Crop rotation in raised garden beds requires some planning. Ask an expert


Although the weather isn’t exactly spring-like in Oregon, it takes a lot of effort to keep people out of their gardens. If you’re one of the diehards, you probably 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 members and Master Gardeners respond to your queries within two business daysDays, but usually less. To ask a question, just go to the OSU Extension websiteType it in and include your county. Here are some questions that other gardeners have asked. What’s yours?

Q: Garden with raised bedsI like to rotate crops, but I am limited in space. Some plants don’t do well when planted together, like onions and peas or carrots and celery. But how long should you wait to plant one of these crops after you have harvested an incompatible crop?

For example, I harvest onions in July/August and would like to use that same bed to plant peas for February. Are there any negative effects on the peas if they are planted in the same place as onions? I harvest cilantro up until early July, and I’d like to plant carrots in that bed in September. That’s only a couple of months apart. Could that pose a problem with the carrots? – Josephine County

A: Rotation of the plants in the gardenIt is possible to manage pests and increase fertility by knowing the family of a plant.

One member of a particular family, such as cucumbers, should be planted in the same spot that squash was planted the year before. This will make it more susceptible to pests and pathogens. This website: Plant Rotation in the Garden Based on Plant Families. ( provides a useful chart that groups plants by families. You will see that cilantro and carrots belong to the same family, Apiaceae. It is best to wait at most a year before planting members from another family, and then return to the original family in the third year.

Another website Use companion planting to practice the good neighbor policy in your garden | Oregon State University discusses complementary planting where growing several species of plants together offers benefits to all.—Sharon May, OSU Extension Master Gardener

You will need to trim any scab from this plant.OSU Extension Service

Q: I have an old pyracantha. I’m wondering if I have pyracantha scab or is this the way that the old berries are supposed to look as the new blossoms come in. You might also have winter weather problems if you notice yellow leaves.

I’ve looked at all the usual sources on the web and don’t have a definite answer.

If it is scab, I’m reading that I need to cut it away. But I don’t want to drastically cut back the plant if it’s just going through its natural process. Any suggestions are welcome. – Multnomah County

A: It is correct, you have diagnosed pyracantha. Because it is a fungus you will need to remove any diseased parts and dispose of them in the trash. Do not recycle. This can be as simple a matter of picking off the scabby or discolored berries or a full branch being pruned.

You should also rake up any fallen leaves, berries. HereThis article focuses on the disease and its treatment. You can use chemicals. There are several options, but only one organic. Look for the “H” with a box around it and the “O” with a box around it to see which ones are available at the store, and be sure to read the directions very carefully whatever you decide. – Rhonda Frick-Wright, OSU Extension Master Gardener

Q: I work on a property with many mature rhododendrons. They’re all healthy and established, but I would like to mulch them to help with moisture retention in the summer and weed suppression.

I’ve read a lot of different things online about which mulch to use, and it seems the most common one I see is pine needles or pine bark. I get that that’s ideal for the acidity factor, but I don’t have any pine needles or know where to get a large amount of pine bark.

I’m planning to get a unit of material delivered. Is fir bark slightly alkaline? Could this be an option? It seems to be available almost everywhere. Could I use something neutral, like garden compost? I’ve heard that if your soil is naturally slightly acidic, you can’t really change that in the end by applying mulch. Perhaps a bark-based compost would be a good choice? I’m aware that rhododendrons have shallow roots and that their roots will grow into the mulch.

I’ve never been told that before by anyone I know, only something I’ve seen online, so that could be totally false. If the mulch was more compost-based than bark, would 2 inches be the correct thickness? Perhaps a bark-based mulch should be about 3 inches. Sorry for all the questions. I would love to hear from experts in the area. – Multnomah County

A: Use hemlock- or fir-based mulches. They are slightly acidic and help to prevent weeds from growing. To add nutrients and lower the pH, you can apply acid plant fertilizer to the mulch before it is applied. Spread the mulch at least 3 inches. Mulching Woody Trees With Organic Materials provides a wealth of information. Compost is usually alkaline, and can also help with weed growth (has nutrients) – Weston Miller, OSU Extension horticulturist

RaspberryOSU Extension Service

Q: Can you tell what kind of disease my ‘Joan J’ thornless everbearing raspberries have? – Multnomah County

A: It is possible that you are suffering from root rot, even though it is still in its infancy. This could be caused by the soil-based Phytophthora fungal organism. I’m attaching an ArticleYou can learn more about it. It is something I have dealt with for many years. It’s not uncommon here.

The idea is to help the plants develop and flourish. Don’t move healthy looking plants to a new area. This spreads the disease.

There’s more information in the article, but there’s only one chemical treatment available for home use, Organocide Plant Doctor at 2 to 6 teaspoons/gal water as a foliar spray; and it is not labeled as very effective. We still have enough raspberries to eat and make jam. Each year I pull out any plants that appear to be completely affected and cut back on infected canes. If you move your strawberry patch to another area, buy new plants. Sorry it’s not better news, — Rhonda Frick-Wright, OSU Extension Master Gardener


Crop rotation in raised garden beds requires some planning. Ask an expert

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