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How to grow heavenly hydrangeas

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07/08/2002
How to grow heavenly hydrangeas

French gardeners are fanatic about hydrangeas. They flourish in gardens all over France (especially in Normandy and Brittany), and are available from florists as potted plants all year round.

Indeed, after the roses' flush of June flowering, mophead and lacecap hydrangeas reign supreme among flowering shrubs for the garden. With their sublime palette of purples, blues, pinks, and deep rose reds, and flowers that slowly age and deepen in color rather than outright wilting, these royal plants have enchanted plant collectors, breeders, and gardeners for hundreds of years. Add to their flower power the fact that, as shrubs, they are never gawky and always fully cloaked with foliage, right down to the ground, and you have plants for which almost every gardener lusts.


Yet perhaps no other popular flowering shrub is the subject of so much confusion and misinformation about the proper way to grow it--and especially, how to get that lusted after blue--rather than pink--color. If you're unfamiliar with growing hydrangeas, you may think the color is a matter of which cultivar you choose. And while a handful of varieties exist which remain pink or rose red no matter how you manipulate them, most hydrangeas act sort of like litmus paper, but in reverse. Grown in acid soil (pH below 6) and properly nourished (see below), most hydrangeas will be blue. In the pH range of 6 to 7, they will exhibit a range of lavenders or purples. At neutral and above, the flowers of the very same plant will be pink or rose red.



This "reverse litmus" effect can be observed among the flowers on a single plant (above left) or even on a single flower (right). Not only mophead varieties of bigleaf hydrangea (H. macrophylla) but also lacecaps and cultivars of other species such as the Hydrangea serrata hybrids exhibit this behavior.

This seemingly magical color alchemy is not as simple to control as is often represented in gardening articles and books. In fact, much more is involved in getting that precious blue to appear than merely creating an acidic soil. In fact, the blue color appears in the presence of available aluminum in the soil. The acidity plays an important role because the aluminum assumes an available form only under acidic conditions. In neutral to alkaline soil, it is bound up with other atoms in a form unabsorbable by your hydrangea.

To complicate matters even further, if you have an abundance of phosphorus present in your soil, the phosphorus will displace the aluminum, making it less available to your hydrangeas. The lesson? Avoid high-phosphorus fertilizers for your hydrangeas.



While the relative pink-blue spectrum of your hydrangea is controlled by soil pH and aluminum, the actual intensity and tone of the colors are determined by the cultivar. While some are so intensely blue (below) as to be almost neon in their effect, others (left) are very pale pastel.

Just what fertilizer should you use for beautiful blue flowers? If non-organic is not a problem for you, use a granular or soluble fertilizer specifically formulated for hydrangeas. If you can't find that, use one formulated for rhododendrons and other acid-loving plants. If your hydrangea refuses to turn blue even though the soil pH is at 6 or below, you can treat it with aluminum sulfate, to ensure that aluminum is sufficiently present in your soil. Use caution, however, as too much aluminum quickly becomes toxic, even to a hydrangea. Do not exceed dosages recommended on the fertilizer package.

Organic gardeners, you should use aged cow manure to feed your hydrangeas. It is the only manure that is mildly acidic. Supplying aluminum organically is trickier. In France, powdered slate (the stone used to make traditional roof shingles) is used. (I am working on sourcing this product in order to be able to offer it on this site.) It contains a slow-release, organic form of aluminum sulfate. That's why hydrangeas planted next to slate-roofed houses are always a brilliant blue; the rainwater runoff from the slate supplies plenty of aluminum in just the right, gentle doses.

If your soil pH is above 6, you will need to lower it. The best way to do this (also okay for organic gardeners), is to incorporate wettable sulfur into the soil. To decrease the pH of a sandy soil by 1 pH point, incorporate 1.2 ounces of ground sulfur per square yard. For all other soil types, incorporate 3.6 ounces of ground sulfur per square yard.

What do bigleaf and serrata hydrangeas need to grow and flourish in your garden? They need a soil that is neither too heavy nor too light. In extremely heavy, poorly drained clay, the fibrous root system of hydrangeas can be prone to rot. On the other hand, hydrangeas in general need lots of water, so they will never flourish in sandy soil. Whatever your soil type, amending with lots of organic matter--rotted cow manure, compost, or leaf mold--will ensure the right soil environment for your hydrangeas.

Hydrangeas will tolerate full sun quite well in cooler areas of the country, if kept well watered. In the northern extremes of their range (Zones 4b to 5), they in fact require sun in order to produce enough flower buds. In the South (Zones 7-10), plant them in at least partial shade. In all cases, try to avoid so-called "frost pockets", or low-lying areas that stay colder than their surroundings. Likewise, plant hydrangeas in a site where they are protected from prevailing winter winds. A large conifer at their backs can provide shade, windscreen, and mild soil acidification all at once.

With their abundance of large leaves and huge flowers, hydrangeas transpire lots of water. Except in rainy, maritime climates, they will undoubtedly require regular irrigation. The best way to accomplish this is with a drip irrigation system. Do not use overhead sprinklers, as this will enormously increase floral and leaf diseases. Alternatively, water by hand at the foot of the plant. Just as with roses, try to wet the foliage as little as possible. Monitor your hydrangeas' water needs carefully; once allowed to wilt, they scorch easily and take a long time to recover and look good once more.

One of the difficulties with all this watering is that in many parts of the country, the water is alkaline. This raises the soil pH, and, well, you can imagine the consequences. The advantage of watering by hand is that you can add a couple of tablespoons of vinegar to your watering can. This simple means of acidification also works very well for hydrangeas grown in pots. For automatic irrigation, acidifying agents are available that can be injected into the system.

Besides how to turn bigleaf hydrangeas blue, the biggest source of confusion is pruning. This especially true in the Midwest, where gardeners are used to growing Hydrangea arborescens, a plant that flowers on new (current year's) wood and which is best pruned to ground level in early spring. Midwestern gardeners treating bigleaf and serrata hydrangeas the same way will be dismayed to find that their plants produce no flowers. That's because these species produce flower buds on the previous year's wood. So, if you prune them down in early spring, you've effectively removed all those potential blossoms.

In the northern parts of their range, bigleaf and serrata hydrangeas need little pruning except for removal of deadwood and spent blossoms. Pruning out wilting flowers diminishes chances for disease and greatly increases the vigor of the plant, no matter which part of the country you garden in. And you don't need to wait for the flowers to wilt; don't hesitate to cut them earlier and bring in for fresh or dried bouquets.

In zones where hydrangeas grow extremely vigorously (mostly maritime zones), you may occasionally thin out the growth by cutting out a few of the oldest branches at ground level in early spring.

North of USDA Zone 6, growing hydrangeas gets to be dicier. Actually, you can grow the plants fine; it's just getting them to flower that's tricky. This is because the flower buds of bigleaf hydrangeas are not well protected by budscales. They are very vulnerable to cold damage in winter and in particular, to the drying effects of winter winds. Siting your hydrangeas in a protected spot is crucial in these zones.

If you garden north of Zone 6, you need to pay careful attention to the variety you choose. As a rule of thumb, the glossier its leaves, the less cold-hardy the hydrangea. Secondly, the cultivars and hybrids of Hydrangea serrata are a better bet for cold-hardy flowerbuds than the macrophyllas. (They're also more drought-tolerant). Finally, there are a few cultivars of Hydrangea macrophylla that will flower on new wood, and so are excellent for northern gardeners. These include 'All Summer Beauty', 'Sadie Ray', 'Dooley', and 'Hamburg'. Good serrata types for the North are 'Benigaku' (very hardy), 'Blue Bird,' 'Blue Billow', 'Diadem', 'Blue Deckle', and 'Grayswood.'



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Products of Interest:
Champagne shrub pruners
Provenal garden and transplant spade

Classic Provenal oval vase

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