CO-Horts Blog

Monday, May 17, 2021

This is Colorado Noxious Weed Awareness week!


Colorado Governor Jared Polis recently designated this week, May 15-22nd, as Noxious Weed Awareness Week in Colorado. For anyone who, like me, considers themselves a Weed Warrior, this proclamation helps highlight the need for everyone in our state to take noxious weed control seriously. And while many landowners are very familiar with the identification of noxious weeds, many home gardeners are not and may inadvertently be harboring these weeds in their gardens.

As a little reminder, noxious weeds aren’t just annoyances in your yard or property: they are non-native, invasive weeds that have a competitive advantage over our vegetation and displace our native plants. Many of them are from other continents entirely and don’t have insects or disease they may have had in their native land that kept them from becoming unruly. Noxious weeds are ones that have been determined by state or local governments to be a threat to local environments and have been placed on a Noxious Weed List. In Colorado, there exist three lists of noxious weeds, and corresponding controls are needed for each.

List A species are weeds that must be eradicated whenever detected in order to protect neighboring communities and the state as a whole. These weeds may or may not be in the state yet, or may be in adjacent states and infestation is probable. Eradicating them as soon as they are found helps keep them from getting a foothold in our state.

A few plants that may be planted in gardens can be found on List A. Myrtle Spurge, Orange Hawkweed, and Purple Loosestrife are all plants that someone thought would make wonderful ornamentals in the yard, but showed quickly that they are aggressive, dominating species that quickly take over our native vegetation when they escape the confines of a garden.

Oxeye Daisies taking over fields
near Steamboat Lake

List B
species are those that are present in the state, but the Colorado Department of Agriculture, in consultation with the Colorado Noxious Weed Advisory Committee, has developed a plan to keep these weeds from spreading. Plants that were once considered ornamentals that have now found themselves on List B due to their aggressive tendencies include Common Tansy, Dalmatian Toadflax, Dames
Rocket, Oxeye Daisy, Russian Olive, and Yellow Toadflax.

List C species are weeds that the CDA will help support efforts by local county and city governments to educate people on their threat and to help use management methods to limit spread. This list includes many common ‘garden weeds,’ like Field Bindweed and Redstem Filaree. List C also includes common mullein, which is spreading along roads, railroads, and trails across the state.

Dalmatian Toadflax on a hillside

Controlling noxious weeds is not only the right thing to do if you’re a gardener or native plant advocate, but also because it is the law. Many of these weeds can be controlled with mechanical, cultural, or chemical means, and using an integrated approach to control using all of these methods is most effective. Of course, keeping the ground covered with healthy vegetation and not giving weeds a place to grow is the best solution, and also gives our native pollinators the best chance at thriving, too.

Wednesday, May 12, 2021

Underused Trees and Shrubs You Can Plant in Your Landscape

By Sherie Shaffer, Horticulture Agent, CSU Extension-Pueblo County

Planting a new tree is a great thing to do, but planting an underused tree is even better! For one, it increases the biodiversity around us. Having a variety of plant and animal species increases the overall health of an ecosystem.

Another thing to consider, is the positive effects using an underused tree would have if there were a pest outbreak. Right now, we are dealing with the potential of the Emerald Ash Borer coming to Pueblo, and many places around the state are currently dealing with EAB outbreaks. Where the EAB is present, there is a potential to lose a whole lot of Ash trees. Since Ash trees have been widely planted for decades this could cause many of our neighborhood trees to be gone. If we had a greater variety of tree species, one pest could not wipe out such a large number of trees.

If you are thinking of planting a tree, look around and don’t choose a tree species that you see already planted near your home. Here are some ideas, from CSU Extension PlantTalk scripts, for underused trees and shrubs you can plant to increase the tree biodiversity in your neighborhood:

  •         Aesculus flava (octandra) or Yellow Buckeye- Has an oval shape at maturity and can grow up to 50 feet tall. Yellow flowers appear around May. Has a great orange color in the fall. Ideally it would like a moist, well-draining soil, and it can adapt to our alkaline soils.
  •         Quercus muehlenbergii or Chinkapin Oak- Grows up to 45 feet tall and can spread just as wide if not wider. The leaves have a nice rust color in the fall. Being an oak, it will have an acorn that will mature in the first season.
  •         Pyrus calleryana ‘Whitehouse’ or Whitehouse Callery Pear- Has a columnar/pyramidal form. The leaves are glossy green, and long and narrow. Leaves turn reddish-purple earlier in the fall than other clones. The white flowers are a little later than other clones of Callery Pear.
  •         Crataegus x lavallei or Lavalle Hawthorn- It grows 15-20 feet tall and has a round to oval crown. The leaves are glossy green in spring and summer and copper red in the fall. It has white flowers that bloom in May or June and a red fruit that will last through winter.
  •         Maackia amurensis or Amur Maackia- Grows to about 20-25 feet tall and wide and has a round crown. The interesting bark peels when mature. It blooms in June and July with fragrant white flowers.
  •         Lacebark Pine Pinus bungeana- Lacebark pine has attractive exfoliating bark in patches of green and brown which makes it a good single or multi-stem specimen tree.Needles are medium to dark green about 3 inches long. It tolerates moderately alkaline soils and is hardy to zone 4 to 5.  This tree will reach a mature height of about 30, with a 15 foot width.
  •     Harvest Gold® Linden Tilia x ‘Harvest Gold’- This hybrid of littleleaf and Mongolian linden develops reliable yellow/gold fall color and is more resistant to linden aphids. As a younger tree itis less prone Harvest Gold Lindento winter sunscald than other lindens. When mature, the bark becomes mottled. You can expect this linden to reach approximately 35 feet tall and 25 feet wide. It is hardy to zones 3-4.
  •         Crimson SpireTM Oak Quercus x ‘Crimschmidt’- This hybrid of white oak and columnar English oak is a good choice where there is a need for a tall, narrow tree. Crimson Spire™ oak has attractive blue-green leaves that are resistant to powdery mildew. Leaves turn rust-red in October for 2 weeks. The leaves then turn brown and persist into early spring. Hardy to Zone 5, you can expect this oak to reach 35 to 40 feet tall and 12 to 15 feet wide.  It has shown some intolerance to wet soils.

In addition to these great underused trees, native species are always a wise choice. You can find a list of Colorado native trees here:  , and a list of native shrubs here:


Monday, May 10, 2021

Aspen Alternatives for the Front Range

By Amy Lentz, Weld County Extension Horticulture Agent

It's tree planting season which means there are choices to be made in yards all across the state. But which tree should you choose?

Today's tree-in-question - the aspen. So many of us want one in our yard to give us that 'Colorado' feeling. From the beautiful white bark to the perfectly shaped and uniform green leaves, or maybe it's the fall color or rustling sound that you just have to have! Before you go out and buy an aspen to plant in your home landscape, I urge you to reconsider if you live along the Front Range or Eastern Plains. These native trees are only native to the higher elevations of Colorado, the mountains and foothills.

Aspen fall color in mountains

Aspens have a few needs that just can't be met down at lower elevations. For one, the soil is much different than that of the higher elevations - it's higher in clay content, more alkaline and drier. The temperatures get much hotter in the summer at lower elevations than what the aspens prefer which can lead to stress. Once an aspen tree is stressed, it can invite other insect and disease problems like oyster shell scale, Marssonina leaf spot or cytospora canker. See PlantTalk Colorado - Aspen Trees (#1701) for more information.

Finally, aspens want to reproduce via suckers and create groves and many home landscapes are just not big enough to accommodate a grove of aspen trees. If you plant an aspen in a small yard, you've probably just planted an aspen for all of your neighbors, too!

Photo: Amy Lentz - Aspen trees in Rocky Mountain National Park. 

So, instead of choosing an aspen tree for your Front Range yard, here are a few interesting alternatives. They won't be exactly the same, but they have similar qualities with much less caution required. 

1. Serviceberry

Serviceberry trees are great for smaller landscapes. They can be purchased in multi-stem form, giving them that 'grove' look without the suckering. They also offer a few things that aspens don't - spring flowers, summer edible fruits and nice fall color. The 'Standing Ovation' cultivar is a taller form, giving it more of that aspen look, whereas the 'Autumn Brilliance' has an intense orange-red fall color.

Photo: PlantTalk Colorado 1213 - Serviceberry

2. Tatarian Maple

Tatarian maple is another tree that can be bought as single or multi-stemmed. The 'Hot Wings' type has guaranteed bright-red summer seed pods creating the appearance of flowering. This tree is better suited to the harsher conditions and low-water or xeric settings once established.

Photo: Adams County Extension Tour the Xeric Garden @ Riverdale Regional Park

3. Redbud

If you like the smooth bark of the aspen, the redbud has a similar appearance in its younger bark but with a more grayish tone. The leaves are larger, but also have that similar heart shape. The redbud also offers a nice display of early spring lavender blooms in most years. Plant this tree in a more protected location from wind and intense sunlight.

Photo: Amy Lentz, taken outside the Boulder County Clerk & Recorder Offices in a very protected northeast facing corner

4. Oakleaf Mountain Ash

If you want something 'mountainy', the oakleaf mountain ash has 'mountain' in it's name! It has low to moderate water needs and has a uniquely shaped leaf, pretty flowers, red berries and nice orange-red fall color. It's not a true ash, so it is not under threat by the Emerald Ash Borer. In fact, it is not known to have any serious insect or disease issues. 

Photo: City of Sheridan, WY - Arboretum

5. Columnar and Fastigiate Trees

So many to choose from! These types of trees are more narrow than wide, making them good for tighter spaces where you might commonly want to plant aspen trees. There are a lot of deciduous trees listed on our CSU Fact Sheet. A couple to consider might be the 'Princeton Sentry' ginkgo or the 'Prairie Sentinel' hackberry. 

Photo: Wikimedia Commons - 'Princeton Sentry' ginkgo

Thursday, May 6, 2021

Climate change and pollinators

 By Irene Shonle, CSU Extension El Paso County

Plants and pollinators have evolved over the millennia to depend on each other. Pollinators need the nectar and pollen that plants provide, and plants need the pollinators to carry pollen from one plant to another to achieve fertilization and seed set. 

Climate change is having a negative impact on both plants and pollinators as the changing temperatures affect each species differently. In some cases the resulting mismatch in bloom time and pollinator activity can mean that some plants don’t get pollinated, and some bees don’t find the food they need at a critical time.

Plants tend to bloom primarily in response to a combination of increasing temperatures and daylight. This is why we see flowers bloom earlier in years with warm springs, and later in cold springs.  Insects also respond to similar environmental cues, which makes sense, given that mutually dependent species would be expected to be fairly synchronized.

European scientists Holzschuh and Kehrberger found that the European pasque flower (Pulsatilla vulgaris) responds to rising temperatures by flowering earlier each year, whereas one of its major pollinators, a solitary bee species, does not quite keep pace by hatching earlier. 

This system may have worked well for ordinary weather vagaries, but the warming of the earth’s climate has caused plant species to bloom an average of a half-day earlier each year. In total, that can result in the growing season of some species now beginning up to a month earlier compared to 45 years ago. Insects aren’t quite keeping pace, although the timing of their emergence is less well understood than with plants, especially for solitary, ground-nesting bees species (as many in Colorado are).  It appears as though insects have a higher threshold temperature for development than those required for plants, so plants are more likely than insects to bloom earlier in response to springtime warming (Forrest and Thomson, Ecological Monographs 2011, Holsuch and Kehrberger, Plos 1 2019).

This has implications for both wild populations of both the plants and pollinators.  If pollination gets missed, early spring ephemeral plants (which are often critical early nectar plants) will reduce in population size. Pollinators that miss the peak bloom of an important plant will be forced to switch to a different species – if another one is even availableavailable (Holzschuh and Kehrberger Plos One 2019).

Wild plums provide early nectar for pollinators

On a home landscape level, we can all help the pollinators by
including a variety of flowering plants that bloom from the very earliest days of spring until later in the season when more is in bloom.  Walk around your neighborhood and open space to see what is blooming when, and take notes, especially of plants that you don’t already have in your yard- such as perhaps quince, currants, plums, creeping phlox or violets.  Highly hybridized plants like tulips and daffodils may or may not provide nectar, despite their showy flowers.

Three lobed sumacs (Rhus trilobata) don't have showy flowers, but are great for early pollinators

There are some early plants that aren’t as showy, but still provide nectar and pollen – such as maples, sumacs and even willows.  If an early spring means the pollinators miss one plant, a diversity of plants with slightly different bloom times will mean that the insects can still find food.

We can all do our part to keep pollinator populations as robust as possible – starting with our own backyards!

Monday, May 3, 2021

Debunking the Myths of Soil Amendments

Posted by: Robert Sanchez, Douglas County Colorado Master Gardener

As we get the outdoor itch from being cooped up all winter as a result of from the pandemic and cold weather, some of us think about going to the local nursery to purchase compost, peat moss, manure, and other soil amendments to get ready for gardening. These are all important elements of gardening but amending the soil has so many misconceptions I thought I would address a few. First, let’s look at the science.

The ideal soil consists of about 25 percent air, 25 percent water, about 45 to 47 percent mineral matter, and about 3 to 5 percent organic matter. Soil texture and soil structure are also important elements. Soil texture refers to the size of particles that make up the soil, including large particles of coarse sand, medium particles of silt, and small particles of fine clay. The larger the particle, the more space for air, but too many large particles allow water to drain away. Thus, coarse sand by itself is not a good growing medium for roots. Scientists refer to the ability of roots to grow in soil as soil tilth. Soil structure refers to how the various particles of sand, silt, and clay fit together. A good soil has a mixture of various sized particles with various chemical and biological components that bind the particles into aggregates. An ideal soil allows air for microbial activity and root growth and retains water. Organic matter in the soil does increase water-holding capacity in the soil.

Now let’s explore a few myths about soil amendments.

Myth #1: Amending the soil will change the nature of the soil. We have notoriously clayey and alkaline (high pH) soil in the Front Range. Adding compost to the soil will not change the nature of the soil. Changing the nature of the soil requires either changing a little bit of the soil over a period of years, such as what nature does on a geologic time scale, or replacing the soil entirely in one fell swoop. The home gardener can speed up the geologic time scale by making changes faster than nature would do, but changes to the nature of the soil would still require years. We can see what it takes to change the nature of the soil by observing new building construction. Typically, construction crews excavate several feet of soil to install utilities and the building infrastructure, removing the top layer of soil that contains most of the dead and living organic matter. As the crews replace the soil to continue with construction, the organic matter is no longer near the surface, living matter has been destroyed, construction debris (gypsum, sand, concrete, and wood) has been added, and crews have pulverized the soil, snuffing out what little life or air that might have survived the onslaught. This process illustrates what it takes to change the nature of the soil. Even adding a lot of compost to a hole when planting does not change the nature of the soil, except for the soil you replaced in the hole, and that is only temporary because the organic matter in the compost will decompose, leaving only the natural soil and decomposed remnants.

Myth #2: Adding lots of compost during planting promotes root growth and plant health. Adding organic matter when planting does indeed promote root growth, but it may not result in a healthier plant. The ideal soil only contains 3 to 5 percent organic matter and filling a hole with 25 percent organic matter (or more) when planting is not ideal. Under good watering and sunlight conditions, the initial results of all that compost appear good. That is, roots show vigorous growth and the canopy of the plant responds accordingly. Then the roots hit the edge of the hole and encounter natural soil with less compost in it. Preferring the compost to the natural soil, the roots circle back into the compost, resulting in a condition called “girdling” roots. The result is an unhealthy lack of spread of the roots into the natural soil. When the compost you initially placed in the hole decomposes, the roots will actually have less access to nutrients because they have not extended their reach beyond the now decomposed compost to absorb more nutrients. In addition, the different soil textures inhibit the natural flow of water, so water falling on one soil texture may not flow into soil with a different texture, leaving some soil dry. Specifically, water cohesion allows water to coat soil and mineral particles, which is one reason organic matter retains water so well. Water moves through fine clay soil through capillary action. Thus, water does not readily move from organic material to clay or from clay to organic material because of the different processes facilitating water movement. Different soil textures do not promote root growth.

Myth #3: Enough application of compost will solve most soil problems. If a planted plant dies, an incorrect assumption is that it did not have enough organic matter. Organic matter is a necessary element for root development and plant health, but it cannot solve all soil problems. Drainage, compaction, sunlight, microclimates, and salt buildup are all problems with soils that organic matter might not necessarily help your plant overcome. In fact, adding too much organic matter, particularly manures, could increase salt buildup in the soil. Soil amendments can be deceptive because in the short term, the added benefits can be seen in the growth of the above-ground portion and overall apparent vigor of the plant, but over the long term, the detriments could outweigh the benefits. Applying organic matter is one aspect of gardening. However, managing other cultural practices, such as planting the right plant in the right place for the drainage, soil conditions, and sunlight, is no less important.

The takeaway from all this is to understand the holistic needs of plants in your garden. Amending the soil is one important part of the holistic view but understanding what it does and does not do can help you attend to your plants much better. For more information on soil amendments, see