The Garden Professors™

Allelopathy Helps Black Walnuts Compete

A walk through the woods can be one of the most peaceful and calming experiences — a place where you can find quiet for reflection and marvel at the beauty of nature. Little do most people know that some plants, especially one specific tree, wage chemical warfare against other plants to keep away potential neighbors that would compete for nutrients and sunlight. In the Appalachian Mountains, the tree most skilled at chemical warfare is the black walnut.

The black walnut tree (Juglans nigra) is a useful, yet often misunderstood tree. Prized for its excellent wood qualities for lumber and furniture, the nuts it produces are either loved or reviled by those who try them.

The flavor of black walnuts is hard to describe. I would say that they have an almost astringent flavor, mainly due to the high level of tannins in them. They aren’t my favorite, but I don’t mind them either. I’ve learned to accept them, unlike during my childhood when you knew which church lady’s cake to avoid at the potluck because you knew that she put black walnuts in everything she baked.

My appreciation for black walnuts grew the year that I was the official nut judge (no joke) for the Black Walnut Festival in Spencer, WV. It was quite an experience — examining and weighing all the entries with a team of high school FFA students who cracked more than a few inappropriate jokes about the situation.

You could tell when someone was picking or cracking black walnuts, thanks to the tannin stains on their hands that just wouldn’t wash off. Black walnuts are a tough nut to crack (literally), so I also remember my grandmother cracking them “the easy way.” She would just pile them up in the driveway and run over them a time or two with her behemoth of an Oldsmobile (you know, the one that had full seats front and back and could hold half the neighborhood).

Black walnut trees have the interesting ability to excrete a chemical called juglone, which makes it nearly impossible for a number of plants to grow anywhere in its root zone. Juglone works by damaging the tiny root hairs on roots that are responsible for taking up a great majority of the water and nutrients the plants use. Research shows that it also interferes with the interaction of the roots with mycorrhizal fungi that aid the plant in taking up nutrients.

This process is not just specific to black walnuts. There are several other plants that do this. The phenomenon, called allelopathy, occurs when an organism excretes something that inhibits the growth of other things around it. You could equate it to the Penicillium fungus excreting a chemical that kills bacteria around it. We harness that chemical to use as penicillin.

Some plants are especially sensitive to the chemical. Many vegetable plants, especially tomatoes, are sensitive. Some plants, mainly those that would grow wild in the woods, are not susceptible. Many grasses also have a hard time growing beneath black walnut trees (tall fescue and Kentucky bluegrass being the exception, except during periods of drought).

Publication with lists of plants tolerant and damaged by juglone

All parts of the tree produce the juglone chemical, so the effects could spread beyond the perimeter of the tree from fallen leaves and branches. I would also suggest that you make sure any fresh woodchip mulch that you use (specifically that from local tree cutters) is free of black walnut. The juglone may break down after composting the wood chips for six months to a year, but I would still be cautious about its use. The wood will release the chemical, killing susceptible plants for a few years in the area where it is applied. Studies suggest that juglone will break down during the composting process, but I would check to make sure by starting a few tomato seeds on the batch of compost to see what happens.

—Garden Professor John Porter is a county extension agent for West Virginia University and writes the weekly Sunday garden column for the Charleston Gazette-Mail Newspaper. This article was originally published October 2, 2015.

You can find John’s writing at wvgardenguru.com and on Facebook and Twitter.

Top Ten Trees and Shrubs with Great Fall Color

Sugar maple (Acer saccharum) in full fall color — Sugar maple (*Acer saccharum*) in full fall color

As promised in my Sept. 9 post of “The Science Behind Fall Color”, I would address trees and shrubs with outstanding fall color. It was hard limiting it to only ten trees and ten shrubs, since I found 5 common shrub species of maples alone, so I cheated a bit and grouped the maples, oaks, etc. into one group so that my list was not entirely all maples.

'Robin Hill' apple serviceberry (Amelanchier x grandiflora 'Robin Hill') — ‘Robin Hill’ apple serviceberry (*Amelanchier* x *grandiflora* ‘Robin Hill’)

I have seen the below plants with reliable fall color in northern, southern and eastern landscapes. These plants “light” up the landscape in autumn. For outstanding, long lasting autumn color, plant the below trees and shrubs with herbaceous plants which bloom in fall such as asters, mums, sedums, monkshood, toad lilies, and Japanese anemones. Do not forget ornamental grasses with their showy seed heads extending the season of color and texture.

Sweet birch, cherry birch (Betula lenta) — Sweet birch, cherry birch (*Betula lenta*)

We used to recommend ash for fall color, but not any more due to emerald ash borer. Japanese barberry and burningbush are tops for fall color, but both species are highly invasive and not recommended. There are more plants with great fall color than the ones below. I would love to hear your favorites!

Top 10 Trees for Fall Color

1) Black gum, sour gum, tupelo (Nyssa sylvatica), orange-red, scarlet to purple, outstanding

2) Maples, especially:

Sugar maple (Acer saccharum), bright yellow to orange-red

Red maple (A. rubrum), yellow, orange-red to bright red

Freeman maple (A. × freemanii), yellow, orange-red, red to reddish-purple

Paperbark maple (A. griseum), dark red to bronze

Japanese maple (A. palmatum), orange, red to purplish-red

Korean maple (A. pseudosieboldianum), deep orange to reddish-purple

Three-flower maple (A. triflorum), orange

Full moon maple (A. japonicum), yellow-orange to scarlet-red

Moosewood, striped-bark maple (A. pensylvanicum), bright yellow

3) Ginkgo (Ginkgo biloba), bright golden-yellow

4) Thornless honeylocust (Gleditsia triacanthos f. inermis), bright golden-yellow

5) Quaking aspen, trembling aspen (Populus tremuloides), bright yellow

6) Oaks, especially:

White oak (Quercus alba), dark red to wine

Red oak (Q. rubra), red to russet

Scarlet oak (Q. coccinea), red to scarlet

Black oak (Q. velutina), dark red

Scarlet oak (Quercus coccinea) — Scarlet oak (*Quercus coccinea*)

7) Apple serviceberry (Amelanchier × grandiflora), yellowish-orange to red

8) Buckeyes, especially:

Yellow buckeye (Aesculus flava), golden-yellow to orange

‘Autumn Splendor’ buckeye (A. × arnoldiana ‘Autumn Splendor’), deep, burgundy-red

‘Homestead’ buckeye (A. × marylandica ‘Homestead’), orange-red

9) Birch, especially:

Yellow birch (Betula alleghaniensis), bright yellow

Sweet birch, cherry birch (B. lenta), bright yellow

Paper birch, canoe birch (B. papyrifera), yellow

10) Yellowwood (Cladrastis kentukea), bright yellow to gold

Top 10 Shrubs for Fall Color

1) Large and dwarf fothergilla (Fothergilla major and F. gardenia), yellow-orange to red, outstanding

Large fothergilla (Fothergilla major) — Large fothergilla (*Fothergilla major*)

2) Common and vernal witchhazels (Hamamelis virginiana and H. vernalis), bright yellow to golden-yellow

Vernal witchhazel (Hamamelis vernalis) — Vernal witchhazel (*Hamamelis vernalis*)

3) Virginia sweetspire (Itea virginica), dark reddish-purple

4) Black and red chokeberries (Aronia melanocarpa and A. arbutifolia), red-orange, wine-red to purple

Black chokeberry (Aronia melanocarpa) — Black chokeberry (*Aronia melanocarpa*)

5) Sumacs, especially:

Shining sumac, winged sumac (Rhus copallinum), bright red to scarlet

Prairie Flame® shining sumac (Rhus copallinum 'Morton') — Prairie Flame® shining sumac (*Rhus copallinum* ‘Morton’)

Staghorn sumac (R. typhina), orange to scarlet

Smooth sumac (R. glabra), orange to scarlet-purple

Fragrant sumac (R. aromatica), orange, red to purple

6) Oakleaf hydrangea (Hydrangea quercifolia), reddish-orange to wine

7) ‘Tor’ birchleaf spirea (Spiraea betulifolia ‘Tor’), orange to reddish-purple

8) Viburnums, especially:

Withe-rod viburnum (Viburnum cassinoides), orange-red, crimson to purple

Blackhaw viburnum (V. prunifolium), reddish-purple

Arrowwood viburnum (V. dentatum), depends on cultivar, yellow, red to purple

American cranberrybush viburnum (V. opulus var. americanum, formerly V. trilobum), yellow to reddish-purple

Doublefile viburnum (V. plicatum f. tomentosum), wine-red

‘Wavecrest’ Siebold viburnum (V. sieboldii ‘Wavecrest’), red to burgundy

9) Cranberry cotoneaster (Cotoneaster apiculatus), deep reddish-purple

10) Virginia rose (Rosa virginiana), deep reddish-purple

Laura Jull, Ph.D.

a.k.a.: The “Lorax”

“Lazy” corn and gravitropism

Inspired by Linda’s post about thigmomorphogenesis, I decided today I would add the word gravitropism to your vocabulary. It simply means growth in response to gravity. Shoots of plants grow up, because they are negatively gravitropic, they grow against the pull of gravity, while roots are positively gravitropic and grow down towards the pull of gravity.

And why is that so important? Well… this is what happens when gravitropism is missing.

To the left is normal old corn. The plant to the right was not sat on by a raccoon or anything, it simply has a mutation in a gene called lazy plant1. I’m not kidding. That’s the official, scientific name for this gene. Geneticists have fun with their names, though fruit fly geneticists are for sure the kings of silly gene names. This gene got that name because, as you can see, without a functioning copy of that gene, the corn plant no longer can detect the pull of gravity and so flops down in a “lazy” manner.

This corn is just odd, of course, with no real value (though it was fun to grow) but similar mutations are what give us some of the “weeping” or trailing forms of popular ornamental trees and shrubs.

Your new word for the day: thigmomorphogenesis

I just finished reviewing 4 manuscripts for three different journals and boy is my brain fried. My private reactions ranged from “I can’t wait until this one is published!” to “If I were to use sheet mulch this manuscript would be my first choice.” Anyway, it was the latter manuscript that got me to thinking about what can go wrong with experimental design, which brings up today’s word: thigmomorphogenesis.

This is a great word for those who enjoy figuring out word meanings by deciphering the (usually) Greek or Latin roots. (This exercise also helps you figure out how to pronounce it.) We have “thigmo-” which means touch, “-morpho-” which means appearance, and “-genesis” which means beginning. String them all together and you get the phenomenon seen when plants respond to mechanical stimulation by changing their growth pattern and hence the way they look.

Wind direction from the right creates an asymmetric hedge.

You can easily see examples of thigmomorphogenesis in everyday life. Look at a line of hedge plants where the plants on the end are more susceptible to wind movement and brushing by people, animals or vehicles. They are always shorter, aren’t they? Plants subjected to chronic thigmomorphogenic forces are generally shorter than their neighbors and thicker in girth. (For a longer discussion about how thigmorphogenesis works, you can read my online column.)

How does all of this relate to experimental design? Well, think about what happens if you are testing a product that requires applying it to the leaves of plants once a week. Your treatment plants are touched every week. How can you know that any changes in your experimental plants aren’t due to being touched? The way you eliminate this source of variability is by treating all of the plants the same way. When you are applying the product to the treatment leaves, you apply water (or whatever the solvent is for the product in question) to the control leaves. That way thigmomorphogenesis remains just an interesting tongue-twister and not a fatal design flaw in an experiment.

Mitchella repens … Partridge Berry … an Evergreen Native Groundcover for Shade

Partridge Berry in its Natural Setting

One of the questions that came up regularly when I was working the hotline at the local county Extension office, is a recommendation for an evergreen ground cover for shady spots. I had the same issue when I created my own shade garden … something that would have year round interest, but complement my desire to emphasize native species, although that was only one consideration.

The solution was literally right next to me, as a walk in my woods revealed with the lovely plant Partridge Berry, or Mitchella repens.

Not only is Partridge Berry (Mitchella repens ) beautiful, evergreen, shade-loving, and native to Eastern North America, but there’s also a fascinating aspect about its flowers and fruit, from a botanical, and evolutionary point of view.

According to the U.S. Forest Service Celebrating Wildflowers website, the “… genus name Mitchella was given to this plant by Linnaeus for his friend John Mitchell, a physician who developed a method of treating yellow fever. The species name repens refers to its trailing or creeping habit.”

Here’s the part I found fascinating: The plant is dimorphous, meaning “occurring in two forms”:

In late spring, two beautiful white flowers (with one calyx) each open their four petals to entice insects to collect their nectar. Each blossom has one pistil and four stamens. The pistil in one is short and the stamens are long. In the other it is just the opposite. … Because of this no flower can fertilize itself–all flowers must be cross-pollinated by insects, and both flowers must be pollinated to get a single healthy berry. A berry will stay on the vine until after the blooms appear in the spring unless a hungry bird finds it nestled among the fallen winter leaves.

How cool is that? The twin flowers produce, together, only one berry.

Here’s a closeup, where you can see residual evidence of the fusion. The berry is edible, and persists through the winter, assuming it is not consumed by “ruffed grouse, northern bobwhite, sharp-tailed grouse, and prairie chicken.

The fruit is also “frequently eaten by raccoons and red fox” and it has been reported that “partridgeberry made up 2.9 to 3.4 percent (dry weight) of the summer and fall diets of white-tailed deer.”

Here’s a picture of the two flowers in bloom.

It’s easiest to spot the plant in its natural setting while hiking in late Fall, or early Winter before snowfall, or early Spring after snowmelt.

Back to the Forest Service article:

Some gardeners consider Partridge Berry a must for winter gardens. During the cold days of late winter Partridge Berry is a treat to the eyes with its deep, dark-green leaves and occasional scarlet berries. In a garden setting this evergreen prefers shade, accepting the morning sun. Partridge Berry is extremely difficult to propagate from seed.

The best way to introduce this native into your garden is through 1 year old cuttings or by division. In the garden situation they will form a thick, substantial ground cover. Once established they are relatively trouble free with the only required maintenance of keeping garden debris from covering the mats.

As always, do not wild collect plants from public lands and only from private lands when the landowner grants permission. Partridge Berry is a commonly available plant from native plant nurseries especially those who specialize in woodland plants.

I love the symmetrical variegation in the evergreen leaves, a bright, light yellow line bisecting each leaf, and the delicate, less visible veins.

It’s a great alternative to Vinca, an introduced species from Europe that appears on invasive species lists in our area.

A Google search will reveal many potential on-line sources for buying Partridge Berry plants, or check with a local nursery, or independent gardening center in the native plant section.

Bad power line pruning

Driving home today, I saw this lovely sight:

Our local utility company has been busy butchering trees around the power lines.

Every plant person I know complains about this, but I honestly don’t think there is much hope for a change. Power companies don’t want limbs falling on the electric cables during storms, and they’re not likely to start spending money to hire real arborists to do the pruning.

What I really wish is that people would start thinking a little more before putting in a tall tree directly under electric lines. I’m sure these went in as cute little babies, I know it can be hard to visualize what a small tree will grow into, but we do really need to do a better job of it. If you are looking to plant, take the time to look up the tree in question and see how fast it is going to grow. Google will usually tell you, and if you are planing conifers, the American Conifer Society has an amazing website which will tell you the growth rates in inches per year of just about any conifer you can imagine. Check it out, and do the math, and see just how fast that little spruce is going to be causing problems before you start digging holes.

Fall is for Planting…Ornamental Grasses! But so is Spring…

I’ve written previously of my adoration for ornamental grasses. A few of you folks in the mid-Atlantic might have heard my “Grasses for the Masses!” presentation complete with lots of arm-waving. As with most of my talks, there’s usually some sort of interpretive dance involved.

Most of our warm-season ornamental grasses are in full gloriousness at the moment. Because it’s autumn! ‘Tis the season to purchase, plant, and enjoy ornamental grasses!

Pennisetum 'Karley Rose' - available at a garden center near you! — Pennisetum ‘Karley Rose’ – available at a garden center near you!

Well, not really. If you’d have purchased and planted them in April or May, you’d only have to do the “enjoy” part now. And your local grower/garden center would LOVE you for it. But most gardeners overlook containers full of 6″ tall Fescue – which is what a LOT of our best grasses resemble in the spring. It’s always been challenging to sell “green” in the spring – consumers want to see and buy plants in flower – so nurseries and greenhouses that supply garden centers do their darnedest to provide said blossoms.

Gorgeous grasses at Babikow Greenhouses near Baltimore, MD.

So we pass over pots full of green grassy things in favor of enticing blooms. Nurseries have picked up on this – many include grasses in their summer/early fall production schedule, making full, fluffy pots for the autumn gardener. This works o.k. for shorter, compact things like fountain grass, little bluestem, etc. But by September, majestic switchgrass, big bluestem, and the like rarely look that fabulous in a one or two gallon pot – the proportions aren’t right; a bit of wind and rain and the situation is ripe for floppage (closely related to splayage). So you’ll probably pass them over. Again. Or maybe…take a second look? Just cut them back and plant away – you’ll enjoy them NEXT fall.

Little ball of horrors

One of the great things about doing a multi-author science blog is that there will be topics about which colleagues will disagree. One of those topics revolves around the best way to prepare woody rooted plants (trees and shrubs) before planting them. This is an area in arboricultural science that is evolving. A search through our blog archives will find many of these posts and for convenience’s sake I’ve linked one from each of us here.

From Jeff (2009)
From Bert (2014)
From Linda (2013)

Rather than belabor the points that Jeff, Bert and I have already made in our posts, I think I can sum up our major difference here: I like to bare-root trees and shrubs completely before planting (so I can correctively prune all flawed roots) while Bert and Jeff prefer a less invasive approach. What we do agree upon, however, is the deplorable condition of the roots of many trees and shrubs that end up in the nursery. Because I do practice bare-rooting trees, I thought I’d use today’s post as a rogue’s gallery of trees that should never have made it to the retail nursery. (All of these trees were ones that I bare-rooted and root-pruned myself before planting – and all are thriving.)

The duct tape around the trunk is where the burlap bag began. While the roots are in pretty good shape, only root-washing could find the root crown of this tree. — The duct tape around the trunk is where the burlap bag began. While the roots are in pretty good shape, root-washing revealed the root crown of this tree (which the “at grade” part of the tree) 10″ below the top of the bag

Confessions of a Lazy Gardener

I admit it. I can’t keep up … I’m not as industrious as most of my gardening friends are when it comes to the effort necessary to manage my 6 acre landscape.

It can get overwhelming, especially when there are previous beds that came with the property that had been neglected for 10 years or more by the elderly lady who owned the property before us, and where perennial weeds are well established.

I make a valiant effort in the Spring, with all the enthusiasm of the new season to clean them up … dig the perennial weeds … plant something new (usually a division, or a naturally layered specimen, from elsewhere, or one shared from friends), but by mid-July or so, I have to redirect my efforts to the places that I’ve created … the shade garden … the rock (mostly sedum) garden … mulching the new trees and shrubs, and of course my tomatoes, so these previous places don’t get the attention they deserve.

But then again, some surprisingly beautiful, and beneficial results can happen in spite of (because of?) the neglect …

This is a “pre-me” bed of mostly Japanese Anemone stretching between two arbors, with Trumpet Vine (Campsis radicans) (pre-existing), and Climbing Hydrangeas (Hydrangea anomala) (me added), anchoring each end.

The Goldenrod (Solidago) and White Snakeroot (Ageratina altissima I think) now dominate, along with Sweet Autumn Clematis (C. terniflora), a non-native introduction that appears on watch lists as an invasive species in our area.

Yet look at the insect life. Scads of hoverflies (some species of Syrphid), who in addition to their role as pollinators as adults, are voracious consumers of aphids in their larval stage.

Scoliid wasp, predatory on Japanese Beetles.

An Ailanthus Webworm Moth (Atteva aurea), which uses the invasive Tree of Heaven (Ailanthus altissima) as its larval source for food.

And my, and most folks favorite these days, honeybees (Apis mellifera)

And if I kept up the weeding of this neglected bed, would there be a self-seeded Hydrangea paniculata available to transplant elsewhere?

So, probably a rationalization to justify my laziness … but looking on the bright side, there can be beneficial results from my neglect.

Why doesn’t my plant look like it did last year?

I get versions of this question often. You have something in the garden, but this year it looks a bit different than it did before. There are a bunch of different things that could have happened to cause this change, and I’ll attempt here to make a complete list of them.

Rootstock

Trees and shrubs, including things like tree peonies and roses, are often grafted, so the part with the pretty flowers or delicious fruit is stuck onto the roots of a different variety, often not as pretty/useful/tasty but more vigorous and/or easier to propagate. Sometimes shoots come up from that rootstock and take over the plant. The best sign will be to see if the shoots that look different are coming up from the very base of the plant. Cut all the shoots with the off-type flowers or fruits as completely as possible to allow the desirable parts of the plant to continue on.

Reversion

This is most common with variegated plants that are chimeras, though there are a few chimeral varieties which have multicolored flowers rather than multicolored leaves. I wrote about what a chimera is here, but the short story is: Many variegated plants are made up of two different cell types living together. If one cell type starts taking over, you loose the bi-colored effect of the variegation. As soon as you notice them, cut out the reverted shoots to keep them from out growing the multicolored parts.

Sports

Sports are chance mutations, and sometimes a flower will just up and change color. Usually, a sport will only change one aspect of a plant, most often color, while size and shape and everything else stays the same. New variegated varieties almost always come into being as sports. Sports will almost always be isolated to just once branch or section of the plant while the rest of the plant maintains the original color. If you like the sport, you might want to try taking cuttings. If it is an attractive sport of a popular plant, it might even be valuable.

Seedlings

Sometimes it isn’t that the original plant changed, but rather that it had some babies, and the babies look different. Confirmed self-sowers like phlox and aquilegia are notorious for this. You get a nice variety, but in a couple years, it has died out and replaced by a few seedlings which usually aren’t as nice. To prevent this, dead head after flowers fade to prevent seeds from developing.

Soil

Hydrangeas famously will change flower color depending on the pH of the soil in which they are grown. However, this is rather an anomaly, and soil conditions have no impact on the look of flowers for the vast majority of plants.

Climate

Some flowers will change their look, sometimes radically, depending on climatic conditions, often getting darker or lighter depending on temperature and the amount of sunshine. So if a plant looks different in a year when it has been unusually hot or cold or cloudy or sunny, it will probably go back to the normal coloration once your weather returns to normal.

Nursery conditions

Often plants you get at the nursery will look quite different once they’ve spent some time in your garden. Many plants from the nursery will have been treated with chemicals called plant growth regulators which are used to keep the plants short and compact. As those compounds wear off, the plants will grow taller and looser. Also, commercial greenhouses are usually warmer than your garden and often exclude UV light which can chance the coloration of a lot of flowers.

Contamination

If your neighbor gave you a clump of their favorite old bearded iris, but after a couple years the iris they gave you is replaced by a different color or a different plant altogether, they likely accidentally included a bit a different, more vigorous plant along with the one they gave you. Your best option here is to lift up the whole clump, wash all the soil off the roots so you can see exactly where one plant ends and the other starts, and just replant the ones that are true to type.

irisglyphosate — A whiff of glyphosate caused this iris flower to bleach out.

Herbicide drift

If you are spraying something like glyphosate (active ingredient in roundup) sometimes a bit of it can drift onto plants, not enough to kill them, but enough to cause them to be deformed or discolored. Glyphosate drift in particular will often cause flowers to be bleached out, white or nearly white. In this case, just be more careful next year, and everything should return to normal.

Forgetfulness

Okay, let’s just admit it. We don’t always remember that we moved something or what we bought or exactly what a new plant looked like last year. Sometimes there is no great scientific mystery beyond the fact that we’re all busy and don’t keep as good records as we know we should.

Joseph Tychonievich