Ask an expert: Steep slopes create landscaping challenges
Gardening season is underground, and you may have questions. For answers, turn to Ask an Expert, an online question-and-answer tool from Oregon State University’s Extension Service. OSU Extension faculty and Master Gardeners reply to queries within two business days, usually less. To ask a question, simply go to the OSU Extension website, type it in and include the county where you live. Here are some questions asked by other gardeners. What’s yours?
Q: I need to remove creeping evergreens on a slope and replace them with appropriate ground covers. They must be low maintenance and drought hardy. – Klamath County
A: One challenge many homeowners face with landscaping steep slopes is soil runoff. The topsoil on hills can easily wash off, leaving an unattractive and muddy mess. In this case, you may want to consider planting your hill with groundcover plants. There are numerous varieties of shrubs and flowers that can thrive on slopes, giving you a better view and reducing the chances of erosion. Just keep in mind that you may need to bring in some additional soil and an erosion mat to establish the plants if the existing soil on the hill lacks the necessary nutrients.
Another popular option is to terrace sections of the slope within your lawn to create flat planting areas. Building several tiers on a hilly property will help you avoid erosion and allow you to add a variety of plants and landscaping elements for a functional and attractive design. Retaining walls can consist of a variety of materials to best fit your space and budget, such as stone pavers, wooden posts or concrete. However, when considering materials that stand the test of time, stone pavers are recommended. Additionally, when installing any retaining wall, it is important to add a good drainage system behind the wall to prevent the wall from cracking or even collapsing.
Getting plants to take hold on a hill can be difficult, as slopes tend to erode quickly due to drainage issues and soil runoff. If you don’t want to terrace your slope, adding rocks of varying shapes and sizes is another way to hold the soil and allow your plants to become established. Adding natural elements, such as a rock garden can give your slope a natural hardscaped look that’s both visually appealing and requires less maintenance.
Whatever method you choose for landscaping your slope, it’s also important to select the right type of plants for the area in which you live and your particular space. For example, deep-rooted plants can help stabilize soil, ground cover plants are great for covering up unattractive spots and ornamental grasses and perennials can be used to add color and texture to your slope.
When selecting your plants, it’s also important to keep maintenance in mind. Plants that don’t require shearing or a lot of annual clean up can save you time in the long run. Here’s some more information. –Chris Rusch, OSU Extension Master Gardener
Q: From the attached photo can you tell me what I need to do, if anything, about the wound on our dogwood trunk? So what you think may have caused it? — Polk County
A: The wound in the trunk is typical of damage from a lawnmower, string trimmer or some other type of collision. The wound appears to be healing, as we would expect. Dressings are no longer considered advisable since they tend to trap bacteria in the wound. With air exposure alone, the tree will mend the wound.
I do suggest pulling the mulch back several inches away from the tree trunk. Also, the irrigation line seems to be too close to the trunk. The trunk does not benefit from soil moisture. The most important place for irrigation is near the drip line of the tree or slightly beyond. That is where the root growth takes place. – Lynne Marie Sullivan, OSU Extension Master Gardener
Q: What’s wrong with my ‘Red Fox’ katsura tree? The leaves feel like it is receiving enough water but the leaves look funny on a few branches. We’re in Zone 8, facing east in Junction City. The tree is on a deep-root watering system with two tubes and is watered for 22 minutes every six days. — Lane County
A: This is most likely the aftermath of the sudden change from cold and wet to blistering heat. We are seeing many leaves with odd colors and worse just now. – Pat Patterson, OSU Extension horticulturist, retired
Q: I have potted camellias and the pH of the soil is fairly neutral. I’d like to lower the pH to 5ish. What is the best way to accomplish this goal? – Yamhill County
A: You can change the pH slowly over time, which allows the plant roots to adjust easily. OSU Extension has an excellent publication, “Acidifying Soil for Blueberries and Ornamental Plants” which is a detailed guide in doing just that – making the soil more acidic. After reading the publication, look for fertilizers for acid-loving plants in your preferred plant nursery. – Anna Ashby, OSU Extension Master Gardener
Q: Could you put into words what is going on with these three creatures – the spider, the bee and the one “waiting in the wings”? – Benton County
A: The spider is a goldenrod crab spider (Misumena vatia). They hang out on flowers (including but not limited to goldenrod) and ambush insect visitors. Looks like it got the honeybee (Apis mellifera) on your sunflower that way and it is feeding on it. The other insect “waiting in the wings” is a little more mysterious. It is a type of true fly (Order: Diptera), but I can’t see enough detail to know for sure what type of true fly it is. Interestingly, there are small flies like that called zombie flies (Family: Phoridae, Apocephalus borealis) that parasitize and kill bumblebees and honeybees. Here is a link to a photo showing one of these flies on a bumblebee.
The adult zombie flies stalk adult bees and lay their eggs inside them. Then the fly larvae hatch and feed internally in the bees altering their behavior and eventually killing them. I’m not sure if that is a zombie fly trying to parasitize the honeybee which is already being fed upon by the spider, but that is one possibility. – William Gerth, OSU Extension Master Gardener
Q: I followed your OSU Extension pamphlet for growing garlic and the bulbs never got very big. I bought good cloves and fertilized and watered a little in the fall. The tops came up, no bulb on top, planted in October and harvested last week. – Clackamas County
A: I’m going to let you decide which reason you might be able to correct for next year in the following article about mistakes with garlic. My best guesses would include planting too early. We usually recommend November for planting here in the PNW. (I see that the OSU pamphlet says September through November, but conventional wisdom says later is better).
My other guess would be our rainy, rainy, rainy spring. There was possibly just too much water in the soil to allow for larger bulbs. I don’t usually dig my garlic until July, so I don’t know how mine have fared this year, but another website says small bulbs may be related to soggy soils and we certainly had that! Don’t give up. Plant what you harvested this year or get new bulbs for next fall, because when you do pull up a huge bulb with big fat cloves there is nothing better. – Rhonda Frick-Wright, OSU Extension Master Gardener