Crop rotation in raised garden beds takes a bit of planning: Ask an expert

The weather may not be very spring-like, but it takes a lot to keep Oregonians 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 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 garden with raised beds and I like to rotate my crops, but I have limited space. There are certain plants that do not do well when planted together (like onions and peas, or carrots and cilantro), but how long should you wait before planting one of these crops after harvesting an incompatible crop?

For example, I harvest onions in July or August, and would like to use that bed to plant peas in February. Will there be any residual negative effects on the peas from planting them in the same bed where onions were previously planted? 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. Would that be a problem for the carrots? – Josephine County

A: Plant rotation in the garden can be based on knowing what family a plant belongs to so you can manage pests and perhaps increase fertility.

When one member of a family, such as cucumber, is planted in a spot where another member of the same family, say squash, was planted the previous year means it will be subject to the same pests and pathogens. This website: Plant Rotation in the Garden Based on Plant Families (psu.edu) provides a helpful chart grouping plants by families. You can note on the chart that carrots and cilantro are members of the same family, Apiaceae. You would want to wait at least a full year while planting members of a different family before returning to the original family the third year.

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

Areas of scab on this plant will need to be pruned away.OSU Extension Service

Q: I have a pyracantha that is a couple of years old. 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. The yellow leaves might be another clue, or just problems from the winter weather.

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 thoughts would be appreciated. – Multnomah County

A: Yes, you diagnosed correctly, pyracantha scab. As it is a fungus, you will need to cut away any diseased parts and throw them in the trash. Do not recycle. This can be as simple as picking off the discolored leaves and scabby berries, or you may include some pruning if a whole branch is infected.

Be sure to rake up any fallen leaves or berries as well. Here is an article about the disease as well as treatment. If you use chemicals, there are several available ones in the above article but only one is 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 at a property with a large number of 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 acidic? Could that be an option? I see it is available basically everywhere. Could I use something more 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. Or maybe a bark-based compost could be good? I know rhododendrons also have shallow roots, and have read 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. Also in regards to the depth, would 2 inches be the correct depth if the mulch were more compost based vs. bark? Maybe a bark-based mulch would be around 3 inches? Sorry for the multiple questions, but I would just love to hear from an expert in the area with their advice. – Multnomah County

A: Using fir or hemlock-based mulches is your best bet. They are slightly acidic and woody mulches help prevent weeds from growing. You could add some acid plant fertilizer before applying the mulch to provide nutrients and help keep the pH down. Spready at least 3 inches. Mulching Woody Plants With Organic Materials has a lot of information. Compost is usually slightly alkaline and also contributes to weeds growing (has nutrients vs. woody products). – Weston Miller, OSU Extension horticulturist

Ask an expert

RaspberryOSU Extension Service

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

A: Although in an early stage, it looks to me like you may be dealing with the very common root rot caused by the Phytophthora fungal-like organism in the soil. I’m attaching an article about it. I have been dealing with it myself for several years. It’s not uncommon here.

Basically, nurse the plants along as best you can and help them drain around the roots by digging trenches along the bed so their roots are higher than the surrounding ground. Don’t move healthy looking plants to a new area. This just 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 get enough raspberries for eating and jam, and I cut back the infected canes each year, pulling out the plants that seem to be entirely affected. If you move your patch to a new area, buy new plants. Sorry it’s not better news, — Rhonda Frick-Wright, OSU Extension Master Gardener