Growing Blue Hydrangea
Keeping Blue Hydrangea Blue Presents Special Challenges, Especially in Zone 4
Those captivating blue hydrangea are so tempting. New varieties that grow in zone 4 are being sold by nurseries everywhere up here, and they are gorgeous. At least in the nursery they are gorgeous. Get them home and plant them and you may discover that you have pink or even pale blooms of both colors. Even worse, after winter you could find that it has died back to the ground or will not bloom. Now I won’t pretend that it doesn’t take some work to keep a blue hydrangea blue and to keep it blooming in zone 4, but it can be done. You need to be sure you can make a commitment to the plant’s care though.
Blue hydrangea (and of course the pinks, which I don’t mean to ignore) are cultivars of the Hydrangea macrophylla family. Many of the hydrangea that we easily grow in zone 4 are Hydrangea arborescens such as ‘Annabelle’ or Hydrangea paniculata such as ‘Pee Gee’ and ‘Tardiva’. These even do well in zone 3. But the Hydrangea macrophyllas, commonly known as “mophead” hydrangea really only thrive up to zone 5 or even 6. Recently growing zones have been changed to reflect warmer conditions. But we are currently into a winter that is considered more “normal”, in other words NOT a zone 5 winter. Marginally hardy plants may be lost this year.
Getting and Keeping Them Blue or Pink
But first let’s address the issue of keeping them blue, or pink as the case may be. Before you even consider a blue or pink hydrangea, get yourself a soil test kit. You need to know the present pH of your soil to determine if you can even alter the pH enough. The blue prefer an acid soil of 5.2-5.5, and the pink prefer alkaline to neutral, a pH of 7.0 or higher. You should also look for the level of aluminum in your soil. The blue blooms require aluminum to be blue, and if it occurs naturally in your soil the plant will be able to absorb it easily. That aluminum is part of what makes your soil acidic. Your soil test should also tell you the level of phosphates in your soil. High phosphate can limit how well your blue hydrangea can absorb the aluminum. It may take a couple of seasons to get your color where you want it due to transplant shock and working with you soil pH. Keep that in mind when selecting a fertilizer for your plant too, get a low phosphate (the second number in the N-P-K).
If your soil is highly alkaline, it probably will not be possible to alter the pH enough to grow a blue hydrangea. Aluminum added in high doses can be toxic. But if the soil is slightly acidic you should be able to lower the pH slightly to appropriate levels. Natural organic materials is the best way to make minor adjustments. Oak leaves, evergreen conifer needles, peat moss and even coffee grounds can be dug into the soil. This will also improve your soil texture. If greater reduction of pH is necessary you can use ammonium sulfate or aluminum sulfate, following the package directions. Elemental sulfur can also be used. In good garden soil add 1 1/2 to 2 pounds per 100 square feet and mix well. If you have clay soil, add the full 2 pounds. Sandy soil requires less, but hydrangea don’t care for sandy soil, so now you have added yet another battle to your fight. After adding amendments, you will not necessarily see an immediate pH change. Over time you should see the level drop, and over more time you will see it rise again. Soil always tends to revert to its’ natural pH level as the amendments you added are consumed. You can see that tending to the pH level is an ongoing event. You will need to test your pH at least annually and add amendments as needed, probably every year or more.
If your soil is very acidic, it is probably not possible to grow pink hydrangea. If is neutral to only slightly acidic you should have reasonable success in raising the pH slightly. As above, you will have to be diligent in testing and modifying your soil. Wood ash is a natural soil amendment that can quickly raise the soil pH. Apply sparingly, it is highly concentrated and too much can not only drastically alter pH but can also burn plants. Dig no more than 2 pounds per 100 square feet into the soil well in fall, then retest the pH in spring. You will have to monitor and reapply every one to three years. Garden lime is most commonly used to raise pH. Dig it in well in fall, as it is much slower acting than wood ash. In good garden soil add 7 or 8 pounds per 100 square feet, if your soil is clay add 8 to 10 pounds. In sandy soil you will only need 3 to 4 pounds, but again, hydrangea don’t care for sandy soil, so yet another battle on your hands.
A few other details about maintaining a constant pH
Using city water can change the pH of your soil, generally neutralizing acidity. Put up a rain barrel.
For blue hydrangea fertilize with a low nitrogen acid fertilizer.
For pink hydrangea fertilize with a low nitrogen and low or no phosphate fertilizer.
Mulch blue hydrangea with cottonseed meal, pine needles or bark, or shredded oak leaves.
For blue hydrangea sprinkle 1/4 cup of 15% concentrate aluminum sulfate around each well established plant and water well in early spring at the first sign of new leaves, and again in six weeks. If the color is not as blue as desired, apply again in fall.
For pink hydrangea sprinkle one pound of lime for every ten square feet and water well once or twice a year.
Grow in a container to make it easier to achieve and maintain appropriate pH.
For pink hydrangea, a high phosphorous fertilizer to inhibit uptake of aluminum.
pH adjustments for bloom color are only effective well ahead of blooming.
Rusty nails, or any rusty metal, buried around the hydrangea will add very minor traces of aluminum to the soil, helping to keep them blue. Pennies can also be used. Here again though, the trick is to have a low enough pH for the plant to be able to absorb the aluminum. And I am not necessarily convinced that enough alum leaches in the soil to make a difference.
Growing in the Cold
There are a number of things working against successfully growing blue and pink hydrangea in zone 4
Hydrangea macrophylla are not fully hardy in zone 4. In zone 4B you may have a better chance. This means that the plant could perish in winter or at least die back to the ground.
Some of them only bloom on old growth, so if it dies back you will not get blooms that season.
New cultivars that bloom on new wood may not have a long enough growing season in the north to produce blooms if they have died back to the ground over winter. Or you may get one quick flush late in the season.
Many break dormancy very early and a late frost will kill the new growth and buds.
There are also a number of things you can do to help
Mulch heavily in November and do NOT remove mulch until all danger of frost has past. Leaving the mulch on in spring will keep the roots cold enough to prevent growth. I would apply winter protection similar to roses, in other words, mound soil up around the base and maybe even erect protection for the branches.
Plant them slightly deep to protect the rootball through winter.
Plant in ideal conditions: evenly moist soil and moderate temperature by protecting from hot afternoon sun. Six hours or more of morning sun is best, but in the north they can often handle more sun.
Plant in a sheltered location protected from winds, eastern exposure is usually the best.
Do not prune in the first couple of years other than removing dead or broken branches. When you do prune, make sure it is immediately after blooming.
Use an appropriate fertilizer as noted above that is low in nitrogen. Apply only in spring and summer, ending by mid August. Never encourage new growth in fall. Better yet, just dress with compost generated specifically for the needs of your hydrangea.
If your plant does die back to the ground, fertilize heavily and water adequately in spring to encourage fast growth. A higher nitrogen and phosphorous content may be appropriate in this instance to encourage the growth. If it grows quickly enough there will still be plenty of time for blooming. But only heavily fertilize early in spring. Over fertilizing will result in good leaf production and few flower buds. The same goes for over watering.
Once well established and blooming each season, deadhead faded blooms to encourage additional blooming. Although time may run out to finish additional blooming. Do not hard prune like an Anabelle, only prune minimally to shape or retain size.
What to Buy
When you go to buy your blue or pink hydrangea, DO NOT buy the beautiful bright blooms in a pot wrapped in pretty foil. They are not garden appropriate. They were grown in a hot house for indoor plant use. Go right back to the nursery and find the ones potted with the shrubs.
If the shrubs are blooming, the blooms will not necessarily stay the same color once in your soil of course. But I would tend to start with the color I want so the color change is gradual to what your soil will produce. It may take longer to see just what your soil will produce, but enjoy your perfect color if only briefly.
If you are buying for zone 4, your selection will be very limited. Likely you will only find Endless Summer varieties. But these were developed by Bailey Nurseries in Minnesota, so they are probably your best bet anyway. If you see other varieties you are interested in be sure to have a serious discussion with the nursery staff before buying. If you read this, you may already know more than some nursery staff. If you do know more or they give you “wrong” answers, move on to another nursery. Don’t be led astray, get the very best plant with the best chance of success.
Good luck! And for the best color results, get going on testing and preparing your soil ahead of time if you can.