March/April 2002 Excerpt

Excerpt from Clematis for American Gardens by Edith M. Malek

Not for nothing are clematis (Clematis spp.) known as the "queen of climbers." They have delicate, finely sculpted foliage, ornate flowers in a multitude of colors and shapes, and many produce interesting and attractive seed heads to close out the year. They are among the most coveted of plants, but their reputation for having demanding cultural requirements has discouraged many American gardeners from giving them a try. Based on my experience with the "queen of climbers," however, they can and should be far more widely grown.

Books on clematis today-most written by authors based in the United Kingdom-still include much of the same basic cultural information in vogue a century ago. Clematis, we are told, need to be planted in organic rich, alkaline soil that drains freely but stays consistently moist during the growing season. They don't tolerate heat well and should be planted so that their roots are shaded but their stems and foliage will receive part or full sun.

These instructions are useful as general guidelines, but they don't necessarily hold true for all clematis selections and in all regions of our diverse country. For instance, I have found that many clematis can be grown quite successfully in neutral to slightly acid soil (down to a pH of 6.5). The reason so many books stress that clematis need alkaline soil is that in Europe clematis were usually found growing in the wild in soil derived from limestone or chalk rock. Since such soils are generally alkaline, it was assumed that clematis needed an alkaline planting location. However, the high level of rainfall most of the United Kingdom experiences helps to quickly leach away the lime present in the soil.

Few areas of the United States have the temperatures and types of rainfall enjoyed in the U.K. Growing clematis in a neutral to slightly acidic site is particularly important for gardeners who live in areas-such as the Southwest-where rainfall is seasonal and accompanied by high temperatures. In regions of low rainfall where the soil already has a high concentration of lime, gardeners may actually need to reduce the alkalinity of the soil by applications of gypsum or elemental sulfur.

EXPOSURE AND HEAT TOLERANCE

And while the oft-heard advice for clematis is "Plant their heads in the sun and their feet in the shade," I have found that most grow equally well where their roots are in full sunlight or in shade. Shading clematis roots with other plants that have aggressive roots can actually stunt the clematis's growth because these plants tend to compete for water and nutrients.

The assumption that clematis are not heat tolerant has led people to believe that they cannot be grown in relatively hot locations. While I wouldn't recommend trying to grow clematis in Death Valley, some selections can be successfully grown in warmer regions of the United States such as those that fall into AHS Plant Heat-Zone 10 and 11. For these especially hot locales I suggest trying the many different small hybrid viticellas, such as 'Royal Velours', 'Venosa Violacea', and 'Etoile Violette'-or the large hybrids such as 'Victoria', 'Star of India', and 'Ernest Markham'. In such warm regions, my advice is to plant these select clematis in filtered or dappled sunlight to help protect them from excessive heat.

GROWING ZONES

One of the problems with relying on references written by British authors is that their experience is in a climate similar to our Pacific Northwest. Thus, little is really known about how clematis fare in our 11 very unique cold hardiness and 12 heat tolerance zones.

Since much of the United States is located closer to the equator than the United Kingdom, plants here are subjected to much greater light intensity. In many parts of this country, we experience far less precipitation than the United Kingdom, as well as higher average temperatures. On the other hand, our northern and mountain regions are subject to harder frosts and longer winters. All these variable environmental conditions have a major effect on how clematis will fare.

The American Clematis Society (ACS) (see "Resources" on page 35) is currently seeking funding to conduct a scientific study of where different clematis will grow in North America. In the meantime, ACS members are being provided with annual charts to help keep track of where different clematis are growing successfully.

PROVEN PERFORMERS

Nearly 300 clematis species have been identified and another 1,000 or more cultivars have been named since Western botanists first classified the genus in the mid-16th century. Many clematis have come and gone since then, but the following 10 selections-most of them over 100 years old-have endured the test of time and are reliable performers for American gardens. Based on my experience, all these clematis can be successfully grown in USDA Plant Hardiness Zones 4 to 10 and AHS Plant Heat Zones 10 to 5.

Clematis 'Ernest Markham'
Although not a centenarian quite yet, 'Ernest Markham' deserves special mention because it is one of the strongest and easiest-to-grow clematis. Its four- to six-inch flowers are a showy, glowing magenta with prominent creases running the length of each of the six tepals. This selection produces flowers on both old and new wood and bloom time can be extended from early summer through early fall if vines are lightly pruned. It grows to a height of six to 10 feet and thrives equally in sunny or shady locations.

Clematis 'Victoria'
Like 'Star of India', 'Victoria' is a cross between C. lanuginosa and C. 'Jackmanii' introduced in 1867. Although not quite as stunning as many of its relatives, C. 'Victoria' is a reliable workhorse that rewards growers with vigorous blooming over many years. Its five- to six-inch flowers are a soft rosy purple with a quilted texture and a pinkish bar running along the center of each sepal. Flowers form on new growth from midsummer to early autumn, so any major pruning should be done in late winter or very early spring. It grows anywhere from eight to 15 feet and is an excellent choice to cover an arch. It thrives equally in sunny or shady locations.

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