Garden docs: Getting to the root of problem with rhododendron

Theresa H. of Santa Rosa asks: The leaves of my rhododendron are curling inward and turning brown, and the plant is wilting. What's going on?

Phytophthora cinnamoni (root rot), is a root rot disease which is usually fatal and often kills a plant very quickly.

The symptoms are a sudden collapse of parts of the plant, or the entire plant, during the growing season. Check the roots and cut into the infected part of the stem. The disease is characterized by the roots being a deep brown color instead of white, like healthy roots should be.

If you scrape away the bark at ground level you will find the cambium layer below the bark has been stained a dark reddish-brown. The disease is caused by inadequate drainage and warm or hot soil temperatures. The combination of these two is often fatal to rhododendrons.

It is most common in areas with hot summers but can occur anywhere with poor drainage. Phythopthora cinnamomii is most active at a soil temperature of 65-75 degrees. Plants in containers are particularly susceptible, especially if watered overhead.

Be sure that the planting area is well prepared with coarse organic matter to ensure good drainage and aeration in the soil. Freshly composted bark has been shown to have some root-rot resistant properties. Packaged mixes for rhododendrons, azaleas and camellias would be good to use.

Make sure that the soil where the rhododendrons are growing does not become compacted by people or animals walking over it.

Growing rhododendrons in shade and mulching with organic matter will help keep the soil temperature down.

If you have a heavy clay soil, the best thing to do is to plant above soil level, either in raised beds or by mounding up the soil around the root of the plant.

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Tom M. of Windsor asks: I would like to use my cold frame this year and grow a few greens, other than the usual Swiss chard, lettuces, etc. Any suggestions?

With a little protection from the elements, you can keep harvesting fresh greens well into fall and winter. Choose frost-hardy crops that continue growing during the transition to colder weather and shorter daylight hours.

Plant them early enough that they're nearly mature by the time the day length drops below 10 hours.

Here are a few good choices for the more unusual fall and winter greens, along with timing tips to help you get the most out of your greenhouse or coldframe.

Claytonia, also known as Miner's Lettuce, is one of the most cold-tolerant of salad greens and is hardy to about 0 degrees. It is easy to grow, is not bothered by moderate frosts and you can get multiple harvests over the course of winter.

The heart-shaped leaves have a wild flavor, and have a crunchy, succulent texture that is especially delicious.

Each stem has tiny white edible flowers. Sow seeds directly into the ground about 1/2 inch apart, 1/4 inch deep, in rows 12 inches apart, thinning to 4-6 inches apart. You can even plant in succession.

Mache, also called corn salad, is a very hardy, low-growing plant that produces tight rosettes of thick glossy leaves hardy to 5 degrees.

Even though it takes up to two weeks to germinate, it is very easy to grow. It was originally selected from a weed found growing in grain fields in Europe.

It has a wonderful mild, nutty flavor that makes an excellent salad with all kinds of fruits. Direct sow once temperatures have dropped below 68 degrees. Plant seeds 1 inch apart in rows 6-12 inches apart.

Beet Greens are extremely winter hardy, surviving with the protection of a greenhouse or coldframe and row cover even in very cold climates, to about 10 degrees.

Bull's Blood variety is the preferred leaf for overwintering salad mixes in unheated greenhouses and coldframes. Because of its hardiness and deep magenta color even in low light conditions, the leaves always look beautiful.

Plant directly in the soil every two weeks until 6 weeks before the first frost date, 1/2 inch apart and 1/2 inch deep, in rows 12-18 inches.

Dana Lozano and Gwen Kilchherr are garden consultants. Send your gardening questions to The Garden Doctors, at pdgardendoctor@gmail.com. The Garden Doctors can answer questions only through their column, which appears twice a month in the newspaper and online at pressdemocrat.com.