Actually, the title above is a total lie: you can’t really make a blue spruce any bluer than it is naturally. None of the tricks promoted on the Web and over garden fences on how to make a blue spruce bluer actually work. That includes adding aluminum sulfate to the soil, pounding rusty nails into the ground at the tree’s base and watering with Epsom salts.
The truth is that the intensity of the blue of a Colorado blue spruce (selections of Picea pungens ‘Glauca’) depends on the concentration of its “bloom,” a powdery white wax that coats its needles. (Note: as used here, the word “bloom” has absolutely nothing to do with flowering!) The needles are, in fact, completely green, but the bloom that covers them gives them a blue appearance. The bloom on spruce needles is exactly like the bloom on a blueberry, a grape or a plum: you can rub it off and see the original color underneath.
Some cultivars of Picea pungens produce a lot of bloom, others almost none. Therefore, the coloration is determined almost entirely by the plant’s genetics and only very little by its environment. So some cultivars are naturally powder blue, others are blue gray and still others are completely green. That’s just the way they are and you can do nothing to change that.
How to Choose
For the very bluest spruce, simply choose the bluest specimen in the nursery. If you’re looking for a tall blue spruce, cultivars ‘Hoopsii’ and ‘Edith’ are among the bluest. Among dwarf, semi-dwarf and miniature varieties, consider ‘Sester Dwarf’, ‘St. Mary’s Broom’, ‘Globosa’ or ‘Lundeby’s Dwarf’ … among others. Already you are very close to your goal.
Be forewarned that, if the label of the plant only shows Picea pungens ‘Glauca’, that is, has no specific cultivar name, it means that it was probably grown from seed and its color will therefore be unreliable. That’s because blue spruce does not come true from seed and even the seeds of the bluest spruces give young plants in a wide range of colors. Yes, named cultivars, produced by grafting a very blue spruce onto a young spruce grown from seed, cost a lot more than no-name spruces, but at least you get what you paid for!
The Right Environment
Of course, the environment does play a certain role. After all, blue spruce grown under poor conditions will be dead and brown rather than blue or green!
First of all, be aware that the Colorado blue spruce is a cold-climate conifer (zones 2 to 6, sometimes 7) and will not do at all well in regions with mild climates. In zones 7 to 9, you might want to consider a blue atlas cedar (Cedrus atlantica Glauca Group), as it doesn’t require (or want) cold winters.
Secondly, to keep your blue spruce as blue as possible, also give it the best growing conditions you can. For example, plant it in full sun in well-drained soil that is not too rich. Full sun and soil not overly rich in nitrogen give needles a bit shorter than usual … and bloom will be a bit more concentrated on shorter needles.
Also, although bloom is waxy and therefore theoretically insoluble, rain does wash some of it free, so spruces growing in regions with fairly dry summers will remain blue longer than those growing in rainy regions.
Finally, avoid spraying your spruce’s foliage with soapy water or horticultural oil (two products often used as pesticides), as soap and oil will melt bloom and turn even the bluest blue spruces green. Of course, new growth produced the following spring will be blue again, but it can be quite a shock to see your tree go from blue to green overnight!
Color Fades… But Is Then Renewed
Bloom eventually wears off, so even the best cultivars are bluer in spring and early summer than in fall and winter. You’ll especially notice the contrast in spring when the new needles expand and are clearly much bluer than the old ones.
Despite all of the above regarding providing the right conditions in order to maintain a blue spruce’s color, the fact remains that it’s the choice of an exceptionally blue cultivar that is by far the most important factor in your quest for a truly blue spruce!