The Garden Professors™

Another good reason to mulch

Posted by Bert Cregg

Researchers often get accused of concluding the obvious. At some point we’ve all scoffed at headlines like, “Study finds cell phones and driving don’t mix” or “Researchers discover high heels make your feet hurt.”* But even when a study demonstrates something we already know, sometimes there is still value in being able to put hard numbers on the scope of the problem – and hopefully spur some action.

A case in point is a recent study by Justin Morgenroth, Bernardo Santos, and Brad Cadwallader at the New Zealand School of Forestry, “Conflicts between landscape trees and lawn maintenance equipment – The first look at an urban epidemic” Urban Forestry and Urban Greening 14:1054-1058. Morgenroth and his colleagues surveyed over 1,000 trees in public greenspaces (parks, cemeteries, campuses) in and around Christchurch, New Zealand (pop. 375,000) to assess the amount of damage to trees by lawn equipment. Their conclusion: Lawn equipment is hell on trees. This conclusion, of course, surprises absolutely no one that has ever looked at trees near turf in a public place on this planet.

Morgenroth et al. claim their survey is the first systematic look at this issue and their data are staggering. Nearly two-thirds (63%) of the trees they surveyed had at least one wound. The proportion of trees with wounds was fairly constant regardless of type of planting (i.e., park, campus, cemetery, roadside verge), though trees in parks and campuses tended to have more wounds per tree than trees in nature reserves or roadside verges.

Not all the news was bad, however. Morgenroth et al. found that grass cut-outs or mulching around trees significantly reduced the number of wounds per tree.

So, not a conclusion that should take anyone by surprise, but some sobering data to put some scale on the size of the problem.

*actual conclusions from real studies

Easy Overwintering

I love growing tender plants as annuals over the summer. But I don’t like buying them again every year, so I try to overwinter as many as I can indoors once frost threatens. However, I have pretty limited windowsill space, so I can’t keep many plants in active growth all winter. Luckily, I’ve found a simple hack that works for a surprising number of plants.

The above begonia is on a high dark shelf. It will sit there all winter, getting essentially no light, and I won’t water it. All those leaves will drop off, leaving nothing but dead looking stems. But come spring, when I put it back outside and water it again, new leaves will start growing and it will come right back.

Quite a lot of tender plants can do this. Just keep them dry, preferably on the cool side (unheated basements are perfect), and they’ll go dormant, usually dropping their leaves, and wait patiently for spring. I personally have done this with both cane begonias (as pictured) and the rhizomatous rex begonias, pelargoniums (the annual “geraniums”), and lots of succulent plants like agaves and cacti. I’ve seen first-hand other people using the same method with great success with brugmansia, bananas, and tender shrubby hibiscus. It seems like it is works most often with plants with thick, woody or succulent stems, but I keep trying it with new things and being surprised when they come through just fine. So if you’ve got some cool tender plant you’d love to over winter, but no window space left, shove it in the basement and see what happens. If it comes back fine in the spring, please comment on this post so the rest of us can learn from your experience!

— Joseph Tychonievich

Academic freedom vs. science-based advice

Those of you that have followed The Garden Professors for some time know that Jeff Gillman and I are relentless in our pursuit of gardening myths to explode. Social media – Facebook in particular – seems to be a natural breeding ground for dumb and/or dangerous home remedies that go viral. Most of these have no basis in actual science and are easy to dismiss. Other recommendations may have some science behind them, but a careful review of the literature often shows that the bulk of research does not support that particular practice or product. These ones are trickier to deal with, and nothing has been trickier for either me or Jeff than compost tea.

Nurseries often carry compost tea products (this one is now defunct)

The two of us have posted extensively on this topic in the last six years: just use the search function over in the left hand column of this blog and type in “compost tea”. You’ll find enough reading to keep you busy for a while. I summarized the state of the literature a few years ago in the now-defunct MasterGardener Magazine and to be honest the accumulated literature hasn’t changed much in terms of generating solid science supporting compost tea use. But its popularity seems to be increasing among landscape professionals and gardeners alike.

I get a lot of questions on compost tea from Master Gardeners in particular, who are bound by their positions as university volunteers to use science-based information. One of their major resources is the state university associated with their program – and recently this has become a problem for WSU Master Gardeners. Because on the Washington State University website you can find one professor who cites the lack of credible, consistent science on compost tea usage and another professor who provides workshops and webinars on making and using compost tea. Master Gardeners are understandably confused about what they can recommend and irritated that their university provides conflicting information. Why, they ask, does the university allow this to happen?

The answer is found in one of the most important values that universities protect: the academic freedom for faculty to speak their minds. Ideally this means that faculty can speak up about topics that are unpopular with university administrators without fear of reprisal, but it also means faculty have a soapbox on pretty much any topic they wish. And that’s whether or not they have any expertise or credibility on that topic. (For a particularly egregious example, one needs look no farther than prestigious MIT who has a research scientist with no expertise in biology or chemistry but who publishes articles in marginal journals linking glyphosate – the active ingredient in Roundup – to just about every known human malady.) Universities tend not step into this fray as it is a slippery slope – who decides what faculty speech should be censured and which should not?

How can Master Gardeners and others decide what information to believe? Well, that’s actually the mission of this blog and our Facebook page and group – to provide the best current gardening science and to help the public increase their scientific literacy skills. Science is not immutable – it advances as credible, published evidence accumulates. When and if compost tea ever becomes a consistent, effective product, we will be the first ones to share that information.

Add one species, get four new ones

Here’s an interesting twist on the whole native, non-native discussion… sometimes the introduction of new species of plants can trigger the evolution of new species of insects! Sometimes, in fact, a whole bunch of them, as is described in the coolest new research paper I’ve read in ages (Actual paper, behind a pay wall) (A brief Summary)

Introduce apples, trigger the evolution of four new species of insect

Basically, there is a fruit fly, Rhagoletis pomonella, native to Eastern North America that lays its eggs on the ripe fruits of native hawthorns. It is part of a whole group of species of flies that each go after a different kind of fruit – blueberries, snowberries and dogwoods each have their own species of closely related fly. When Europeans arrived and introduced non-native apple trees, the hawthorn fly started laying eggs on the apples as well, and got the name of apple maggot. But here’s the crazy bit: The hawthorn flies didn’t just expand their diet, they actually evolved to a new race, a new species in the making, that live exclusively on apples.

These flies have very brief life spans, so the adults must emerge at exactly the right time or there won’t be ripe fruit to lay their eggs on. But apples and hawthorns ripen nearly a month apart, so the apple targeting flies have evolved to emerge several weeks earlier than the original hawthorn flies. In addition to diverging in time of emergence, the two types of flies have changed their preferences in smells. The original fly is attracted to the smell of hawthorns, and avoids the smell of apples, while the new flies show the exact opposite behavior, each homing in on their target host, be it new or old.

The final piece of these two types of flies becoming two different species is that they each now mate only on the fruit of their tree of choice. This is important, because now the apple and hawthorn flies don’t interbreed due to their preference of mating location, and being a reproductively isolated group is the most commonly accepted definition of a species. Now the two types of flies will continue to diverge, as the lack of interbreeding means more and more genetic differences between the two populations will build up over time.
All of this is very cool, and has been long understood. Here’s the EVEN COOLER part from this new research: The divergence of one kind of fruit fly into two is cascading through the ecosystem. There are three species of parasitioid wasps that lay their eggs on the hawthorn fruit fly that have diverge into new forms that specialize in the new apple fruit fly. Just like the fruit flies, the timing of their life cycle, their preference and avoidance of the smell of the ripe fruit, and their mating habits have shifted to create different apple and hawthorn specific races. So where there was one fruit fly and three wasps, the introduction of the European apple has lead to the evolution of one additional fruit fly, and three new wasps.

I’m not sure what import this has, if any, in the ever raging native-versus-exotic debate in horticulture, but it sure is cool – the evolution of new species happening right before our eyes.

Joseph Tychonievich

Just post some pretty pictures or something

Posted by Bert Cregg

A little over six years ago Jeff Gilman called me out of the blue and asked me to be part of a new blog about the science of horticulture that he was embarking on along with Linda Chalker-Scott. I was reluctant – I didn’t know much about social media at the time and was plenty busy already. Jeff explained that he was recruiting others and how we would rotate posts. I asked Jeff, “What if I don’t have anything to write about?” Jeff replied, “Just post some pretty pictures or something.” So with that long ago conversation as backdrop, here are a few photos from this week’s trip to Hidden Lake Gardens near Tipton, Michigan.

Entrance to the Harper Collection of Dwarf and Unusual Conifers

Taxodium distichum “Pendens’ cones

Pinus contorta "Chief joseph'. With apologies to Joseph Tychonievich... — Pinus contorta “Chief joseph’. With apologies to Joseph Tychonievich…

Sassafras albidum fall color

The long and winding road... — The long and winding road…

Sciadopitys verticillata 'Joe Kozy' — Sciadopitys verticillata ‘Joe Kozy’

Abies koreana 'Icebreaker' — Abies koreana ‘Icebreaker’

Taxodium distichum 'Pendens' — Taxodium distichum ‘Pendens’

Moss magic

In my opinion, no coastal Pacific NW garden is complete without moss softening the edges of a rock garden or nestling between paving stones. Now that the rains have returned, mosses are lush green sponges, absorbing sound as well as water. They are the finishing touches to our native landscapes.

A few months ago, however, mosses looked quite different. With our particularly hot and droughty summer, mosses were brown, dry and brittle just like our lawns. But unlike those dead blades of grass, the mosses were only in a state of environmental dormancy. All it took to revive them was water.

Here’s a patch of moss in our home landscape during a hot dry spell. It’s dry and brown:

Here’s the same patch of moss 20 minutes after I watered it:

It's a garden miracle! — It’s a garden miracle!

How can mosses recover so quickly? Well, mosses are one of the most primitive groups of land plants still in existence. They lack a true vascular system, so their “roots” are only anchoring structures – they don’t absorb water. Instead, water and nutrients are taken up over the leaf surface. As soon as water hits the leaves, it’s absorbed and literally throws the switch to turn everything back on. Leaves expand, chloroplasts start to absorb sunlight, and the photosynthetic machine is humming along.

In fact, my undergraduate major advisor was a bryologist (one who studies mosses). Jack Lyford’s lab was stacked ceiling-high with shoe boxes. Each box contained a different species of moss – completely dried out of course. All he had to do was take out a piece and place it in a dish of water. Within minutes it was fully functional and ready for study.

So make room for some moss in your garden. It’s a tough and fascinating little survivor.

A Resilient Citrus Tree Rebounds

Cit3Spring1

Sad Citrus

The last two winters have been pretty brutal on my citrus trees. Their winter home is the enclosed, but unheated, south facing entrance foyer. Usually, this is a perfect spot. Sunny, and with temperatures usually in 45-60 degree range. But when the polar vortex brought record cold to the Mid Atlantic region back in February, they were hit hard, and I had my doubts that this 13 year old specimen would survive.

Happy Citrus

But it bounced back pretty well, after a season in the sun, so I figured it should be rewarded … I’d give it a new home, replacing its split container … and document the process here.

Prep Area

Drainage Holes Drilled

Whew! No Pebbles in the Bottom!

Rescued Parsley

Uh Oh, The Dreaded Circling Root.

Snip Snip

Wood Chip Mulch, of Course

Voila! Ready to Move Inside

Native vs Exotic: Not as simple as it seems

Lots of discussion recently over on the Facebook side regarding the recent publication in Ecological Letters by Karin Burghardt and Douglas Tallamy, “Not all non-natives are equally unequal: reductions in herbivore β-diversity depend on phylogenetic similarity to native plant community.” http://onlinelibrary.wiley.com/doi/10.1111/ele.12492/full
While there are certainly some things to nitpick in the paper (see Linda’s comments on the Facebook discussion), I think this paper may go a long ways re-shape, and possibly even begin to end, the debate over native versus exotic.

How was the study conducted. In 2006 Tallamy’s group established a series of test landscape plantings. Each planting fell into one of four groups: non-native congeners (species that are not native but have native relatives in the same genus); non-native non-congeners (plants from non-native genera), native congeners and native non-congeners. In 2008, when the trees were about 6’ (1.8 m) tall, they conducted a census to identify and quantify the adult and immature insect herbivores they collected. They analyzed the data to determine the amount of insect herbivore diversity within each planting type. Specifically they looked at what ecologists refer to as beta-diversity, the amount of species diversity among sites. If you’re interested and want to learn more check out https://methodsblog.wordpress.com/2015/05/27/beta_diversity/

So, what did they find? Like every good study on host-insect interactions; the answer is, “It depends.” (BTW, if you’re following along at home the key figure in the paper is Fig. 3). When Burghardt and Tallamy looked at the differences in diversity between adult herbivores on native and non-native congeners, they found no difference. When they looked at differences in diversity between immature herbivores on native and non-native congeners, they found no difference. When they looked at differences in diversity between adult herbivores on native and non-native non-congeners, they found no difference. When they looked at immature herbivores on native and non-native non-congeners, they found a small but statistically significant difference, with higher total diversity for native non-congener.

As an aside, it is also instructive to look the version of Figure 3 presented in the article’s supplemental materials, which has been re-scaled to include zero. Including zero on the scale helps to give a better perspective on the actual variation among means. It’s a little like the “Truth in lending statement” that comes at the end of your credit card bill.

So, a possible alternative title for the paper could be, “Do native or non-native plants increase herbivore diversity? Most of the time it doesn’t matter.” That said, I think this paper makes a number of contributions and will start to shift native versus non-native debate, and perhaps even signal the beginning of the end. First of all it demonstrates that that non-native species of native genera contribute equally to herbivore diversity. However, I think some of the most insightful information in the paper is buried between the lines and in the supplemental information attached to the online version of the paper. The authors briefly mention that they also looked at guilds (i.e., chewing insects, sap feeding insect, xylem feeders, etc.). Once again the answer of whether natives contribute more to species diversity is, “It depends.” For xylem feeders, for example, diversity was the same for congeners and non-congeners.

To me, this is the level of resolution we need to work to gain a true handle on the situation. I’m not an entomologist and I’ve never played one on TV but I’ve been around these questions long enough to know that different types of insects are attracted to or repelled by different plants by different mechanisms. In one case it’s an attraction pheromone, in another it’s a defense chemical, sometimes it’s leaf toughness or a tree is able to produce enough resin to drown boring insects. An old axiom states that ecosystems are not more complicate than we think; ecosystems are more complicated than we can think. As this paper demonstrates, to think that all the complex interactions between plants and insects can be boiled down to something as simple as native or exotic is hopelessly naïve.

Cactus grafting fun

I’ve been grafting cactus this summer, and made this:

It is a seedling of the gorgeous hardy cactus Echinocereus reichenbachii, grafted onto Pereskiopsis spathulata, an odd, leafy cactus I wrote about earlier.

Why do this? Other than the fact that it is darn cool? Well, because that vigorous, fast growing rootstock pumps a lot of energy into the cactus grafted on top, making the grafted cactus grow a LOT faster than left on its own roots.

grafted cactus start This is a (terrible, blurry) picture what the graft looked like when I first made it back in July. Just three months later it has grown to enormously, while the seedlings I left on their own roots look pretty much the same. I’ll let it grow on the graft for a while, then probably next year some time, cut it off, and move it into the garden, getting me to a reasonably sized plant in a reasonable amount of time.

So… if you want to speed up the growth of a pokey cactus, try grafting it. The process is crazy easy, lots of fun, and very thoroughly explained here.

Pretty in Pink

It’s October. Fall is such an underrated time in the garden, and much pink can be found. In fact, flashes of pink are everywhere!! Got my ma’ams grammed last week; thanks for the reminder, NFL.

Muhlenbergia ‘Pink Flamingo’.
Aye yi yi. Alleged hybrid between M. capillaris and M. lindheimeri. Five feet tall and as wide, huge plumes of pink. Looks like nothing important the rest of the year, then, blammo!!! Sorry, folks north of Zone 6. Actually, it only works here (Z. 6a) because of outstanding drainage; it’s planted in a pile of gravel. Mine has lived through two winters with -20 F days. Place where the sun will rise or set behind it for maximum effect. Bunny the Whippet not included.

Salvia involucrata – Rosebud Salvia
Big ol’ gal that will not favor you with blossoms until September. Absolutely not hardy here, or anywhere north of Zone 8. Take cuttings, ’cause baby she’s worth it. The furry, hot pink flowers will thrill any hummingbirds left zipping around (I read ours the riot act this weekend, they have GOT to hit the road soon). Note there is some hullabaloo as to S. puberula vs. S. involucrata vs. some hybrid amongst the two. Will report back.

Chrysanthemum x whatever ‘Venus’ .
Am so tired of the taxonomic uncertainty. Chrysanthemum…Dendranthemum… Whatever you call her, ‘Venus’ is a wonderful “real” garden mum (not those heinous meatball things) that brings the pink blooms in September, then fades to palest of pink, but not before every bee in the neighborhood visits. Fairly compact (2-3’) and pretty darn hardy (Zone 5). Tuck Venus amongst things you know will be done before fall – bee balm, phlox, etc. to keep the show going!

So there you have it, some pink for our October gardens. In loving memory of my sister Carlene.