By Alex Tisthammer
It is easy to find lists identifying plants to seek out for harsh environments, low-water gardens, poor soil, and more, but rarely is there discussion about what you shouldn’t plant. There’s some gray area when talking about this subject because there are some plants that don’t do well here and will die for obvious environmental factors, then there are plants that don’t grow here naturally, can be grown, but will need extra care and careful selection of the planting site. There are varying degrees of this, some things just require extra watering, while others need more consistent fertilizer schedules, the addition of amendments into the soil, wrapping with burlap in the winter, or planting the plant in a specific, protected area.
Some plants should be avoided because our soil does not provide the nutrients they need. A classic example is Autumn Blaze Maple, which is not recommended for northern Colorado. This type of maple is notorious for suffering from iron chlorosis. Most iron in our soil is in an insoluble form, making it inaccessible for plants to intake. So, for plants that need high levels of iron, they become chlorotic and will eventually die. This is also the case for Pin or Maple Oaks, which have high iron needs.
Our weather also provides certain challenges that aren’t worth fighting. Heather is a tempting shrub to grow, but it does not grow well here as a perennial. It is native to Scotland, Ireland, Russia and Scandinavia, places that are consistently cool and humid, quite the opposite of Colorado. Our extreme temperatures paired with drying winds usually cause these plants to die over the winter. Our dry windy winters are one of the main reasons why many plants struggle here. Boxwoods and arborvitae are two popular types of evergreens that are used heavily in other parts of the country but tend to struggle here due to winter wind desiccation and getting planted in areas with too much sun.
You also need to consider diseases and pests when deciding on what to plant. A tree that was widely grown in this region but is no longer planted is the Black Walnut. It is very susceptible to a fungus called Thousand Cankers Disease. As this fungus spreads through the tree it kills areas of the bark, blocking the flow of water and nutrients, usually killing it. One of the more recent trees that is no longer recommended is the American Ash. These beautiful trees known for their fall color are unfortunately being attacked by the Emerald Ash Borer that was found in Colorado for the first time in 2013 and almost always leads to the decline and eventual death of mature Ash.
Finally, we’ve got plants and trees like the Russian Olive that just do too well here, creating a different problem. They are on the Invasive Species list due to how easily they spread and how they choke out other native species. They are also very hard to control due to their deep root system and being able to re-sprout from the root crown.
Now that we’ve covered the plants that truly don’t do well here, we can move on to the plants that can be successful if given extra care. Blueberries are popular, and with good reason, nothing beats eating fruit you’ve grown yourself! Blueberries are, however, particular about their growing conditions, preferring very acidic soil, which we do not have. We recommend planting in a planter like a whiskey barrel, making it easier to control the acidity of the soil. A soil mixture that is mostly peat will be acidic enough to keep the shrub happy, and feeding regularly with an acid loving fertilizer helps, too. Acidifying your soil and using an acid-loving fertilizer is a common practice when trying to grow plants that aren’t found here naturally.
Japanese Maples are another plant that benefit from this practice. Their acid needs are not as significant as blueberries, but you’ll still want to pick a soil amendment that is more on the acidic side, like cotton burr compost. They also have very delicate leaves that dry out easily, so they need to be planted in an area that is protected from wind and gets 4-5 hours of morning sun or all day filtered sunlight. Winter watering is also essential as Japanese Maples do not tolerate periods of drought. Any plants that are from humid or cooler regions are going to need winter watering, as well as more consistent moisture in general.
Red Dawn Redwoods and Nootka Cypress are two conifers that prefer higher moisture and need to be planted in very specific conditions. They both need moist, yet well draining, slightly acidic soils, and they require protection from wind and enjoy shade at some point of the day. When visiting the Missouri Botanical Gardens, they had incredible oak-leaf hydrangeas, azaleas and rhododendrons, almost as big as small trees. When looking into growing these here, I discovered that they all bloom on old wood and in order to get them to flower here they would need to be wrapped in burlap and mulched/insulated to prevent winter dieback so as to not lose the flower buds to our freezing winter temperatures and wind. These are also plants that need winter wind protection, well draining soil, and to be shaded from our afternoon sun.
All of these plants are beautiful and well worth the effort but for some gardeners it is too much trouble. It’s beneficial to plant things that do well in your area, even better if they are native and low water. But if you think you have the perfect area for something and want a challenge, many uncommon plants are worth trying if it is plausible to create the right conditions for them. If any of these sound interesting to you, come into the nursery and talk to one of the Outside Staff. We would love to help you determine if you have the adequate growing conditions to have one of these unique plants!
Originally published on October 26th, 2022. Updated on March 3rd, 2023.