Author: Linda Chalker-Scott

I’d intended to write the column earlier in the year, but it’s as relevant now as it was in the spring. This post will familiarize you with how unseasonable weather can affect your plants. Though I’ll be focusing on my own location in Tacoma, the phenomena are global. You just have to pay attention to what happened last week, last month, last year in your own location.

Our spring started out wet and cool, which is nothing new. But it was REALLY wet and REALLY cool compared to normal. This meant that our trees and shrubs had plenty of water to fill their expanding leaves and blossoms – but the lower than normal temperatures affected leaf growth. These dwarf Japanese maples had lots of leaves, but they were tiny! And they stayed that way, because once the leaves begin to lay down cuticle, they don’t expand any longer, even when it gets warmer. These maples put on a second flush of growth in the summer. Look at the difference in leaf size, determined solely by ambient temperature.

We had an abundance of flowers on our fruit trees – so dense that I put off pruning some of our heritage apple trees so we could get an even bigger crop (our black Angus love apples). But summer rolled around and…virtually no apples on ANY of the trees. What happened?

Well, that cool spring ensured that most of our pollinators were late to emerge from overwintering. I had wondered about them in the spring, as I could only see a few pollinating flies and no bees. But sure enough, we had almost zero pollination. No apples this year. Next year if the weather is similar I’m going to try using my battery-powered leaf blower to pollinate these trees. I’ll take pictures.

Fast forward to summer – for us, a record-breaking drought (again). Our temperatures weren’t as high as last year, but we still had very hot weather and no rain. For our landscape it’s not a problem, as we have well water for irrigation. But those gardeners who have little or no supplemental irrigation may very well find that their woody plants and perennials don’t perform very well next year: perhaps fewer flowers or branch dieback will appear. This is due to root dieback that happened all summer in unirrigated conditions. The damage is only seen in the following spring, when there aren’t enough roots to supply water to emerging buds.

Being able to predict the impact of specific weather events on your landscape plants is key to avoiding misdiagnosis and subsequent misuse pesticides or fertilizer in a futile attempt to rescue them.

Oh, and if you are wondering about the photo at the top, you’ll have to look at a post from 2009 to see what’s going on.

Irrigation bags, often called “tree gators,” are durable plastic bags used for irrigating newly planted trees. These projects have been discussed here and here and I still don’t like them as they don’t consistently benefit trees and often create conditions conducive to pests and disease. Plus, as the blog title suggests, their aesthetic attributes are nonexistent.

Newly installed trees and shrubs generally need to have supplemental water, period. It doesn’t matter if they are “drought tolerant” species – any plant needs sufficient water to establish roots. And where automated irrigation systems aren’t possible, there are many products that promise to deliver water to the establishing root system. Unfortunately, they often deliver other things as well, including pests, disease, and early death.

To be fair, many time these trees die because they were poorly planted: we know that improperly amended soils, structurally compromised root systems, inadequate root preparation, and/or poor installation are the leading causes of young tree failure. But anything that covers the trunks of young trees and reduces air flow and light exposure will, over time, create a dark, moist, and reduced oxygen environment that’s damaging to the bark of young trees. Wet, damaged bark allows opportunistic pests and pathogens to invade.

Until a few weeks ago, I had not seen any irrigation bags that I actually thought might work. These bags are installed on stakes away from the tree trunks, and they deliver water to the area where tree roots need to grow, enhancing root establishment. It took a trip to Malmö, Sweden to see this innovative approach and my immediate reaction was “why hasn’t anyone thought of this before?”

There are many types of irrigation bags, from sleeves to donuts, but none of them are as good for tree or soil health as a thick layer of arborist wood chips. When wood chips can’t be used for some reason, irrigation bags set well away from the tree and actually kept full of water might be a good solution.

A week or so ago one of my “friends” sent me a link to a new journal article that claims plants can “see.” (The use of quotes here indicates that plant vision is suspect, as is the friend status of the person who sent the article.) Of course, dissecting the claims in this article became an all-consuming task for the next several hours. And rather than writing off those hours as never to be reclaimed, I decided a blog post would at least set those thoughts down to save other skeptics the time.

The article can be found here; it reports on the ability of leaves to mimic other leaves. While the concept of leaf mimicry is not new and has been seen in agricultural weeds for decades, this article goes a step further in claiming that plants can actually see the leaves they are to meant to mimic.

But let’s back up a bit to explore leaf mimicry, which is a thing. Leaf mimicry serves to protect plants against herbivory and other types of removal (like weeding). This phenomenon was reported decades ago where agricultural weeds were shown to change their morphology to more closely resemble the desired crop. The benefit is obvious: if a weed looks like a crop plant, it is unlikely to be removed through hand weeding. Likewise, if a weed resembles a poisonous plant, herbivores that are visual learners will avoid these weeds. When some plants of a species are disproportionately allowed to survive (i.e., not eaten or removed), they reproduce better. Higher reproductive capacity means more offspring: this is the process of natural selection. We can even see this in dandelions in our lawns and gardens.

One astounding leaf mimic is Boquila trifoliolata (a tropical woody vine). This vine can be found on several host trees, where it mimics the leaves of each host and thus avoids herbivory (this short article by Gianoli and Carrasco-Urra is worth reading).

The article I’m currently dissecting doesn’t report on field observations of mimicry; instead, it looks at an indoor situation where B. trifoliolata is grown in the presence of artificial leaves. The authors claim that the leaves on the living vines began to take on the shape of plastic leaves on artificial vines located on a shelf above them. Despite Gianoli and Carrasco-Urra’s earlier speculations that horizontal gene transfer or volatile chemical signals might trigger the mimetic response, these authors propose that plants can see the artificial leaves and adjust their leaf morphology accordingly. They base this hypothesis on papers written over a century ago that suggest plants have ocelli (“little eyes”) as a way of sensing light. Of course, a century ago we were decades away from discovering pigments such as phytochrome and cryptochrome, both of which inform plants about light conditions in their environment.

There are a lot of problems with this paper; it would take me a separate blog post to critique the Materials and Methods section alone. But the biggest red flag for me was the following paragraph:

This reflects significant author bias: the experiment didn’t work in the winter, so they did it in the spring and summer to see if they got results they liked better. And apparently they did.

I’d like to propose a couple of different reasons that these leaves may have changed shape in the summer and not the winter:

1. Summer months are hotter and brighter than winter months. The experimental leaves were exposed to increasing heat and water loss compared to the shaded control leaves. Newly expanding leaf morphology changes in response to changing environmental conditions.
2. Under increasingly hot temperatures, plastic releases volatile chemicals, many of which are toxic. Leaf morphology has been demonstrated to change in response to air pollutants.

This is a deeply flawed article based on a poorly designed experiment and reflects significant author bias in the interpretation of the results.

And just for more cowbell, here is Christopher Walken’s take on plant ocelli.

“In the Spring a gardener’s fancy lightly turns to thoughts of…plant shopping!”

If Alfred, Lord Tennyson had been an avid gardener, I am sure he would have included the above line in his poem “Locksley Hall.” I certainly look forward to visiting nurseries and plant centers in the spring to see what new goodies await. But my enthusiasm is tempered with caution – because bad things can lurk in otherwise perfect plants. I posted a four-part series way back in 2009 (the first year of our blog) on inspecting nursery plants.

I strongly recommend you review these posts before you buy – they are 13 years old but the information is still 100% valid.

Part 1: inspecting the root flare and trunk.

Part 2: inspecting the roots.

Part 3: avoiding suckers.

Part 4: avoiding poorly pruned young trees.

Today’s post will add some new nursery nightmares to avoid at all costs.

Free complementary gift!

Make sure you’re buying a cultivar and not a nutrient deficiency

There are lots of interesting cultivars out there with unusual foliage. This dogwood is not one of them. Interveinal chlororis is a symptom of foliar nutrient deficiency – either iron or manganese – most likely caused by excessive phosphate fertilizer.

Fusion

Fusion can be innovative in music and cuisine. Not so much in plants.

You can’t say they didn’t warn you

Back to nature

The scion of grafted plants is rarely as vigorous as the rootstock. Usually you have to wait a few years for the rootstock to take over, but there’s no waiting with these weeping silver birch specimens! But given how hideously trained these trees are, maybe it’s better that they will be slowly subsumed.

Just don’t do it. Please.

Four years ago we moved to the family farm (where I grew up) and we’ve enjoyed restoring the 1 acre landscape around the farmhouse. Given that the residential part of this farm is surrounded by pastureland, there is a continual influx of weed seeds into our managed beds. While our thick applications of arborist wood chips have kept out many weeds, they still pop up where mulch hasn’t been applied yet or is too thin.

One of these weeds is Hypericum perforatum (also known as Klamath weed or St. John’s wort), a species native to Eurasia. The latter common name can confuse gardeners, as there are several ornamental species of Hypericum also called St. John’s wort, but H. perforatum is easily identified by the perforations in the leaf. This invasive species is a problem for our cattle, as Klamath weed causes photosensitivity when it’s consumed and can be toxic in large amounts.

In the last few years H. perforatum colonized our stockpile of native soil waiting to be used in our raised beds. It was a small enough infestation that we could pull it all up, but a closer look revealed that some shiny metallic beetles were already busy feasting on the leaves. Putting on my IPM hat, I first needed to identify these interesting beetles. It didn’t take long to find out they were a Chrysolina species.

Chrysolina hyperici and C. quadrigemina (or St. John’s wort beetles) are also native to Eurasia and are specialist feeders – they only feed on Hypericum species. They were imported as biological control agents several decades ago and have been effective in controlling dense populations of St. John’s wort. C. quadrigemina in particular has been reported to feed on both ornamental and native species of Hypericum but not to the extent of causing significant damage.

Both species of the St. John’s wort beetle feed on the leaves, where they also lay thousands of eggs. The larvae that emerge from the eggs are voracious feeders and can defoliate dense stands of St. John’s wort. Like other animals that eat Hypericum perforatum, the larvae become photosensitive and generally feed before sunrise to avoid damage.

Since biological control agents depend on the presence of their host, it’s important to retain a small population of the host. And because this particular beetle is a leaf feeder, one can remove the flowers of the plants to reduce reproduction, but maintain the plants to support the beetle.

Many other introduced, invasive weeds can be controlled using carefully researched microbes and insects. Some of these biocontrol agents may already be found in your area – so it’s important to avoid using insecticides and fungicides, in particular, to conserve these garden assets.