Evaporative Cooling

I don’t think there is anyone who hasn’t experienced an extreme heatwave this past month. Rin, who is spending the summer in Savannah, experienced their first East Coast heatwave (with accompanying high humidity) a couple of weeks ago. Here in the West, we too have had our share of extreme heat, though not the humid kind. Our backyard weather station hit 116 degrees several times this week, and I instantly start sweating the moment I walk outside.

I have discovered recently that I am a ‘sweater.’ I don’t mean that I’m always sweating; but yes, when the circumstances are right, I will sweat more than the average person. I learned this because Tom and I have joined a local gym. The reason for joining began with injury recovery, but quickly morphed into something else - another tool to build our resilience. We are planning a month-long walking trip next summer for our 25th anniversary, so we are both working hard to build muscle, improve balance, regain agility, and increase flexibility. We’ve added all kinds of different exercises to our daily routine; simply hiking up a hill is not going to prepare us for hiking 16 miles a day for weeks on end.

So now we are in training. And man, when I train, I sweat. I mean not just in the usual places. When I’m done with, say, a spin class, my calves are slicked with sweat. And after a TRX class, my forearms are dripping. I suppose it could be embarrassing, but I don’t look at it that way - I consider it a sign of a healthy vascular system that is performing one of the jobs it has uniquely evolved to do.

Sweat is a beautiful human adaptation. Chimpanzees and macaques have sweat glands, but humans have 10 times the amount that they do and are the sweatiest among the great apes. Scientists have discovered that “the higher density of sweat glands in humans is due, to a great extent, to accumulated changes in a regulatory region of DNA that drives the expression of a sweat-gland-building gene.” This happened through repeated mutations and contributed to an evolution of higher sweat gland density in humans. We are meant to sweat! It’s our primary way of cooling ourselves. As the water in sweat evaporates, the surface of our skin cools. This is true of any evaporative cooling. A liquid will remove latent heat from a surface, and that evaporating liquid will cool the air around it.

Since most animals do not sweat, they have to cool themselves in other ways, and sometimes they too use evaporative cooling. During this heat wave, we’ve been closely watching the behavior of our backyard bees on our water fountain.

Bees use water to cool their hive. Some worker bees are tasked with finding, collecting, and bringing water back; it is spread in a thin surface over the surface of the comb and the bees then fan their wings to evaporate it. A hive may use a quart of water a day in the hot months for this purpose.

Honeybees are not the only ones who do this. Some wasp colonies (many wasps are social insects and, like honeybees, live in large groups) use water the same way. They collect it and spread it on the surface of their nests. Today, while filling the one of the water bowls at the school garden, I watched both paper wasps and yellow jackets collecting water from the edges.

This is one of the most important ways we can help insects; we can place shallow bowls of water around our gardens and yards. Put a rock in the bowl, or several rocks, so that the insects don’t drown (they have poor depth perception). If mosquitos are a worry, change the water daily or weekly (it’s good to do this anyway to keep the water clean). Birds will love this, too.

Many insects will also get water from the soil in your garden, so it’s nice to have a bare space which you keep wet for this purpose. Butterflies especially adore a muddy spot.

Another creature in our garden that appreciates evaporative cooling is the chickens. Chickens, like dogs, pant when it’s hot, and this past week they’ve been panting from dawn until full dark. I actually spray them with water, and though they seem to dislike me doing that, they really love when the ground in their run is wet. So when the temperature is over 100, I go out several times a day and spray them, and the dirt in the run, thoroughly. Immediately afterward, the chickens will congregate in the wet place, and they really perk up.

Chickens also dig holes in the dirt to find the cooler place under the surface. Many creatures do this, too. While dogs and coyotes pant, owls use something called ‘gular fluttering’ which is flapping the loose skin under the throat to move air over the throat cavity. Vultures urinate on their legs to keep cool, another form of evaporative cooling!

That last fact makes me appreciate anew the way we humans use sweat to keep ourselves from overheating.

Stay cool, everybody.

Don't Forget...

To order your garlic (for fall planting) NOW!

Because I was late ordering last year (I ordered in summer rather than spring), I ended up getting a variety of garlic that I’d never planted before, and as you can see from the above picture, the bulbs ended up being quite a bit smaller than the type I prefer (which is Inchelium Red). It’s ok, the garlic will be fine, and home-grown garlic of any kind beats the pants off of anything you can buy in the store. But still, I wish I had my old faithful hanging on the ceiling racks.

So - don’t be like my 2023 self - be like my 2024 self and make sure you order now! I actually order it in March for delivery in October. My favorite place from which to order seed garlic is Filaree Farm in Oregon. They have so many different varieties. I like the softnecks so I can braid them for hanging, but if you like garlic scapes, you’ll want to order hardnecks.

Anyway, our garlic harvest was late this year, in early June rather than late May, and then the stalks hung on the garage racks for a month in order to thoroughly dry out. This past weekend, I braided them for hanging in the house. It’s a fun task.

Then I cut up all of the old cloves left from last year, all of which were bolting. I cut out the green shoot, put them in the food processor and made a paste, then spread it on a sheet and put it in the dehydrator. After drying, I whizzed it in our dedicated spice grinder (an old coffee grinder) and voila, we’ve got our garlic powder supply for the next year.

The smell of the house on ‘garlic’ day is really something. It feels good to provide ourselves with this necessary kitchen ingredient each year. Garlic is a beautiful and easy crop to grow, it’s used in practically everything we cook, growing it ourselves saves us money and time, and it looks great hanging up in the house.

Follow-up to 'Relevance'

You might remember my post about my experience at the Chesapeake & Ohio Canal Historic National Park. I am pleased to report that I received a response today from Tina Capetta, who is Superintendent of the park:

“I just received your correspondence a few minutes ago, and please let me apologize for the delay in its reaching me and for the poor response to your question that you received at Great Falls. The park has identified that we lacked sufficient scholarship about the indigenous people who were and are associated with the Potomac River Valley, and we commissioned a study.  In 2020, the College of William and Mary launched a Tribal Affiliation Study for the C&O Canal National Historical Park.  It identified dozens of tribes that were affiliated with the valley, and park staff have done outreach to all of the identified tribes.  While most of them no longer consider themselves affiliated with this area, we are working to establish strong relationships where we can.  We will use information that we learn from the tribes and from the study to help inform updated exhibits throughout the park.  We have identified that the person you spoke to was an intern.  Clearly, there is training gap that needs to be remedied, which we will do promptly. Thank you for calling this to my attention.”

I am satisfied with this response, and very glad that the park is working on fixing the problem. This cements for me that letter-writing, and advocacy in general, is not a moot exercise.

Special Visitors

Many years ago, I planted a line of three dogwoods (not our native species; rather, Cornus sanguinea) just outside our front porch. This is the part of the garden I call our ‘woodland’ garden, because it gets dappled shade all day from our mature trees. I have a lot of natives here (various Ribes, spice bush, coffee berries, etc, as well as some true geraniums, things that can handle dry shade). I wanted the dogwoods because of their bright red stems, but I haven’t been pruning them correctly I guess, because they have yet to show the flame color their name suggests.

Anyway, I can see them from our living room. Tom and I were sitting on the couch chatting yesterday and I was looking out at them and admiring the sun shining through their leaves when I realized - hang on a second, what am I seeing? - those leaves aren’t supposed to look lacy. But they do now, and for a very good reason - one that makes me super happy. They are being used to build the nests of leaf-cutter bees.

image credit: Planet Bee Foundation

Leafcutting bees (Megachilidae family) are solitary native North American bees who use soft leaves and flower petals to create nests for their young. The female bee finds a long channel or tube, for instance in wood or in a hollow stem, and painstakingly creates chambers for her larvae, depositing some bee bread (a little mound of pollen) and an egg in each one. Each chamber is separated by a wall made up of chewed leaves and mixed with resin or mud. The bees spend the winter as mature larvae in the chambers; in spring, they pupate, then chew their way out of the nest and go off to mate. The adults are active only in spring and early summer; most of their lives are spent in the cells as larvae.

We have at least 75 species of leafcutter bee in California. They are generally smaller than honeybees, tend to be more of a grayish color, and carry pollen on their bellies rather than on their legs like honeybees. They are wonderful pollinators, and in fact there is an introduced species that is a major pollinator of alfalfa and is economically important. The family Megachilidae also includes Mason bees and Wool Carder bees.

The ‘damage’ to the margins of the leaves is quite slight and doesn’t hurt the plant at all. The bees are extremely gentle and in fact, in all my years of taking pictures of bees in my garden, I’m not sure I’ve ever gotten a photo of one. I’m delighted to find this evidence of their existence in my little ecosystem and I hope I get the privilege of finally meeting one.

Biodynamic Accumulators?

As a teacher, I’ve learned that if I state a claim during a lecture, at least one student is going to question it. I’ll get asked, “What’s your source?” or, “What’s the history behind that information?” or, “What’s the data that backs that up?” I have learned to love those questions, because it has made me a better teacher. I don’t dare write a lecture without exploring, and citing, every angle of every claim I make. This is important work and I’m glad to do it; especially because science frequently changes as more experiments are conducted. After all, it does no one any good to learn old science.

But all of us have ideas about gardening that we’ve heard so often that we take them for fact, whether they are or not. Things like:

• Putting eggshells around your plants will protect them from snails (1)

• Putting a layer of rock in the bottom of a container will increase drainage (2)

• Giving plants a lot of space will quell any competition for resources (3)

• Gathering up leaves and other plant matter from under plants will help them grow better (4)

You get the drift.

So, when a student recently said to a class, “Plant comfrey! The deep taproot will bring minerals up from the lower levels of your soil!” my inner alarm went off. I hear this all the time. Where did this idea come from? Is it true?

Comfrey, and other plants like nettles, are widely known as ‘biodynamic accumulators.’ There is no definition for the term because it hasn’t actually been defined yet. It came to prominence in the Permaculture world, as part of ‘plant guilds’ - that is, plants that are placed near each other to provide different benefits to the whole. For example, you might plant a fruit tree, and beneath it place a leguminous bush to provide nitrogen, chives to repel insects, and yarrow to provide biomass for mulch. Comfrey is often included in these guilds for its oft-lauded ability to ‘bring up nutrients,’ whatever that means.

Wait, what DOES that mean?

Well, all plants bring up nutrients, right? The nutrients in the soil are dissolved in the soil water and become the soil solution, which is where plants get the nutrients they need for growth.

How do roots form? When seeds germinate, they form a primary root from the radicle of the embryonic tissue. This helps the plant establish, so that it can unfurl its seed leaves (or ‘cotyledon’ leaves) and begin to photosynthesize. Once the plant is established, it usually forms one of two root systems, depending on what kind of plant it is - a tap root or fibrous root systems. Tap roots have a central root with little side roots coming off of it. The purpose is to anchor the plant in the ground and/or to act as a storage place for nutrients (think of a carrot). The fibrous root system has lots of adventitious roots that move across a wide area to find more surface pockets of nutrients, and cannot store nutrients long term. Confusingly, a very established fibrous root system can also act as an anchor for the plant (think of corn).

image credit: Zassou Garden

We could get much deeper into the weeds here (no pun intended) but I want to keep it simple so we get to the point of this post. Quite simply, plants have adapted different approaches for their needs, but all of these different kinds of systems (whether tap or fibrous) additionally have fine hairs which gather the moisture and nutrients. These fine root hairs will take up whatever is available, in whatever location they happen to grow.

So that means that the roots, wherever they are, whatever kind they are, are taking up nutrients. I suppose, then, depending on the kind of soil you have, there could be different nutrients in different levels. Perhaps you have a very shallow top soil that is depleted in nutrients; it might be good in this case to include deep-rooted plants in your garden to make those subsoil nutrients (minerals from rock, mostly) available. Most tap roots develop a greater number of associations with fungal networks, which break down the rocks at that lower level, so it makes sense that these deeper tap roots are finding different minerals and nutrients. Shallow roots, however, are getting more nutrients from the top layer of organic matter, whatever is available from rotting plant material on top of the soil, as well as from the biological ‘poop loop’ happening with the little creatures who live in the soil.

So yeah: Deeper roots bring up different nutrients. That part is true. But do you need a special plant, like comfrey, to act as this biodynamic accumulator? Is comfrey the only option for this? Most folks are using comfrey as a nutritive mulch; they grow the plant to bring up large amounts of nutrients which are then stored in the leaves (they claim - which again sounds the alarm - doesn’t a tap root store the nutrients in that large root, like a carrot?), and then they chop the leaves, lay them down as mulch, and those nutrients are then brought into the top layer of soil, for the fibrous roots to ‘mine’ for nutrients. At least, I think that’s what the gardeners who buy into the whole ‘accumulator theory’ are doing. Of course, all mulches eventually break down, whether it’s straw or wood chips or sawdust or grass clippings or comfrey leaves. And in that process, they feed the microbial life in the soil, which then poop out the nutrients in a form that is available for the plants (and is taken up by roots). The question then becomes: Do comfrey leaves provide more nutrients as a mulch than any other plant matter?

Well! Turns out there has been a recent study on exactly that question. The study used USDA’s ethnobotanical and phytochemical database to compile peer-reviewed nutrient concentration data across thousands of plant species. They set a threshold of 200% of average for a plant to be called a ‘dynamic accumulator.’ What they found is 340 plant species that showed nutrient concentrations high enough to qualify. This is impressive (plants are always so surprising, aren’t they, in so many wonderful ways?), but what’s very interesting is which, and how many different, nutrients the plants accumulate. The scientists compiled all the data into an online tool called Dynamic accumulator database and USDA Analysis. Here you can see which plants ‘brought up’ which nutrients and in what concentrations.

The next step in the study was to choose six promising species to trial for two years at a community farm. They chose dandelion, lambsquarters, red clover, redroot amaranth, Russian comfrey, and stinging nettle. They wanted to use these plants in different applications, such as liquid fertilizer and mulch production. Here are their key findings (this is a direct quote from the study):

- Plant tissue nutrient concentrations are tied to soil nutrient concentration. Dynamic accumulators are well-suited to extract specific nutrients from fertile soil, but they aren’t going to create nutrition that isn’t there. Therefore, dynamic accumulators should be regarded as one useful part of a larger nutrient management plan.

- That said, even when grown in poor, unamended soil, lambsquarters surpassed the dynamic accumulator threshold for potassium, and comfrey surpassed the threshold for both potassium and silicon.

- Previous studies have shown stinging nettle to accumulate calcium at concentrations above the thresholds. These new findings show that not only does it accumulate a lot of calcium, but it also has a high nutrient carryover rate, resulting in calcium-rich liquid fertilizer and mulches.

So there you go: Biodynamic accumulators are a real thing, and comfrey is certainly one of them. But, one must consider: Does the soil even need these particular nutrients? For instance, much of the Bay Area already has plenty of calcium in the soil. In this case, a dynamic accumulator like stinging nettle, which accumulates high levels of calcium, may not be necessary. Also, none of the six tested in these trials provided a large nitrogen benefit, but I suppose we already know what does provide that, and that is legumes.

I find the first finding the most important: This is only one part of a soil nutrient management plan. Adding plenty of organic matter to your soil, whether comfrey leaves or nettles or wood or straw or manure or chaff or whatever, will feed the microbiology in the soil which will in turn make it bioavailable for our plants. It will also form stable aggregates which will create long-term health. Living roots in the ground, a great diversity of them, will do the same. Eschewing pesticides of all kinds will also help the soil nutrient profile. Lots of soil cover will also prevent evaporation, leading to greater soil moisture and soil health.

Bottom line: Plant comfrey if you want to. It’s a beautiful plant and as long as you get the Bocking 14 variety, it will be well-behaved in your garden. Insects love it and it thrives as an understory plant for larger species. I myself have just planted 50 root cuttings under the trees in our orchard, if only to provide another living root in the ground, improving the soil profile.