WSU Clark County Extension

Photo collage of trees and WSU Master Gardener Plant Sale

Garden Mastery Tips
WSU Master Gardeners of Clark County

January/February 2006

Hydrangeas


Photo of hydrangea in garden

Hydrangeas, with their gorgeous large blooms and lush healthy foliage are easy to grow in Southwest Washington. Our temperate climate, abundant rain and naturally acidic soil provide the ideal habitat. These long-flowering and low-maintenance shrubs are hardy in zones 5 to 9 and often have a 50 year life span. They can complement any garden, large or small.

The plants we see most often are different seedling forms of hydrangea macrophylla, commonly known as mopheads and lacecaps. Both forms originated in the high humidity and acidic soil conditions of the wooded and maritime provinces of Japan and China. The large, dome-shaped mopheads found favor in 19th century France where they were cultivated from forms imported from the Orient. Lacecaps were likely several steps removed from natural hybrids and also found their way to European hybridizers. Both varieties seem to be of mixed parentage, making an accurate genesis impossible.

Hydrangeas are excellent garden plants. They come in a wide range of sizes. Dwarf varieties (as small as 18’ in height and width), a few varieties of climbers (up to 50’ high) and plants between 3’ and 10’ are readily available from local nurseries. Numerous landscape possibilities include siting under high cover trees and in combination with rhodies, azaleas, and fuchsias which have different bloom times. Small moisture and shade loving plants such as heuchera and hosta will do well in proximity to hydrangeas. Planting a bed or border of plants of the same variety or using them as a hedge in a shaded area can be useful in a larger garden. With careful selection and location of plants, I have combined eight hydrangeas with all of the above mentioned varities amid 170 roses.

Photo of hydrangeas in vase with other flowers

Flower color of many hydrangeas is controlled by the availability of aluminum and the acidity of the soil. Normal rainfall in the Pacific Northwest results in acid soil. A pH of 6.0 to 6.5 is sufficient to develop blue flowers. A lower pH will stimulate more intensely blue flowers. Hydrangeas producing pink, purple or red blooms will still produce their usual color with a ph of 6.5 to 7.5. White hydrangeas will remain white. The central fertile flowers of lacecaps may vary in color. A soil analysis will determine aluminum deficiency. A judicious one-time application of aluminum sulfate per growing season can be added to turn flowers blue. Early spring is the best time to apply. A word of caution – follow directions on the package as an excess of aluminum damages plant roots. If the spring application does not work, another one can be added in late fall, but then skip the following spring.

Hydrangeas will grow in most soils, but are happiest when moist and well-drained. When planting in heavy soil, dig a 24’ deep hole and add 6’ of rich organic material to the bottom. The hole should be 3 times the width of the root ball and when back filling, use one half of the original soil mixed with an equal amount of organic material. Thoroughly water the plant and add compost to fill in any low spots. Create a slight mound around the root ball to avoid water collecting. If more settling occurs during future watering, add a bit more compost. In our area it is best to defer planting until the last chance of frost has passed. Also avoid planting in severe heat (higher than 80 degrees). Hydrangeas prefer early morning and late afternoon sun. Prolonged mid-day sun stresses most plants and will scorch leaves and damage blooms. When replanting, wait until the plant has finished blooming in the fall. Prune and dig a large root ball with lots of roots before it becomes dormant. Most of the feeder roots are near the surface. Replant immediately and water well. Hydrangeas can be readily divided and replanted successfully.

Hydrangeas need little or no feeding depending on soil and naturally occurring nutrients. Plants bloom best when slightly stressed. High level nitrogen fertilizers inhibit blooms on many varieties. Heavy green foliage with few blooms indicates that excess nitrogen is being applied. The most effective way to feed these plants is to apply a dry organic fertilizer in early spring and again in the fall. Look for a formulation that is roughly N-5 P-7 K-2. Some of these organics will also contain beneficial soil microbes, mycorrihizae, calcium and sulfur. By their nature these blends are slow release products and so minimize any negative effects nitrogen has on blooms. P-7 (phosphate) and K-2 (potash) encourage budding and blooming. If sulfur is present it adds slightly to the acidity hydrangeas crave.

Hydrangea in the fall
Mophead Hydrangea in the fall

Hydrangeas in Southwest Washington are minimally affected by most pests with the exception of deer which savor the young, tender green buds and foliage much as we might a salad. Wire cages or fences are the most effective defense. Deer can devastate unprotected plants in a day or two. Slug and snail damage can also be a problem as they chew extensively on new foliage. There are organic baits that are very effective and will not harm children, pets or other wildlife. Aphids, thrips and spittlebugs are attracted to soft new growth on some plants but do minor damage. All three of these insects may be washed off with a strong jet of water or sprayed with a solution of 1 quart of lukewarm water and 1/3 teaspoon of liquid castile soap. Powdery mildew may appear during periods of warm days and cool evenings and may be quelled by applying a few ounces of skim milk in a quart of warm water sprayed on just the affected area. Occasionally blackspot will affect hydrangeas. Just cut off the affected leaves. The same holds for rust. The only damage I have experienced has been from slugs and snails and I have learned that by treating the problem early in the year (January or February), the garden can be free from these pests. Frogs, toads and geese also help contain them.

Most older hydrangea varieties bloom on ‘old wood.’ Pruning severely means fewer blooms the following season. There are some new varieties that bloom on both new and old wood resulting in blooms twice during the summer. The purposes for pruning hydrangeas are to keep them appropriately sized for your garden and to remove dead or diseased wood. Both of these purposes can be accomplished by removing some of the oldest blooming wood at the base of the plant just after the bloom has faded in the fall. If you choose to prune in spring, dead wood is more easily identified and removed. After the last hard frost it is possible to reduce branch size by cutting back to just above the third leaf node that is forming. You may wish to remain on the safe side and wait for leaves to show so you don't prune back too far. Repeat bloomers may be cut for size in either spring or fall.

Resources

JoyCreek Nursery, 20300 NW Watson Road, Scappoose, OR 97056 (503)543-7474 http://www.joycreek.com

Lawson-Hall, Toni and Rothers, Brian, Hydrangeas, A Gardner's Guide.

Dirr, Michael A., Hydrangeas for American Gardens. Timber Press, Inc. Timber Press, Inc.


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