(Editor's Note: This article was originally published on December 20, 2008. Your comments are welcome, but please be aware that authors of previously published articles may not be able to promptly respond to new questions or comments.)
One of the most popular broad-leaved evergreens for gardeners are hollies. Their glossy, deep-green foliage is appreciated all year. And for the Christmas season, nothing looks nicer than an arrangement featuring holly stems adorned with bright red berries. If you are into attracting birds to your garden, hollies are excellent as they are eagerly sought by robins and waxwings (much to the dismay of those who want to enjoy the berry display through the winter months!) Traditionally in Europe, the English holly, Ilex aquifolium, was the holly species of choice. In the U.S., the American holly, I. opaca is a similar counterpart, while in China, it's the Chinese holly, I. cornuta. Unfortunately, northern gardeners (zone 5 or colder) could not enjoy these trees and shrubs since the previous ones mentioned are mostly rated for zone 7 or mild areas of zone 6. There is a super hardy (zone 4) species from China called I. rugosa but unfortunately, that species is rather rangy with far less attractive foliage. Thankfully, holly are rather promiscuous and will readily hybridize between species. Plant breeders took advantage of this and created a cross between the hardy but inferior I. rugosa and the very attractive but more tender I. aquifolium. The resulting offspring had the best qualities of both parents; excellent foliage on a dense, compact, hardy (zone 4) shrub. These hybrids are commonly called the blue hollies or Ilex X meserveae, in honour of the creator of the first blue holly, Mrs. F. Leighton Meserve of St. James, New York, way back in the early 1960s.
At this point I should point out that hollies are what botanists call dioecious plants; plants are functionally either male or female. Both produce small, rather insignificant white flowers in late spring. If a male and female are close enough, the female flowers will be fertilized by pollinating insects, resulting in the red berries we associate with hollies. Males are very effective at producing pollen, so one male plant can ‘service' several nearby females. In fact, the males can service several closely related species, resulting in hybrids, of which the blue holly are an example.
Flower and leaf details from 'Blue Prince'
The earliest released blue holly were the cultivars ‘Blue Boy' and ‘Blue Girl', released in 1964. These earliest selections, while not as dense as later introductions, are still worthwhile plants. Both grow to 3 m tall and wide, but can be clipped to keep them smaller. Released in 1972 and 1973 respectively, were ‘Blue Prince' and ‘Blue Princess'. These are the standard blue hollies offered in the trade. Plants are very dense with deep-green, glossy foliage and attractive purple stems. Both may reach to 4 m in milder areas but rarely exceed 2 m at their northern range of hardiness. They may be clipped to maintain shape and denseness. ‘Blue Princess' is not only among the most productive berry producer of any holly species or hybrid, their berries are also among the darkest red. For something a little different, you can try ‘Golden Girl', whose fruit are yellow rather than the traditional red. More recently have come the selections ‘Blue Maid' and ‘Blue Stallion'. Both grow faster than ‘Blue Prince/Princess' and can reach even taller. They are reported to be hardier than the previous cultivars. ‘Blue Stallion' has the least spiny foliage of any blue holly selections. Perhaps the loveliest hybrid is ‘Blue Angel'. It resulted from backcrossing I. X meserveae to I. aquifolium. The resulting offspring look even more like English holly but alas, has inherited some of that species tenderness, thus is only reliable in zone 5.
Above are pictures of 'Blue Boy' and 'Blue Princess' (whole plant and fruit details)
Above are some details of 'Blue Angel', 'Blue Maid' and 'Golden Girl'
If variegated foliage is your preference, then try to obtain ‘Honey Maid', ‘Honey Jo' or ‘Gretchen', all which sport yellow-margin foliage. These are all females and produce red fruit, providing a wonderful contrast to the variegated leaves. The newest introductions, developed in Germany, are ‘Castle Wall' (male) and ‘Castle Spire' (female) which exhibit a fastigiate to pyramidal habit not unlike Hick's Yew. These selections grow 2-2.5 m tall but only 1 m wide. For limited space, you can try ‘Little Rascal', a dwarf which will reach 0.5 to 1.5 m. The foliage of this selection turns purple in winter.
Included among the blue hollies are hybrids between I. cornuta and I. rugosa. The two most popular selections are ‘China Boy' and ‘China Girl'. Both can reach 3 m and are perhaps even a tad hardier than the standard blue hollies noted earlier. They are certainly more heat-tolerant. A characteristic inherited from I. cornuta are their leaf margins which arch downwards giving the leaves a distinctive cupped appearance. Also included among these hollies is ‘Dragon Lady', a hybrid between I. pernyi and I. aquifolium sometimes referred to as I. X aquipernyi. This one has a strong upright, pyramidal habit and may reach 6m! To reiterate an earlier note, any of the male cultivars noted above will pollinate any of the above noted females.
Shown above are foliage details of the cultivars 'Honey Maid', 'China Blue' and 'Dragon Lady'
Cultivation is quite simple. Full sun is best for maintaining a dense habit and maximizing fruit production, but part shade is certainly tolerated. Although hardy to zone 4, avoid windswept locations. The soil should ideally be moist yet well-drained, with a slightly acidic pH. Uses include foundation plantings, informal hedges and spiny barriers. Hollies are surprisingly resistant to both insects and diseases. With relatively few broad-leaved evergreens suitable for northern gardeners, the blue hollies have become indispensable landscape plants and a much appreciated addition to our winter gardens.
I would like to thank the following people for the use of their pictures: chicouchi3 ('China Boy'), hczone6 ('Blue Maid'), growin ('Blue Angel') and TBGDN ('Blue Girl').