CO-Horts Blog

Tuesday, September 29, 2015

More From the Cottonwood Front

Posted by Mary Small, CSU Extension in Jefferson County
Dropped branches 
A few blog articles ago, I wrote that many cottonwoods were shedding leaves early. This was due in part to foliar diseases resulting from a wet spring. Now, it seems, they are shedding small branches and causing a second round of worry from their owners. (Is it winter yet?!)

This phenomenon is known as “cladoptosis”, one of those “osis” words that strikes fear into the hearts of many.  Fortunately, cladoptosis is a naturally occurring twig drop common to many tree species, including cottonwood. And, wouldn’t you know, it happens in the fall alongside leaf drop.
"Ball" of dropped branches
Take a look at the bottom of the fallen, pencil sized branches underneath the tree. You’ll see nice, clean cut with a rounded, raised center. If you tried to reattach the dropped branch to where it was originally connected, you’ll see the two pieces make up a ball and socket arrangement. This is a big clue that the tree was an active player and the branch drop wasn’t caused by squirrels chewing or storm breakage or something else.

"Socket" on tree branch
Before the branch fell from the tree, specialized cells created a separation layer where a break occurs. The cells also created a protective layer covering the opening in the branch remaining on the tree. This helps protect against moisture loss.  

Researchers are unclear why cladoptosis occurs. It's thought to be stress related. Affected trees may be suffering from problems such as soil compaction issues or drought.  The tree is stressed, so it “cuts its losses” by shedding less vigorous plant material, enabling it to better support more vigorous portions. Cladoptosis may also function like leaf drop, a sort of “planned senescence”.  

Bottom line: don’t worry about this type of branch drop, the tree is not dying. Try to improve water applications and reduce compaction in the tree’s root zone to improve its overall health. Don’t be surprised if cladoptosis pays a visit next year, though!

Thursday, September 24, 2015

Native plants for fall color

By Irene Shonle, CSU Extension in Gilpin County

Colorado’s fall colors are often overlooked or underrated, especially by people who feel that only the splendid reds of the upper Midwest and East Coast “count.”    “Yeah”, they say wistfully, “Colorado’s fall colors are just, well….not the same.:

Okay, maybe they’re not the same, but I would like to say that I, for one, LOVE our falls.  Perhaps there aren’t the vast maple forests, but when I see translucent aspen leaves lit by a slanting sun and contrasted with our blue blue sky, it makes my heart sing.

And, let’s not overlook that some aspen have a genetic tendency towards reds, especially some years.

But, aspen aside, there are many fantastic native shrubs and even perennials that can provide fall color.Take smooth and three-lobed sumac (Rhus glabra and Rhus trilobata ), for example. Both provide nice fall reds.  Smooth sumac is the more fire-engine red, but beware of its rhizomatous and aggressive nature, and only plant where you have lots of room. 

Rhus trilobata
There’s red-twig dogwood (Cornus sericea AKA Swida sericea).  It has lightly fragrant white flowers in spring, lovely red fall color, white berries for birds and –bonus!—red twigs for winter interest.

Golden Currant (Ribes aureum and Ribes odoratum) have fragrant yellow flowers (R. odoratum especially), edible black berries and a gorgeous red. I particularly like pairing it with Western sage (Artemisia tridentata) for the color contrast.

Waxflower (Jamesia americana) is a Plant Select ® Selection, and in good years, turns beautiful reds and purples.

One of my favorite underused plants is native mountain-ash (Sorbus scopulina).  While this may be most appropriate for mountain communities like mine, it is still a gorgeous plant.  I love how the pinnately compound leaves become a mosaic of greens and orange and reds in the fall.  The addition of orange berries can’t be beat!

Our native chokecherry is also no slouch when it comes to fall color.  Plus, it has glossy black edible berries to boot!  It can sucker quite a lot if grown in moist conditions, so beware.

Shrubs are great, but there are native perennials that provide color lower down.
Wild geraniums (Geranium viscosissimum and Geranium caespitosum) develop luminous red leaves in the fall.

Sulphur flower’s leaves turn bronze-red in the fall and are evergreen (ever-red?).  The rust-colored seed heads in the early fall provide a nice contrast.

Leafy cinquefoil (Drymocallis fissa) usually turns nice shades of red and green in the fall when it’s not too dry in August and September.  This year, it looks a little crispy since it’s been so long since it’s rained.

Little bluestem (Schizacyrium scoparium) is a fine grass for fall and winter interest.  The cultivar “Blaze” has been selected for particularly fine fall color.

This is just the beginning!  Get out there and look around, and enjoy the subtle and often not-so-subtle colors of a Colorado fall.

Tuesday, September 22, 2015

Aspen, cottonwood leaf diseases not lethal to trees, but could impact fall colors

(posted by Irene Shonle)

A Colorado State Forest Service Press release as a follow up to Mary Small's post:

FORT COLLINS, Colo., – Some stands of aspen and cottonwood trees across northern Colorado and along the Front Range won’t be their most picturesque this fall, due to leaf spot diseases that benefitted from an unusually wet spring and early summer. For about the past month, foresters have been seeing an unusually high degree of leaf blight in the mountains and along the Front Range, as far south as Aspen, the Collegiate Peaks and Colorado Springs.

At least two fungal diseases are to blame for the leaves now showing significant spotting or dark splotches. Marssonina leaf spot is caused by the Marssonina fungus and is the most common leaf disease of aspen and cottonwoods in Colorado. The disease can be identified by the presence of dark brown spots or flecks on leaves, which can then fuse into large, black splotches on severely infected leaves. Also active now, mainly on Colorado’s cottonwoods, the Septoria fungus initially causes tan spots that become irregular brown-to-black spots coalescing to cover much of the leaf by late summer.

“The good news is that these diseases rarely cause any permanent tree damage or death,” said Dan West, entomologist for the Colorado State Forest Service. “But this is the highest level our foresters have seen in many years for some parts of the state.”  

These fungi typically develop during wet, cool weather and generally cause only aesthetic tree damage. However, severe disease outbreaks occasionally cause some defoliation or dieback on impacted trees. West says trees with these leaf spot diseases can display noticeably less vibrant colors and can drop leaves prematurely, but often are mixed in among stands of healthy trees, so there should still be plenty of beautiful Colorado foliage for fall viewing. 

Fungicide sprays can prevent new leaf spot infections in the spring, but will not cure trees that are already infected. Because these fungi overwinter on fallen leaves infected the previous year, West says the best management option for homeowners this fall is sanitation. Get rid of any diseased material by raking up and disposing of infected leaves and twigs, to reduce spread of the disease next year. Also, always try to keep tree leaves as dry as possible to reduce future incidence of leaf spots by watering early in the morning rather than at night, and by keeping sprinkler patterns set to prevent over-wet leaves.

Regarding landowners with larger areas of aspen or cottonwood – of a scale that makes most management options impractical – West says maintaining space between trees helps prevents moisture from lingering too long on the leaves.

“This is a natural phenomenon, and despite how they look, these trees should leaf out again next year,” he said.

For more information about leaf spot concerns for aspen, cottonwood and other poplar species, go to

Tuesday, September 15, 2015

Giant Sequoia

Posted by Robert Cox, CSU Extension - Arapahoe County
How many times have you heard “ you can’t grow __ in Colorado”?  

Giant Sequoia is one of those plants – Sequoiadendron giganteum doesn’t even sound like a tree that could survive here….but there are several growing in landscapes along the Front Range – in Fort Collins, Boulder, Denver, Englewood and Colorado Springs.    Pueblo and Grand Junction may have a few growing within their city limits too.  
The Gardens at Spring Creek, Fort Collins.  Photo courtesy Jim Klett.

If you’re a little tired of the standard landscape conifers – pinyon, ponderosa and Austrian pines, Colorado spruce, arborvitae and juniper – Giant Sequoia may be worth a try. many plant names have four vowels in succession? That alone makes planting  one worthwhile. 
Needles are somewhat juniper-like
Interesting cones
Now for the Extension disclaimer:  Giant Sequoia in Colorado is unlikely to achieve the height and girth of those growing in California’s National Parks, some of them over 250 feet tall and 30 feet diameter at the base.    (Well, OK, if one eventually becomes that big here…don’t call to brag or tell me I was wrong).  

Giant Sequoia - California

Giant Sequoia is rated at Zone 5-6 hardiness, but may tolerate -20 degrees once it is well-established. The cultivar ‘Hazel Smith’ is touted as the hardiest of all Giant Sequoias.  You’ll probably have to order it or any Giant Sequoia from an online mail-order nursery. 
Plant in spring or early summer to give it some time to establish before winter arrives.     Ideally, plant it in a sunny, protected spot, in well-drained soil.
Young tree in Fort Collins park.  Photo courtesy Jim Klett.
To view photos and locations of Giant Sequoia in Colorado, see