Hardy rhodys can survive deep freeze

It’s hard to beat rhododendrons for magnificent spring displays of color. Their huge flower clusters in almost jewel-tone colors add so much to the spring garden.

However, last winter was a reality check on how fragile they can be in our Inland Northwest gardens. A lot of folks lost plants outright or flowering buds to our minus 20-degree weather.

With a little planning, however, rhododendrons can continue to be an element in our landscapes. If you find you have to replace plants, here are a few steps that will help reduce future losses.

First and probably most important is to select varieties that are truly hardy in our climate.

With more than 800 species and more than 10,000 named varieties of rhododendrons known to botanists, rhododendrons have been found thriving in some seemingly inhospitable places around the globe, including places like ours.

A quality rhododendron should have a tag on it that lists growing requirements, including the temperature it is hardy to. Resist the urge to look for a particular color before checking this hardiness rating. After you have identified the hardiest varieties, then pick the color you like best.

As a rule for my garden, I tend to pick plants that are hardy to at least minus 15. As a result, I only lost one plant last winter, an Elizabeth that was good to minus 5. However, I did lose the flower buds on several other plants. The flower buds are usually the first part of the plant affected by cold.

Two of my favorites are the PJM and the Northern Starburst, hardy to minus 25 and minus 30 respectively. The PJM is as tolerant of heat as it is cold and is widely planted in our area. It puts out a cloud of almost electric blue purple blooms in early April. Northern Starburst is an improved form of PJM with larger flowers and thicker leaves.

The second step to success with rhododendrons is providing the right growing conditions.

Rhododendron roots are fine and require consistent watering and a great deal of oxygen to stay healthy. As a result, the plant needs a soil rich in organic material to hold water and coarse sand or gravel that lets it drain quickly enough to allow air back into the spaces between soil particles. If you have a clay soil or poor drainage, consider planting them in raised beds.

Rhododendrons need an acidic soil to grow well. Because the soils in our region tend to be alkaline, it is important to use a fertilizer specifically for rhododendrons and one that can restore acid levels. Apply fertilizer early in the spring before the flower buds begin to swell and again after the plant finishes flowering. Avoid fertilizing any later than early July so that new growth has time to harden up before winter.

Water rhododendrons generously throughout the growing season and especially when it gets hot in July and August and late in the fall before the ground freezes. Consider putting all your rhododendrons in one area of your landscape so you can water them efficiently but not over water other plants.

Provide afternoon shade for the plants. Their leaves are large for the most part and the hot sun can pull a lot of moisture out of them. If the soil is dry and the plant can’t keep up with the loss, the leaves begin to get a bit crispy very fast.

For basically the same reason, provide wind breaks from the winter winds (usually from the north). Rhododendrons do not lose their leaves in the fall and continue to function through the winter. If we get a cold, windy period, the leaves are still transpiring moisture. If the ground is frozen, then the roots can’t replenish the water and the leaves dry up and turn brown.

Wind breaks can be provided by fences, buildings, other shrubbery or burlap or wooden panels. Do not use plastic as a wrapping because the plant can’t breathe. You can apply sprays such as Wilt-Pruf in the fall to seal the leaves and reduce water loss.

One reason the PJM and Northern Starburst varieties are so hardy is that they have fairly small leaves and can roll up very tightly when it gets cold. By rolling them up, they reduce the surface area that can be affected by the drying winds and buy themselves some protection.

Plants that were damaged by last winter’s cold and are still showing some life may be salvageable if you act quickly. Rhododendrons do most of their growing right after they flower, which is right now. If you can catch them in the next couple of weeks, you can take advantage of this.

Start by cutting back all the damaged stems to just above a set of green leaves. If there aren’t any seemingly living leaves, cut it back to the main trunk and take your chances. The plant may be able to draw on some latent buds on the stem. New growth should appear within a few weeks.