An Oregon city's requirement may point to expensive root barrier installation before replacing street trees. File photo. Staten Island Advance/Annalise K

Gardening season has started, 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: The city where I live now requires a root barrier if you plant a tree in a parking strip. Our strip is 4 feet long and 6 inches wide. Is there a type of small tree that we could guarantee would not threaten the sidewalk or the curb?

For example, the city lists possible trees such as a ‘Fireburst’ paperbark maple, a kousa dogwood or a Japanese snowbell. It gives a list of 30 such species that it approves. The trouble is that it requires a root barrier that costs around $500 or more per tree. We have 6 trees to replace. The root barrier also ruins the current irrigation system.

I’m hoping to get validation that certain trees would not be a threat and therefore do not need a root barrier. (The smallest tree on their list is 20-by-15 feet tall.) – Yamhill County

A: Trees are a wonderful addition to the landscape until there is some sort of problem with them. Then they become a headache. I am assuming that you have existing trees the city is requiring you to remove and replace.

The list of street trees the city provides is specifically trees that are deeply rooted and thus unlikely to cause the sidewalk to buckle. I am also assuming that the city has currently specified the root barrier and that it is not an old requirement that has since been superseded. Has the city told you where you have to purchase the root barriers? Or of what materials they must be constructed? If not, a simple Google search revealed a much cheaper source from a forestry supply company.

Unfortunately, we cannot guarantee or certify that any particular tree cultivar would not cause sidewalk damage at some point in the future. Municipalities in general have very specific guidelines for a reason.

Here is a list of Street tree planting FAQs from the city of Portland, which explains how the city came up with its list of approved trees and why those particular trees. (Sometimes knowing the “why” makes stipulations easier to swallow.)

Your best bet is to talk to city employees who are responsible for street tree governance to see how much leeway you have and if they have any ideas on mitigating the expense of root barriers and replacing irrigation systems. I’m sorry to not have better news for you. Since caring for new trees is in your future, this OSU Extension publication, Selecting, Planting, and Caring for a New Tree, contains information which you will find helpful for the success of the young trees.

– Anna Ashbey, OSU Extension Master Gardener

Mason bee box. File photo.

Q: I have my mason bee tubes out to hatch and there are mason bees hatching. They are solid gray, gentle, and crawl on your hand, etc. Also, I’ve seen some tiny bees.

There are also some larger bees that are considerably more aggressive, don’t like me around, and seldom land. They look like honey bees, but haven’t stung. And they seem to insist on returning to the straws.

I have the straws in a clean paper food-type container, closed, with ½- inch holes drilled. The bees explore the clean wood block but don’t seem to stay. I’m experimenting with a clean wood block (that comes apart), a container of paper straws only and a container with cardboard tubes with straws.

Any advice? Am I worrying too much? I have the wood block protected from my work station, where I usually enjoy watching and being with the more gentle mason bees. – Washington County

A: I suspect you have horned-faced bees. These are an introduced bee that has become quite widespread in the Portland area. They shouldn’t be any more aggressive than the O. lignaria are, so I suspect you may have encountered them at a bad moment.

Generally new wooden blocks are relatively unattractive to bees. To help encourage nesting, make sure you have an area with exposed and moist clay close by (and I must emphasize keeping the clay moist, so add water regularly). Also, make sure you have good early blooming shrubs close by, like Oregon grape or rosemary. – Andony Melathopoulos, OSU Extension pollinator specialist

Q: I have a pieris that has developed dark browning of the leaves and I’m wondering if it is disease, parasite, neglect, or age. The browning appears to begin at the base of the leaves and spread towards the tip. I’ve attached a photo. Last year I think it was fine, and there is another pieris (different variety) about 40 feet away that looks fine. Is there anything I should be doing for it? – Lane County

A: Your pieris is suffering from leaf spot. Leaf spot is a fungal disease that is characterized by necrotic spots on the leaves of a plant. It overwinters on the diseased leaves and wet weather only enhances a plant’s possibility of developing leaf spot.

To help control leaf spot, make sure the soil is enriched with organic material. The soil should also be moist, not wet, acidic and drain well. Avoid watering your pieris overhead to keep the leaves from remaining wet for extended periods of time. Being continually wet contributes to the development of leaf spot. Also remove all leaves and plant debris from around your plant. Disease can survive the winter in debris and continue to inoculate your plant. Removing debris can also help to reduce the chance of spread to other plants that are susceptible to leaf spot fungus.

To help keep your pieris healthy, besides the soil requirements mentioned above, make sure it is planted in full sun to partial shade and is protected from winter winds. For more on pieris, see the following website, Landscape Plants - Pieris japonica. – Jan Gano, OSU Extension Master Gardener

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