Answers to Your Common Hydrangea Questions

Hydrangeas may be the most popular shrubs for home landscapes in Maine. Two common questions we receive are (1) when to prune them and (2) how to change their bloom colors. We've put together the following tips to help you get the most out of your hydrangeas:

When to Prune Hydrangeas

For many gardeners, pruning hydrangeas can be a daunting task. Knowing when your plant forms flower buds is the key to successful pruning and thus better flowering.

Hydrangeas That Bloom on Old Wood

There are two kinds of hydrangeas: those that flower on old wood and those that flower on new wood. Hydrangeas that bloom on old wood include most bigleaf, mountain, oakleaf, and climbing hydrangeas. They start to form buds in late summer and fall for the following season, and they carry their buds through winter.

Pruning these plants after the buds are formed and anytime until they flower the following summer will result in few flowers and perhaps none. Even the reblooming hydrangeas carry buds from the previous year, so the same pruning advice applies to them. If you need to prune old wood hydrangeas to control height, we recommend doing it at the end of July or early in August. This gives the plant time to grow, form buds and harden off before winter.

If you want to prune off dead wood in spring, we suggest letting the plant tell you what is alive or dead. Wait for the leaves to emerge in spring, then prune above the new growth, removing any stems that show no signs of life.

Hydrangeas That Bloom on New Wood

Hydrangeas that flower on new wood, such as panicle and smooth hydrangeas, are the most cold-tolerant and reliable flower producers in the Hydrangea genus. These hydrangeas set buds and flower in the same season, so their buds avoid the perils of winter.

Autumn or early spring pruning will not affect flower production. It can be very difficult to tell whether your hydrangea blooms on old wood or new wood just by looking at it. It's better to keep plant tags and know the species you're working with. You can also always ask one of the staff members here at Estabrook's.

How to Change Your Hydrangea Bloom Colors

Hydrangeas can bloom in a range of colors depending on several factors. Pay attention to the following growing conditions to ensure you get the blooms you want.

pH Isn't the Only Factor

The color of the blue or pink flowers on bigleaf and mountain hydrangeas is determined by the soil in which they are planted. It's a common belief that the pH, which is the measure of soil acidity, is the main factor that turns flowers blue. While pH plays an important role, it is the aluminum, which is present in most soils, that makes the flowers blue. Think of pH as a faucet in the ground that allows aluminum to flow. A pH of 5.5 or lower opens the faucet, and the aluminum becomes available to the hydrangea root system. The plant moves the aluminum from the roots to the flowers, which turns them blue. As the pH rises, the faucet starts to close and eventually shuts off, making the flowers pink. The aluminum is still in the soil, but it becomes unavailable to the plant because of soil chemistry.

Potted Hydrangeas Can Be Deceiving

Many container-grown hydrangeas have little or no aluminum in their soil, which is why you see many red and pink flowers at the garden center. Most red-flowering potted hydrangeas will be dark blue to purple once they are planted in soil with a lower pH. Alternatively, hydrangeas that have lighter pink flowers in pots usually turn light to medium blue once planted in ground with a lower pH. White flowers do not change color regardless of soil chemistry.

How to Use Amendments to Change Flower Color

To turn blue flowers pink, add lime to raise the soil pH. To turn pink flowers blue, you need to lower the pH by adding aluminum to the soil. There are many products available at Estabrook's that can lower the pH of your soil, but one example is aluminum sulfate.

You should consider getting a soil test before applying these products to know what pH and mineral content you are working with. Apply lime or soil acidifier in fall or spring; while they might take a year to kick in, their effects should last for a few years after that.