Q: Our development has many (hundreds) of rhododendrons as part of the landscaping. What time of the year is best to do regular pruning (shaping)? Is there a bad time of year?

Also, after a few years, the bushes get too big for their positions in the landscaping. How often should they be pruned back to a smaller size? And how long should we expect that the plant can remain in the same location?

A: The best time to prune rhododendrons is immediately after blooming, since the plant’s creation of next year’s blooms begins immediately after this year’s fade.

Although the general rule is not to plant trees or shrubs in a place where they will overgrow their allotted space (height or width), if you need to control their mature size or reduce legginess, the guideline is to prune only one third of the stems each year, leaving at least one or two growth nodes (where other branches or foliage emerge from the stem). This leaves adequate foliage to create food (carbohydrates through photosynthesis) for the plant until the next year.

Woody landscape plants aren’t meant to be moved, especially if planted in the right place initially. Some rhododendrons in Japan have existed for centuries. The species is a hardy one, and curtailing its natural lifespan is a human choice, not one required by nature.

Kris LaMar, OSU Extension master gardener

Is that mold in the soil?

Q: I’m weeding under a Japanese maple, and in the soil is what looks like white dusty mold or mildew. I searched the internet for what it might be and found conflicting results. Can you tell me what it is and whether I should be concerned?

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A: Never fear, this is a great fungi growing in your bark. It is mycorrhizal fungi, which is actually working with your plants. It attaches to roots and then spreads further into the soil looking for nutrients. When nutrients are found, it sends them into the plant roots and the plant uses them to grow. In return, the plant roots produce an exudate that feeds the fungi. A true symbiotic relationship.

Sheryl Casteen, OSU Extension master gardener

When is lawn reseeding time?

Q: What is the best time to dethatch and reseed our lawn? We are going to overseed it. Does that change the time of year? 

A: We generally recommend dethatching in April when the grass is actively growing and the temperatures are typically warm enough at night that any newly sprouted grass seeds won’t be killed off.

The only risk of overseeding too early is that the germinating seeds could get nipped by a cold snap. Then you would have to seed again. Check for a 10-day forecast when temperatures stay well over freezing and, ideally, when you’ll get regular rain to help with germination and establishment.

Brooke Edmunds, OSU Extension horticulturist

What’s the best way to move blueberries?

Q: We need to move established blueberry bushes and an established red currant bush this month. Should we prune them first? Is it OK for them to be in a pot for a few days? Anything special we should know about root care during the move?

A: Starting with root care: Blueberry roots are often right on the surface, with some roots going down, but most going out around the bush. So dig a wide circle around the bushes (both blueberry and currant), going down at least 12-18 inches. That will give the roots soil to hold on to.

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Dig your new holes twice as wide as what you dug out and the same depth. You want the bushes to be exactly at the same soil level as they were before.

Add some peat moss and elemental sulfur to each hole for the blueberries and mix with a trowel. This will lower the pH in the new planting area. A small bucket of peat and a cup of sulfur should do it.

Add some compost and set the bush in the hole. Tamp the soil around the bush gently. You want air pockets for the roots to breathe, but not large ones. Water if needed. Add sawdust around the blueberries, starting 5 inches away from the base of the bush.

The currant bush needs a lot of compost but no sulfur. They also like well-drained soil. I would add compost only to the bottom of the hole and then add compost mulch the same way as the sawdust for the blueberries.

Blueberries like very acidic soil, around 4.5-5.5 pH. Currants like less acidic soil, around 5.5 to as high as 6.5. We have had very hot summers the past two years. Plan on watering all the bushes on a regular, weekly basis, making sure the soil is moist down to about 5 inches. Shade cloth may be necessary if you plant them in full sun. Look for brown spots on the leaves in the summer; that may indicate sunburn. If you see that, put shade cloth over the plants.

The less you handle or disturb the roots, the less shock they will have, so I don’t recommend storing them in a pot for any length of time. Delay pruning for a month in order to make sure your plants are not in shock from being transplanted. Then the only pruning I would do is taking out dead wood and making sure the center of the plants have airflow. Once they leaf out, there needs to be air in the center of the bush to stave off fungal growth.

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S.C.

What to plant where wild blackberries once grew?

Q: I have a hillside above my yard that was overgrown by Himalayan blackberry. I had them cut back, but need advice on what to plant there instead. It gets full sun.

A: If they were just cut down, they will come roaring back and the few plants you add won’t be able to compete with them. There are fabrics designed to smother out an area, but even then, the berries will attempt to escape at the edges. But leaving the area bare will probably lead to serious erosion of the bank.

Replacement plants would need to be fairly sturdy, woody small shrubs. Creeping ones have the best chance. Consider: Arctostaphylos uva-ursi (kinnikinnink), Calluna vulgaris (heather), Ceanothus thyrsiflorus var. repens (creeping blue blossom), Euonymus fortunei (fortune’s spindle), Euphorbia amygdaloides var. robbiae(wood spurge), E. amygdaloides ‘Purpurea’ (spurge), Hypericum calycinum (Aaron’s beard), Mahonia repens (creeping barberry), Symphocarpos albus (snowberry), Vinca major (periwinkle).

Suggested planting distance is 3–4 feet apart. Space closer on poor soils and in difficult situations. In good conditions, effective cover should be achieved in about two years. Meanwhile you will have to mulch with wood chips and weed as needed.

Another option that might work more quickly is to plant grasses — not turfgrass, but creeping ones such as mondo grass. Or you could plant biomats that peg down to the soil and are pre-seeded with stabilizing plants.

Pat Patterson, retired OSU Extension horticulturist

Ask an Expert is an online question-and-answer tool from Oregon State University’s Extension Service. To ask a question, visit extension.oregonstate.edu/ask-expert.