Hydrangeas are woody shrubs brought back from Japan and introduced to the American scene sometime in the 1730s. A few Frenchmen started raising seedlings in pots and discovered that while the plant was young, it would produce large mop head blooms that enchanted gardeners and onlookers and became wildly popular. Some hydrangeas are special in that, unlike other plants, the color of their blooms can change dramatically depending on their growing conditions.
Hardy in zones 6 through 10, hydrangeas tend to be a bit fussy about where they live. The color is in the soil.
Varieties of Color
If you have a white hydrangea, appreciate it for what it is. No matter what you do to the soil, it will remain white except for a touch of pink or deep red that sometimes appears on the florets as it ages.
There are, of course, pink varieties and blue varieties of hydrangeas that can be purchased from nurseries and growers. Even a pink hydrangea will lose its lovely pink color if the soil is not just what it needs.
TIP: Our expert gardening advisor Rachel Kline adds, "When grown in alkaline soil, the blooms are pinker. In acidic soil, the bloom color is bluer. Even if you purchase a hydrangea in bloom, there is no guarantee that it will bloom the same color once planted in your garden."
Depending on the variety of hydrangea and uncontrollable weather patterns, the intensity of color varies from plant to plant if not head to head. There’s nothing you can add or or take away to achieve a more vibrant color. Fertilizing once or twice a year boosts the health of your hydrangea, which in turn may give it a bit more brilliance.
Adjusting the Color
Let's say you have a pink hydrangea and want it to bloom in shades of blue. What do you do? Keeping a pink hydrangea blue in an alkaline soil requires constant attention to what’s going in the ground around the plant. As soon as soil conditions revert back to the natural state for your area, it will revert back to a pink color.
Blue hydrangeas require aluminum in the soil for them to bloom the color of a newborn’s eyes. Hydrangeas take up aluminum in the soil more easily when the pH is low. Check the pH level of the ground around the hydrangea plant. For a blue flowering hydrangea, aim for an acidic pH level of 4.5 to 5.5. Get advice from the a reputable nursery concerning the best ways to test soil.
TIP: Rachel suggests, "If your pH is very high, you can spread garden sulphur around the hydrangea at the rate of 1/2 cup per 10 square feet. You can also add organic matter to the soil to reduce pH. Supplement soil with coffee grounds, grass clippings, pine needles, or fruit and vegetable peels. The aluminum sulfate you add next will also lower the pH. It is important to have your water tested as well, so it doesn't undo the work you've done to maintain a low pH level. Your water should have a pH no higher than 5.6."
Add aluminum sulfate to the soil around a plant that is at least 2 to 3 years old. You can purchase aluminum sulfate at garden stores. Follow the directions exactly and measure carefully. Authorities usually recommend a solution of 1/2 ounce (one tablespoon) aluminum sulfate per gallon of water to be applied to the plants once a month throughout the growing season. Make sure plants are well hydrated before watering with aluminum sulfate. Too much aluminum sulfate can burn the roots of your plant. Fertilizers will aid in keeping the soil at the pH level needed, but most fertilizers contain a mixture of various nutrients and minerals with large amounts of phosphorous and nitrogen. Both elements create fabulously pink hydrangeas, but a desire for blue points us in another direction. Use a low phosphorus preparation, such as 25/10/10 (the middle number indicates the phosphorus), or a single element fertilizer like potash or muriate.
If you've done your homework, followed the directions, and added needed elements to the soil, your hydrangea should be ready to begin its chameleon change from pink to blue. You won’t wake up the day after adding aluminum sulfate to the soil and find a miraculous change from pink to blue--it takes time. If you let the soil go and stop paying close attention, it will revert back to its alkaline natural state with pink hydrangea blooms dancing on the stems.
Other factors come into play that affect the colors, such as weather, amount of sunlight, and genetics. Even after following all the guidelines and suggestions, you may find that your hydrangea gleams purple in the light of day. An easy solution to the dismay of not being able to produce blue hydrangeas in your garden is to plant them in very large containers where keeping the soil at the perfect pH level is less taxing.