetables alike) specifically because of our cool summers. As I write this in Noe Valley in late June, it is completely overcast, misting a bit, and maybe 60 degrees outside. SF gardeners tend to have more success with a small subset of summer veg varieties, like cherry tomatoes as opposed to full-sized, and optimal varieties are highly dependent on the particular location, since our city has some ridiculously small microclimates.

Climate change, however, might start changing those calculations. One recent study estimated that by the end of this century, San Francisco’s climate will be more like San Diego’s, where fruiting crops are currently much more suited. Tree crop producers in the state are already seeing some damage to their yields from fewer chill hours in the year. (“Chill hours” are hours during which the temperature is 45 degrees F or below; all fruit and nut trees except citrus require a certain range of chill hours for proper leaf and bloom production.) And while San Francisco soil might be warmer in 50 years, weather, and crucially rainfall, will also be more unpredictable. For trees, whose fruit output comes years after planting, that’s a serious challenge. But the odd rainy year followed by a San Diego-style summer could greatly expand our options for San Francisco-grown cukes and tomatoes. A silver lining.

To support public health and education: grow trees

By Lauren Lewis

When you stumble upon a street where the sidewalk trees are so big they form a full canopy over the road, it just feels good. (I’m picturing 24th Street in the Mission as an example, from Mission to Potrero.) Something about the completeness of the shade, or the feeling of outdoor enclosure, is rare and lovely in an urban setting. Because they are big, long-lasting plants, trees provide structure and large-scale greenness in a city, and it’s very noticeable when they’re lacking. Some interesting recent research has tried to identify the more nebulous benefits that humans derive from trees, and city governments, including San Francisco’s, are starting to prioritize the urban forest.

IMG_5393In 1982 the government of Japan began a campaign encouraging “shirin-yoku”, or forest-bathing, to promote public health. Since then, various studies have shown physiological benefits, like lower cortisol level and lower blood pressure, from time spent in a forest as compared to city. To me this is somewhat unsurprising, though, given the noise and stresses of the city as compared to a natural setting. What’s really interesting is work that has tried to pinpoint trees as the health-promoter, compared to other natural elements. One such study found that high school students whose view out the cafeteria windows was filled with trees showed higher academic performance than those whose views were mostly other landscape elements like lawns. Thoughtful landscape design that mimics wild nature can have benefits that go beyond aesthetics.

Looking at the same idea in reverse, i.e. the public health impact of lost tree cover, yields similarly compelling evidence. When a beetle infestation caused 100 million trees to die across the eastern half of the United States a few years ago, a forest service researcher looked at how human mortality in those areas was affected. He found that infested counties had significantly more deaths per year from cardiovascular and respiratory diseases than uninfested counties, controlling for other factors. Even controlling for income, race, and other factors that impact health, the tree mortality provided a significant explanation for human mortality.

San Francisco’s 13.7% tree canopy is lower than most other major American cities, including Chicago, LA and NYC. Some of this is attributable to the fact that San Francisco’s land was never a treed environment; it was dominantly a shrubby, grassy landscape. Especially in the western part of the city, the fog, wind, and soil properties create a challenging environment for trees. According to Allegra Mautner, tree care manager at Friends of the Urban Forest (our city’s awesome urban forest nonprofit), trees that are planted in the western part of the city have only a 50% survival rate, as compared to the norm of 80%.

But now that we’ve got a dense urban environment on this land, trees are crucial for the all the benefits they provide: quality-of-life, human health, habitat, heat-control, carbon sequestration etc. The city government recognizes this, and in 2015 the Board of Supervisors adopted the Urban Forest Plan to begin building a stronger and larger urban forest. In the 2016 election that effort got a boost when SF voters approved a proposition that moves responsibility for street trees from property owners back to the city. A funding challenge still exists to fully implement the switch, but this change has the promise to bring more consistent and better care to our street trees.

Striving for ant-human mutualism

By Lauren Lewis

The concept of ecosystem services gained recognition and common use among ecologists and policy makers only within the last 20 years or so. Ecosystems services are the benefits that humans collectively receive from the functioning of ecosystems. It’s the natural processes happening around us that we don’t control but that make the world as we find it. There are many examples (trees respiring oxygen, bees pollinating crops, etc), but specifically in an urban environment, the insects around us arguably provide some of the most crucial ecosystem services. They’re barely noticeable, but hugely present.

By numbers, ants are a massive presence in cities and they provide the crucial service of eating our trash. One study estimated that there are 8000 ants for every rat in New York City and 2000 ants for every human. The research team for this estimate is led by Amy Savage, a PhD ecologist who is a leader in the effort to understand non-human urban species and their interactions with each other and with us. Her work in NYC showed that arthropods, mostly ants, carry away and digest as much discarded food as all vertebrates combined (that’s rats, squirrels, etc). Ants are active cleaners of our cities.

In the garden, we’ve long recognized that ants are among the myriad creatures necessary for healthy soil; ants aerate soil by digging and they help break plant material down into smaller pieces for digestion by worms. But a recent study illuminated another fascinating benefit of ants for soil: the “aggregate mulch” churned up by burrowing carpenter ants can reduce evaporation from the soil surface. The burrowing action of these ants leaves soil clumps with a particular texture behind them. Greater thickness of this aggregate mulch layer was associated with lower soil evaporation rate, particularly in the hours right around noon, when evaporation is highest. At Small Spot Gardens we are big advocates of mulch for its ability to reduce water needs, so now we know that supporting ant populations could be right in line with that goal.

Various ant species behave in widely divergent ways, so supporting ant populations can mean a lot of different things, but overall, there’s a consistent theme of leaving some wildness for ants to use. For epigeic ants, those who inhabit only the top layer of soil rather than burrowing and nest in existing cavities, presence of appropriate nesting materials is crucial. One study of epigeic ants and their nesting preferences found that ants showed a preference for certain cavity types (“large” 6mm wide hollow twigs) and that all nest types were more abundant in forest sites than in vacant lots or gardens. The findings suggest that efforts to mimic a forest environment (with leaf litter on the ground!) can support ant life.

The Bay Area is home to over 100 species of ants, but unfortunately the introduced Argentine Ant has driven a lot of them away by outcompeting them here and across California. The threatening presence of the Argentine Ant is all the more reason to see your garden space as a potential haven for other ant species. The plant litter you decide to leave behind instead of transferring to the green bin might mean home for a native ant colony.