Leaves are green, right? And we’re all supposed to know why because we studied photosynthesis in high school. However, since many of us don’t remember everything we learned in school, here’s a quick and easy refresher.
Green cells carry on photosynthesis, the process plants use to convert sunlight and carbon dioxide into usable energy. So leaves have to be green. If they weren’t, the plant would die. At least, that’s what they told us in school. Except… they aren’t always green. In fact, they come in a variety of colors – yellow, purple, silver… and blue.
Let’s take a look at the latter. Of course, no plant has true blue leaves. The proper term for this foliage color is glaucous, meaning bluish-gray or bluish-green. It is caused by a waxy white coating called bloom covering green leaves. If you take a “blue” leaf and rub your fingers over it, the bloom comes off, revealing the underlying green. Buds and stems can also be covered in bloom. And what are blueberries but purple fruits covered with waxy bloom?
A Useful Coating
In nature, everything has a purpose. The most common “use” for bloom is to protect plants from excess sun. Visit a seashore, desert or mountain top – somewhere where nothing filters out the sun’s rays – and you’ll see many plants with waxy bloom. Filtering out the sunlight, it allows the plant to carry out photosynthesis with no risk of sunburn.
Bloom repulses insects. Studies show that predatory insects prefer blueberries when the bloom has been removed. And you’ve probably noticed that most blue hostas are slug-resistant. Bloom also creates a barrier against salt thereby protecting seashore plants from salt damage. But it wears off over time.
A Colorado blue spruce (Picea pungens ‘Glauca’) is bluest when its new needles first appear in spring. By winter’s end, it is fairly green. Blue hostas are also bluest in spring. By fall they too become quite green.
Plants exposed to rain lose their bloom faster than those protected from the elements. To keep blue hostas as blue as possible, plant them in the shade beneath trees.
Products that melt wax can also remove bloom. Spray a blue spruce with horticultural oil or insecticidal soap and it turns green – temporarily. In the new year it will again produce blue-green needles.
Contrary to popular belief, nothing can make a plant bluer than its genes intended. Beliefs that Epsom salts, rusty nails, aluminum sulfate and acid fertilizers make blue spruces bluer are urban legends. Don’t believe them!
Blue Foliage in the Landscape
Pale blue is a soothing color that can help create an intimate corner. Use it as a soft contrast to nearby dominant greens. It is most appreciated in deep shade where pale color adds the impression of sunlight piercing through overhanging leaves.
The Japanese often incorporate blue foliage into their landscapes to represent blue water or blue sky. And since blue flowers are so rare in nature, the famous blue borders seen in British gardens rely heavily on blue foliage.
Finally, if you want to create a desert-like environment or recreate a sand dune or pebble beach in your yard, blue foliage is a natural choice. That’s where you find it in nature.
Blue Plants to Discover
The best-known blue-foliaged plant is the Colorado blue spruce (Picea pungens ‘Glauca,’ Zone 2). It has dozens of even bluer cultivars, including many dwarf forms.
The concolor fir (Abies concolor ‘Candicans’) is a silvery-blue fir (hardy to Zone 4, USDA 4-7) and many junipers (Juniperus, most hardy to Zone 2 or 3, USDA 3-8) are blue or gray-green. These range from creeping ground covers to small pyramidal trees. There are even some fairly blue larches (Larix x eurolepis ‘Blue Weeping’, Zone 2 (USDA 4-6), is an example).
There are hundreds of blue hostas (Zone 3, USDA 4-8), ranging in size from only 25 cm (10″) in diameter to over 2m (6.6′)! They do best in shade to partial shade. Given too much sun, they too turn green.
For sunny spots, the choice is much wider. The oyster plant (Mertensia maritima), a Canadian native, is a low-growing creeper with powder blue leaves and true-blue bell-shaped flowers. A seashore plant, the oyster plant is salt-tolerant. Needing good drainage, it will even grow in sand or pebbles. 15x50cm (6×20”). Zone 1 (USDA 3-7).
Sea kale (Crambe maritima) looks like convoluted cabbage with thick powder blue leaves and honey- scented white flowers on branching stems. Originally a seashore plant, it is salt-tolerant. Try eating it – it tastes like cabbage! 60x60cm (2×2′). Zone 4 (USDA 5-9).
The cabbage leaf coneflower (Rudbeckia maxima) is a tall 1.2-1.8m (4-6’) prairie plant with steely blue leaves. Give it full sun in any well-drained soil. Zone 4 (USDA 5-9).
More Blue Plants
Many sedums have glaucous leaves, including Hylotelephium cauticola and S. dasyphyllum. Itsy-bitsy S. reflexa ‘Blue Spruce’ is the bluest of all with thick needlelike powder blue leaves and yellow flowers. For best results, give it sun and good drainage. Zone 3, USDA 5-9.
Rue (Ruta graveolens) is a charming perennial/medicinal herb with deeply cut blue-green leaves and frothy yellow flowers. Some people are allergic to its sap. Give it sun and average to poor soil. Zone 5.
Pinks (Dianthus spp.) are best known for their abundant, crimped, often scented flowers in white, pink and red, but they also produce clumps of grass-like leaves that are blue-gray to blue-green in color. Try Dianthus ‘Frosty Fire’, a Canadian hybrid with red flowers and silvery-blue leaves. It grows best in well- drained spots in sun. 10-15cm x 20-30 cm (4-6″ x 8-12′). Zone 3, USDA 3-9.
Many of the smaller bleeding hearts (Lamprocapnos spp.) also have blue-green leaves, especially the fem-leaf bleeding hearts ‘Candy Hearts,’ ‘Burning Hearts’, ‘King of Hearts’ and ‘Ivory Hearts’). They bear white or pink to red heart-shaped flowers all summer. 25-30 cm x 30-40 cm (10-12″ x 12-16″). Zone 4, USDA 3-9.
There is certainly no lack of blue-leaved grasses, as any visit to the seashore will demonstrate. Wild ryes (Leymus spp. and Elymus spp.) grow wild on sand dunes throughout Canada. Giving their all to preventing erosion. In culture, Eastern native Leymus arenarius ‘Blue Dune’ and ‘Glaucus’ are widely available. These spreading grasses will attempt to take over your county. Control them by planting tem inside a root barrier. Cities use these salt-tolerant plants along streets where road salt would kill off most other plants. 60-90 cm (2-3’) x indefinite. Zone 4, USDA 5-9.
Blue oat grass (Helictotrichon sempervirens) is like a giant blue fescue and requires the same growing conditions. 60-70cm (2-2.3’) x 60-90 cm (2-3’). Zone 4, USDA 4-8.
If you’re looking for something taller, Indian grass (Sorghastrum nutans ‘Sioux Blue’ 1.5-2m x 0.6-0.9m )[4.9-6.6′ x 2-3′], Zone 4, USDA 2-9) and blue switch grass (Panicum virgatum ‘Dallas Blues’ and other cultivars, 1.2- 1.5m x 0.8-0.9m [3.9-4.9′ x 2.6-3′], Zone 4, USDA 3-9), are non-invasive blue-leaved native grasses to try in sunny spots.
The very bluest annual is cabbage (Brassica oleracea). Some cultivars, like True Blue, are steely blue. Kale (also B. oleracea) can also be a blue color. Try growing some of these beauties in your flowerbed and you’ll be absolutely thrilled with their stunning color. Besides, cabbage comes with a bonus. You get to eat it at the end of the season!
Your grandma used to grow opium poppy (Papaver somniferum) for culinary and medicinal use (poppy seeds). Its huge, often double or deeply cut flowers come in a wide range of colors and its stupendous blue-green foliage has cut wavy edges. A snap to grow, just sow it directly outdoors in a sunny spot in spring or fall.
Go ahead, opt for blue plants. I guarantee you won’t be singing the blues!
Larry Hodgson published thousands of articles and 65 books over the course of his career, in both French and English. His son, Mathieu, has made it his mission to make his father’s writings accessible to the public. This text was originally published in Gardens Central magazine in September 2010.