Seed Power

By Lauren Lewis

One of the defining characteristics of plants is unfortunately a limitation: their inability to move around. They have to reach out from where they are to find water and mates, or let those resources come to them, and they’re certainly disadvantaged when it comes to escaping danger (although many have evolved ingenious compensating strategies). Because of this limitation, the seed, the small but mobile plant part, holds great power. The seed has arguably the greatest power over a plant’s survival: it determines where the plant will forever live and grow by “choosing” when to germinate. Recent groundbreaking research has illustrated the almost decision-making power seeds possess for this purpose.

IMG_5067As a general rule, seeds move through space in a dormant state, waiting until conditions are right for germination. The timing of germination is crucial- the environment might be too cold if germination is too early, but too late and the plant will likely be outcompeted by surrounding plants who are already bigger. Seeds respond to environmental cues by producing the hormones abscisic acid (ABA) and gibberellin (GA), which promote dormancy and germination, respectively.

In a 2017 study of Arabidopsis, a fast-growing cress common in plant research, researchers mapped the 3-4000 cells in a seed and found that different cells produced these two antagonistic hormones, and that these cells were clustered in a dormancy group (ABA) and a germination group (GA) separated but near each other, and in the root tip of the seed. This layout is somewhat analogous to a “brain,” where differentated cells can send signals between each other, and the space between them is meaningful. The production and transport of more dormancy hormone (ABA) in this “brain” center of the seed maintains dormancy, and greater germination hormone (GA) stimulates germination. The researchers found that the physical separation of the cells is key because it allows the plant to more precisely respond to variations in temperature, which is an environmental signal of changing season.

Discoveries like this “brainy” cell layout in seeds are finally helping us appreciate agency that plants are capable of. In a managed setting like a garden, plants have us to determine their fates through propagation techniques. A plant off on its own appears to have a comparative disadvantage, but happily, recent science is giving us a better understanding of a plant’s own active role in its rebirth and survival.

Gardens for the protection of the SF Bay

By Lauren Lewis

The winter rainy season is a happy time for our gardens, and a less happy time for the San Francisco Bay. That’s because heavy rains, which typically happen a handful of times each winter, bring more water than Bay Area cities’ sewer systems can handle, and the result is more minimally-treated wastewater making its way into the Bay.

Most of San Francisco has a combined sewer system, which means that sewage and stormwater travel through the same pipes. Under normal circumstances, all that combined wastewater goes through primary treatment (basins for removing settled heavy junk and floating light junk) and secondary treatment (use of microbes to consume organic matter) before being discharged into the bay. When there’s a storm, there’s more rain in the system than can be treated in the secondary treatment plants, so the discharged water retains all its organic matter, like bacteria, and people who come in contact with the water are more likely to get sick from it. In the 90% of the city with a combined sewer system, the key mitigating factor is how much water we can prevent from entering the storm drains.

In the other 10% of the city, stormwater and sewage travel through different pipes, and stormwater flows directly to the bay, untreated. In these areas, the key to protecting the bay is improving the cleanliness of the water going down storm drains.

In both cases, there’s a variety of practices that the city and its residents can use to keep some stormwater from getting into the bay. The most basic way is plant-covered ground. Even bare soil allows for more water infiltration than cement or asphalt, but a well vegetated piece of land can do so much more than soil alone. The plants hold the soil in IMG_4694place, so the water doesn’t wash it away. Plant roots, dead plant matter, and all the soil organisms plants support make soil more porous, so more rainwater can soak in.

“Rain gardens” situated where rainwater gathers and flows are an inexpensive way for the city to prevent some stormwater from reaching drains, and if you look carefully you’ll notice more and more of these popping up around the city. Bits of sidewalk torn out and replaced with gardens can have a similar effect (in addition to the obvious side-benefit of beautification!). The city of San Francisco partners with Friends of the Urban Forest to do this, and you can spearhead a sidewalk garden with FUF’s help.

If you’ve read any of our previous blog posts, you’ve probably noticed some repetitive themes for how we garden and how we want others to garden, and this post is no different. Our advocacy is for dense and diverse plantings, hidden organic matter, and location-appropriate plant choices- practices that improve the water-infiltration capacity of soil, and also apply to all the various topics of our previous posts. What a wonderful thing. When you copy nature’s patterns in the controlled environment of a garden, your garden and the much bigger world it’s connected to all benefit.

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.