By Deb Lebow Aal
Thanks to Kristine Johnson for her assistance on this article.
A few years ago, we published two articles on Colorado native plant gardening myths. They were wildly popular, and the issues discussed in those articles still surface when talking gardening, so, they bear repeating. I am updating the original posts with new information. It also bears repeating that I am not a scientist or a professional landscaper, botanist, or horticulturist. I am just an avid gardener who reads gardening books and scientific studies on plants like they are novels.
Many of these myths pertain to all gardening, not just gardening with native plants. And, there is usually no right or wrong answer to a gardening question. The answer is usually “it depends.” But, here are some answers backed up by science.
Myth 1: Needles from pine trees acidify your soil
No they don’t. A common misconception is that since pine needles from pine trees are acidic, when they drop, they will acidify your soil. Fresh pine needles taken directly from a tree are slightly acidic, and after a few days on the ground, are not acidic at all. Scientists have looked under 50-year old pine trees, with 50 years of dropped pine needles, and have not found the soil underneath the tree acidic. It turns out that adding pine needles to top-dress your soil is a good idea. They will help enrich your soil and will not blow away. See this post by the Colorado State University El Paso County Extension.
Myth #2: Always deadhead your perennials
Once a plant’s blooms are past their prime, the plant directs its energy towards producing seed and fruit. Conventional advice is to deadhead – or remove spent blooms – so that the plant can direct its energy instead to producing more blooms, foliage, and roots. While it is true that deadheading some perennials will give you a second bloom, that is not always good advice. Leaving your spent blooms on your plants is good for wildlife and can add winter interest. If you always deadhead your perennials, you are robbing birds and insects of important shelter and food. Also, letting seed fall to the ground encourages self-seeding which may be desirable. That said, cutting some perennials back in the spring before they start “waking up” can make the plant stronger and less leggy. But, that is not the same as deadheading; that is spring cleaning your garden.
Myth #3 : Wood chip mulch takes nitrogen away from your soil/plants
Well, yes and no. Wood has a high carbon to nitrogen ratio. So, for wood to break down, it needs to take nitrogen from somewhere. The early stages of wood decomposition are aided by fungi, which add nitrogen to the wood by taking some from the top inches of soil. This is done above most plant roots, so it will not affect your plants, unless they are seedlings. Wood chips are not recommended for vegetable gardens, for this reason. When bacteria enter the process, they pull nitrogen from the surface of the soil. Again, this does not affect your plants. In the later stages of decomposition, wood chips actually add nitrogen to the soil.
Myth #4: Gravel mulch is too hot and adds heat to urban areas
Yes, the heat-island effect is a real thing. Urban areas are often 5 to 10 degrees hotter than surrounding areas because of all the concrete, asphalt and buildings. Too much impervious surface and lack of green space is not a good thing. But, gravel mulch does not have to be part of the problem. Gravel mulch is pervious, allowing water to percolate in, which is a good thing. In fact, while the top of the gravel mulch may be hot, underneath, the soil and roots of the plant stay cooler than they would in wood chip mulch. Gravel mulch conserves soil moisture very well – better than you might expect. So, half-inch size crusher (sharp, not rounded, pea gravel) gravel, laid 2-3 inches deep is recommended, and an excellent mulch for Colorado native plants. Do note that gravel mulch is a mined product and mining has serious environmental consequences. So, if you want to use gravel mulch, best to look around for recycled materials – gravel that someone doesn’t want anymore. For more information, see this CSU Extension fact sheet, Mulches for Home Grounds.
Myth #5: The color of gravel mulch you put down matters
We’ve heard that you should avoid using very dark or white mulch around plants that may be sensitive to heat that can be generated from absorption in the former, and reflected by the latter.
I could not find any scientific studies on whether dark or light colored mulch – gravel or wood – will make a significant difference. However, in our warming climate, it may be getting too hot for dark mulches. I found one Reference to dark gravel in the CSU extension document Colorado Mountain Gardening Basics – 7.244. It states, “…pea gravel may increase the soil temperature particularly if it is dark colored. So, in the early spring, when we may want to increase the soil temperatures, a darker color may be preferable. But certainly by summer, that is not what we want. And, since you are not going to change out the color of your gravel by season, I am opting for light colored mulches. (I have a preference for light to medium-colored gravel for my native plants. They do well in it.)
Myth 6: We need landscape fabric/weed cloth to keep the weeds down
The landscaping trade uses lots of landscape fabric/plastic/weed barrier cloth. In fact, it’s hard to talk them out of it. Landscape fabric (LF), is unnecessary, unsustainable, and in fact does not prevent weeds. Okay, in some cases, it may retard them for a year or two, but they’ll make their way in. In fact, LF deprives the soil of beneficial processes. Let’s break it down.
Soil is a living entity. It needs water and billions of tiny critters to be healthy. Putting any type of barrier down (aside from cardboard or newspaper, which will break down over time) disrupts the natural cycles in the soil. And I know of zero projects where LF has prevented weeds from sprouting. First, some of the harder perennial weeds to get rid of will eventually grow through the barrier. Second, many weeds will seed right on top of the barrier; their roots become enmeshed in the fabric, making them much harder to get rid of. Finally, LF tends to silt up, lowering water infiltration over time. Properly applied mulch is a better option. Green mulch – living plants bunched tightly, is an even better option. Don’t leave room for the weeds! For more on landscape fabric, see Maintaining a Water-wise Landscape published by Utah State University Extension.
Myth 7: Holes in my leaves indicate a problem with my plant
Not always, and in fact, rarely. Holes in your leaves probably mean you have leaf cutter bees and/or other small insects/pollinators. It is a healthy sign of biodiversity and life in your yard.
Myth 8: Fall leaves need to be bagged and hauled away
By now, we all know to leave the leaves. Yes? Then why do I see so many leaves bagged up in plastic bags to be hauled away? Leaves are great, cheap mulch, provide shelter for overwintering insects, and it is just a shame to trash them.
Myth 9: Weeds need to be trashed
Unless they have flowered, I dig my weeds up and put them right back into the landscape, as mulch (in wood mulch, not gravel mulch). Many weeds are good nutrient accumulators, and will not harm your landscape. Dandelions, for example, are a calcium accumulator, so the leaves of the dandelion plant make a good mulch (but not the flowers!).* If you are practicing sustainable landscaping and making as little trash as possible, by all means put the non-flowering weeds back into your landscape or into your compost pile. That said, I do not put any part of the bindweed plant back into my landscape or my compost. It’s too smart!
Myth 10: Butterfly Bush (Buddleia davidii) is a good plant for butterflies.
Well, no. And, sure, there will be butterflies on that butterfly bush, but there are many better plant choices if your goal is to attract butterflies. Butterfly bush (Buddleia davidii) is a brilliant name and a deceptive marketing ploy. Butterfly bush actually has a few key strikes against it. It:
- Does not attract as many butterflies as other plants
- Is not a host plant for butterfly larvae, which makes it not a great plant for the butterfly life cycle; and
- Is not a native plant. In fact, it is an invasive plant on the east coast of the United States and on the DO NOT PLANT list. Originally from China, it has crowded out native plants in warm climates, and has become a noxious weed, spreading aggressively.
If you want to host butterflies, plants in the milkweed family (Apocynaceae) are important plants. Asclepia tuberosa, butterfly weed, with its gorgeous orange flower, is my personal favorite. It is unfortunately easy to confuse butterfly weed with butterfly bush. There are many lists out there of plants that attract butterflies, including in our Attracting Butterflies with Colorado Native Plants article.
Myth 11: Compost should always be added to soil when planting plants.
The topic often comes up as to how much compost should be added to your soil for native plant species. Often, landscapers use a 50:50 ratio of compost to soil when planting, often with good results in the short term. Here’s the scientific literature’s response:
From CSU Extension’s Colorado Mountain Gardening Basics Paper #7.244:
- “Native plants are often adapted to leaner soils (lower in organic material) and may ‘flop’ or have shortened lifespan in well amended soils.”
- “Chicken manure is deadly to native plants because it is high in phosphorus. Mushroom compost kills the soil bacteria.”
From the Native Plant Revegetation Guide for Colorado, published by Colorado Parks and Wildlife (CPW):
- “Most native plants in Colorado have evolved to thrive in low nutrient soils and will not benefit from high doses of nitrogen nearly as much as competing non-native weeds.”
- “Colorado soils typically have low phosphorus levels and native plant requirements for phosphorus are small.”
From this CPW guide, we know that Colorado native plants don’t need extra nitrogen or phosphorus. An over-application of any soil amendment, including compost, leads to an overabundance of nutrients. Further, manure- or biosolids-based composts can be high in salts. A judicious application (1-2 inches) of a plant-based compost as a top dressing helps retain water for more mesic plants (plants which need or tolerate more water). Applying as a top dressing (i.e., as a light mulch) rather than mixing into the soil also protects the structure and tilth of the soil. For more information, please read Using Compost in Colorado Gardens and Managing Soil Tilth, both by Colorado State University Extension.
From “The Myth of Soil Amendments” by Linda Calker-Scott, a professor at Washington State University:
- “Eventually, amended planting holes will have negative consequences for plant health.”
Her argument is that the roots circle down into the amended soil, not reaching out into the neighboring non-amended soil, which inhibits the plant’s growth.
- “…there is a multi-million dollar soil amendment industry that has little interest in debunking this myth.” She reminds us that ideal soil has only five percent organic matter by volume. In Colorado, that number is even lower.
This fact — that a plant’s roots will want to stay in the amended soil arena — is also the reason that bare-root planting is gaining in popularity. See our video on how to plant bare-root native plants.
So, I am convinced you do not need to amend your soil when planting native plants in Colorado. But – there’s always a but – if you are planting where the soil has been degraded so badly that there are no nutrients in the soil, e.g., the soil has been scraped away for a new build, you may want to help the plants a bit. But, just a bit. And, realize that it takes time to rebuild soil structure, organic matter, and nutrients, and that moderation, top dressing, and a plant-based compost or a mulch of fall leaves is your best approach.
Myth 12: We need more large trees on the Front Range
This one can probably be debated. We always need more data.
Historically, as a short-grass prairie ecosystem, the Front Range was pretty much devoid of trees, save for cottonwoods in riparian areas, and some scrub oaks. Once you get a little elevation gain, we of course have more variety in trees. If you are a native plant gardener, do you want more trees in your yard? Well, it depends. We did write an article on the pros and cons of trees on the front range.
The City of Denver can give you a dollar amount that trees contribute to the economy in ecosystem services – citing the facts that well-placed trees will save heating and air conditioning costs, sequester carbon, provide stormwater benefits, actually increase the precipitation in and around Denver, and reduce the aforementioned heat island effect of the built environment, etc, etc etc… They don’t even get to the benefits for wildlife. While it is true that trees require watering in our climate, the water they require is outweighed by the benefits.
Equity impacts our current canopy cover. Areas with lower income and or higher percentages of people of color have lower tree canopy cover, for complex reasons. The impacts are clear, though; heat island impacts from climate change, health impacts from lowered air and water filtering of pollution, and more.
Often when decision makers weigh trees, they consider large, non-native, Eastern shade trees which require significant space and which can be expensive to maintain. They require pruning, once large, and taking one down is very expensive. However, there are native tree options that are really good for a variety of landscapes. Trees like Chinkapin oak (Quercus muehlenbergii), Bur oak (Quercus macrocarpa), and Gambel oak (Quercus gambelii) are great options. Chinkapin and Gambel don’t get too big, and all are great for wildlife, don’t use too much water, and when placed appropriately, can provide a lot of the benefits the City of Denver quantified.
Beyond those, consider that smaller native trees and larger native shrubs provide tremendous habitat benefits, can provide the cooling impacts of transpiration, and when placed near western walls, shade. Small to medium size trees to recommend include:
- Box elder (Acer negundo) — on the larger size but popular for its fall color.
- Big-toothed maple (Acer grandidentatum)
- Thin leaf alder (Alnus tenuifolia)
- Western water birch (Betula occidentalis) in rain gardens (they have higher water needs)
- Bristlecone and piñon pine in low water sites (Pinus aristata and Pinus edulis)
- Netleaf hackberries (Celtis reticulata) — great wildlife trees and not well known
Recommended larger shrubs include:
- Serviceberry (Amelanchier alnifolia)
- Hop tree (Ptelea trifoliata)
There are many more recommended medium and small sized shrubs and all of these native trees and shrubs should be considered for their cooling, their biodiversity impacts, and their smaller and more manageable form. Read Native Shrubs for Colorado Landscapes and Native Trees for Colorado Landscapes by Colorado State University Extension.
Rather than fixating on the impact of gigantic trees non-native to our region, I would like to see a comparison of how much carbon deep-rooted native prairie plants sequester compared to these trees. I have yet to see that comparison here in Colorado. Given the water savings of native plants, it may be worth considering them as a tree alternative. Here is an article that argues that increased drought and wildfire risk may make grasslands more reliable carbon sinks than trees. And a discussion thread on this topic that details the complexity of this issue.
I don’t think we have an answer to this one. I think we can safely say plant more native shrubs and don’t skip over our native deciduous trees just because many of them are smaller in stature or slower growing.
So there are the 12 myths we just busted for you! Many of these myths have been around for a long time but now we have the science to debunk them.
*Dandelions. We’ve all encountered those who believe that dandelions are a good source of nectar early in the season for pollinators. While they do provide some nectar, it is substandard nectar, so that is not a great reason to leave your dandelion flowers out. There are much better sources of nutrition for early pollinators.