Featured Plant of the Week (Hydrangea)
Hydrangeas are a genus of plants that consists of about 70 species. Lacy flower heads of blue, pink, purple, and white blooms characterize the plant, which comes in variety of forms. Depending on the species they may be shrubs, trees, or climbers that hitch a ride on taller hosts. But what is amazing about hydrangeas is that their flowers can change color over the course of their blooming period. What's more is that it's possible to control this color change and make the flowers change to pink or blue, or anything in between!
Hydrangea is native to the Americas, and Asia where they show the most diversity and where the majority of the species originate as well. The species most people are familiar with is Hydrangea macrophylla, which is commonly called Big Leaf Hydrangea, or French Hydrangea. Despite what the name implies this species is actually native to Japan, though it's now cultivated around the world.
Hydrangea macrophylla cultivars fall into two broad categories: mopheads and lacecaps. Mopheads have large dome-like inflorescences that resemble pom-poms, or as the name implies the head of a mop. Lacecap flower heads are flatter, and consist of a core of diminutive fertile flowers surrounded by a ring of larger but sterile blooms. While they are usually sold as potted plants, they grow to become large shrubs with long woody stems.
The unique trait of changing colors is a feature of mainly H. macrophylla. The blooming flowers of these hydrangeas can change from pink to blue, or vice versa based on the conditions of the soil. With some effort these conditions can be controlled, though the intensity or paleness of color is largely down to cultivar. Also, hydrangeas can only be induced to change blue or pink. White hydrangeas can't be changed, and true red is usually not possible (usually just a very dark pink).
The main way of forcing the color change is to adjust the pH of the soil the plant is in. pH is a measurement of the acidity on a scale of 0 to 14. It's a bit counter intuitive, but the lower the pH the more acidic something is, while the higher the number the more 'basic' (or alkaline) the substance. Pure water is 7.0, smack in the middle and totally neutral. When the soil is made more acidic (the pH is lowered), the hydrangea will turn blue, and when it's made more basic (the pH is increased) the plant will turn pink instead.
The reason pH changes the color of the flowers is because hydrangeas are what scientists call 'hyperaccumulators.' That means they soak up more 'contaminants' from the soil than other plants, in this case the important one being aluminum. When a hydrangea's potting medium is high in aluminum the plant absorbs it and gives it the bluish hue. When the soil is low in aluminum it turns pink. By controlling the pH of the soil, the amount of aluminum can therefore be controlled. (For those that remember a bit of chemistry: pH is a measure of how many hydrogen ions are in a solution; hydrogen ions bond with just about anything, including aluminum. So if there are lots of them, they snatch up all the aluminum before the hydrangeas can soak it up).
The best way to change the pH of the soil is to use a combination of fertilizers and special compounds. To get pink you need less acidity, so adding dolomitic lime a few times a year to the potting medium should do the trick. The goal is to reach a pH of about 6.0 to 6.2. Using fertilizer with a lot of phosphorus also helps to raise the pH. A ratio of around 25/10/10 works (phosphorus is the middle number). To get blue the soil must become more acidic, so adding fertilizer high in potassium works well there. A ratio around 25/5/30 works nicely (potassium is the last number). You can also add aluminum sulfate to your water during the growing season to boost the blue color. A solution of ½ oz per gallon is recommended.
This all may seem technical, but with a little investment and time to get the materials you can exert a great deal of control over the color of your hydrangeas. Dolomitic lime, aluminum sulfate, and pH meters are all readily available at major retail outlets with gardening sections, and are quite affordable. However, it can be harder to control the conditions if the hydrangea is planted in the ground, where it will be subject to the natural acidity of the local soil. Keeping it in a pot will help to isolate the plant and give you more control over the color.
6" and 5" Hydrangeas are available now at BloomRite® Gardens in white, blue, purple, and pink. They cost $10.95 and $7.95, respectively. As always call ahead to determine exact availability, as our inventory rapidly changes week to week.
