It is often said that hydrangeas are not only one of the best-loved garden plants but also the most understood. Although, I think trial and error is sometimes a better way to learn about gardening, here are a few insights into the broad category of plants called hydrangeas.
Hydrangeas are generally thought of as shade plants, but that isnât entirely accurate. All hydrangeas do best if they get some sun – 4 to 5 hours a day of full sun means strong stems, healthy growth, and the best flower color.
Thereâs no getting around this one: hydrangeas love water. Their roots are quite fleshy and donât go very deep into the soil. Your hydrangeas should not be allowed to dry out, particularly during hot weather. You may need to water them daily during the summer. All hydrangeas benefit from a 2″ layer of mulch around their roots. This will help to keep the moisture in the soil. Someone once pointed out to me that âhydraâ is close to âhydro,â which means water in Greek.
Hydrangeas are one of the most trouble-free garden plants in terms of pests and diseases. Slugs may eat the leaves but wonât affect the plantâs overall health and vigor. There are a few fungal leaf spot diseases that may infect hydrangeas, but these are highly dependent on the weather and watering practices. Leaf spots thrive in rainy seasons and/or on plants that receive water from overhead (such as with an automatic lawn sprinkler system). If your plants get leaf spot one season, carefully clean up all the foliage as it falls in autumn and dispose of it in the trash. Try to adjust sprinkler heads so that they do not hit the plantâs foliage.
Though hydrangeas donât need to be pruned, they can be pruned if you understand how they flower. Two terms that are important to understand are ânew woodâ and âold wood.â These refer to how and where the plant makes its flower buds.
A plant that flowers on ânew woodâ will form flower buds for that summer on the growth that it begins to produce when it emerges in spring. Smooth hydrangea and panicle hydrangea both flower on new wood.
A plant that flowers on âold woodâ can only produce a flower bud on a stem that has been growing for a year or longer. Hydrangeas that flower on old wood form their flower buds for the following season while they are flowering in the current season, so the flowers youâll see in 2016 were actually created by the plant in 2015. Big-leaf hydrangeas, both mophead and lacecap, flower on old wood.
Plants that flower on ânew woodâ can be pruned in fall, winter, or spring, and still produce flowers that summer. The best time to prune these types is in early spring. Pruning later in the season will result in smaller flower clusters that appear later in the season.
Plants that flower on âold woodâ cannot be pruned without sacrificing some of the flowers. Any time you remove a stem of this type of hydrangea, the growth the plant puts out to compensate will have to live for a year before it is mature enough to produce a flower bud. It is best to avoid pruning these plants altogether by choosing an appropriately sized plant and siting it so that it can reach its full potential.
Some plants flower on both old and new wood. Endless Summer Hydrangeas would be an example of a hydrangea that blooms on old and new wood. Since they bloom on last yearâs growth (old wood) as well as the current seasonâs growth (new wood), you will get the most flowers by protecting the flower buds on the old wood. To do this, do NOT prune or cut back your shrubs after August 1st. Leaving the fall blooms on your plants over the winter provides winter interest, and ensures you arenât removing buds that will become flowers in the spring and summer.
Hydrangeas do especially well when fertilizers are effectively used in spring or early summer. Follow the package instructions when applying fertilizer and be sure not to use too much. Over-fertilizing can cause hydrangeas to grow big green leaves, but stunt bloom production.
Fertilizing some hydrangeas will change the color of the flowers. Raising the pH of the soil can lead to pink blooms, lowering the pH can turn the flowers blue. There are several commercial and natural ways to raise and lower pH. To raise pH you can add lime or wood ashes. To raise the pH you can add Sulphur or coffee grounds.
Hydrangeas, specifically Hydrangea macrophylla, may benefit from an additional layer of mulch in the winter. Mound the mulch or leaves around your plants at least 12â high to protect the flower buds that will bloom early next year. In the spring, do not remove the mulch too fast; wait until all danger of frost has passed before uncovering to ensure beautiful blooms from old and new wood. Hydrangeas are very susceptible to frost damage. If frost is predicted in the spring, consider covering your plant with a sheet or pillowcase.
Kathleen Carr is the owner of The Growing Scene, Inc., a garden center and landscaping company. She can be reached by calling 815-923-7322 or emailing her at kathleen@thegrowingscene.com Have a gardening question? Please contact her. She will address it in an upcoming column.
