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Hydrangeas

Folkert Siemens Folkert Siemens

With their imposing blooms, Hydrangeas are a favorite among gardeners. Here’s how to plant and care for these popular ornamental shrubs.

Origin

Hydrangeas are upright, primarily deciduous flowering shrubs or climbing plants that give their name to the Hydrangeaceae family. There are around 80 species that are native to East Asia, North America and Chile. They primarily grow in moist, deciduous forests in lime-deficient, humus-rich soil. The genus’ botanical name comes from the Greek, meaning ‘water vessel’ - a play on the high water requirements of many species.

Hydrangeas are slightly toxic to humans and some animals such as dogs and rabbits. However, Hydrangea poisoning is very rare. The risk is minimal when the plants are handled properly.

Appearance and Growth

Hydrangeas grow as shrubs and form broad, upright crowns. They tend to have many base shoots with a light gray to ochre outer layer that feels like bark. New shoots are initially green before turning brown, and generally do not develop into terminal buds in French and mountain hydrangeas (Hydrangea serrata). The stems rarely exceed 6.5 feet in height, and most species and varieties remain considerably smaller. One exception is the Climbing Hydrangea (Hydrangea petiolaris). With the aid of its aerial rootlets, it can climb up tall façades in partial shade. The leaves of most Hydrangeas are ovate to elliptical, arranged oppositely. Some species have a sharply serrated edge. The lobed leaves of the Oakleaf Hydrangea (Hydrangea quercifolia) boast a spectacular orange-red color in fall, as long as the plant is grown in full or indirect sunlight in soil that is not too nutrient-rich.

Oakleaf Hydrangea (Hydrangea quercifolia)

Oakleaf Hydrangea (Hydrangea quercifolia)

The leaves of the climbing Hydrangea turn golden yellow in fall. The long to broad panicles of wild species feature white, pink, red or blue sterile ornamental blooms along their edges to lure insects. The less impressive green fertile flowers are found in the center. In most garden hybrids, even these flowers have been turned into ornamental blooms, rendering the plants sterile. Climbing Hydrangeas, Hydrangea sargentiana, Oakleaf Hydrangea and Mountain Hydrangea all have this original flower structure. Hydrangeas mostly flower from early July, but nurseries often offer plants that have been forced to bloom by Mother’s Day. The flowering period lasts into September, but the inflorescences are still decorative after this. The slowly wilting ornamental flowers first turn pale blue-green before developing reddish tones in fall.

Location and Soil

Most species prefer to grow in partial shade, but some can withstand full sun. In any case, loose, humus-rich, moist and slightly acidic soil with a pH between 5 and 6 is important.

Planting

The roots of newly purchased Hydrangeas frequently show spiral growth as the pot is often too small. As Hydrangeas are actually shallow-rooting plants, this spiral root growth will hinder the plant’s development once planted, so must be removed before planting out. To give the shrub a good start, the planting hole should be large enough and the soil sufficiently loosened so that the roots can spread out unhindered. You should also ensure enough distance from other shallow-rooting plants. Place the Hydrangea into the planting hole so that the top of the root ball is level with the top of the soil. Fill the gaps with soil and press down on the substrate well to secure. Finally, generously water the freshly planted Hydrangea. Incidentally: Regular watering is particularly important in the first few days after planting.

Care Tips

The botanical name ‘Hydrangea’ translates to ‘water pitcher’, and for good reason. Sufficient watering is the key to ensuring hydrangeas thrive. Rainwater or softened drinking water is best for watering. If a Hydrangea’s flowers appear to hang low at midday on hot summer days, this is a clear sign that the plant urgently needs more water.

Watering Hydrangeas

Hydrangeas need sufficient water to thrive

When caring for Hydrangeas, keep the following in mind: Hydrangeas are heavy feeders, so they need sufficient nutrients. Special Hydrangea or even Rhododendron fertilizer is the best option for fertilization. If you prefer an organic fertilizer, you could opt for cattle dung pellets. Incidentally: Hydrangeas are only fertilized up to the end of July so that new shoots can mature by fall, ensuring that the plant is prepared for winter.

Blue Hydrangea flowers

The flowers of some French and Mountain Hydrangea varieties change color based on how they react to the soil: They bloom pink at higher pH values, and blue at lower values. For bright blue flowers, the soil’s pH value must generally be around 5. The more acidic it is, the more aluminum ions the plant can absorb - these are responsible for the blue flower pigment. To support this blue coloration, you can water hydrangeas once a week with a solution of 0.1 ounces of aluminum sulfate (alum, available at drugstores) per 0.22 gallons of rainwater while the flowers are developing. You can find multipurpose solutions at specialist gardening stores that contain alum as well as fertilizer.

Blue Hydrangea flowers

Blue Hydrangea flowers don’t appear on their own, but rather by using the right fertilizer

Pruning

How to prune your Hydrangeas depends on how the plant is growing. French and Mountain Hydrangeas, which form their flower buds in the previous year, should be pruned in spring, cutting only the old inflorescences and any shoots that have frozen off. You can also trim the stems a little if required. Smooth Hydrangeas and Panicle Hydrangeas form their buds on new sprouts during spring. All flowering stems from the previous year can therefore be pruned down to two bud eyes. These plants react to strong pruning with particularly rigorous growth and large flowers.

Overwintering or Winter Protection

Most French and Mountain Hydrangeas tend to freeze back in cold winters. However, losing the whole plant is very rare: the stems tend to come back with vigor the following spring after pruning. In the case of ongoing black frost, it may be helpful to cover the plants with fir twigs or winter fleece to ensure successful overwintering.

When growing Hydrangeas in pots, the containers should be protected in fall with coconut matting or placed in a wooden box with bark mulch and the crown wrapped in fleece to prevent freezing. A shady location on a terrace, ideally tight to a wall and protected from wind, is essential.

Use

Hydrangeas are ideal for partially shaded planting groups under larger woody plants and shrubs in cottage or country house gardens. They form a calm contrast to light-hungry roses in planted areas. They’re also suitable for narrow planting strips around buildings as long as the location is not in full sun and offers some protection. Hydrangeas are also popular container plants for terraces and balconies. Due to their high water requirements, they should be placed in a partially shaded area of a terrace and generously watered daily.

In partially shaded beds, Hydrangeas can be combined with box hedging and various shade-tolerant shrubs such as Hostas, Masterwort (astrantia) or Astilbe. These plants can also be paired with shaded grasses. Their flowers are beautiful in vases and also make great decorative dried displays.

Preserve your Hydrangea flowers with glycerin

With their intense colors, Hydrangeas turn a summer garden into a sea of flowers. Sadly, normal Hydrangea flowers often loose much of their color as they dry, and become brittle. But this little trick ensures they hardly loose any pigment, while petals retain their delicate feel.

You’ll need 3.5 fluid ounces of glycerin from the drugstore, 7 fluid ounces of water, a container and a knife. Make a fresh diagonal cut to the Hydrangea stem to create the largest possible surface area for fluid absorption. Mix the glycerin with the water and place the Hydrangeas in the solution. The stems will absorb the mixture and store it in the petals. The water will evaporate, leaving the conserving glycerin behind. If you don’t mind little beads of glycerin on the bracts of the inflorescences, the process is complete and you can either leave the Hydrangeas in the vase or hang them upside down to dry. The result is a very decorative, long-lasting Hydrangea bloom.

Important Species and Varieties

The most significant Hydrangeas for gardeners are garden varieties of French Hydrangea (Hydrangea macrophylla). There are now countless hybrids, created by crossing various species. For example, the Endless Summer group is very popular as it boasts the first reblooming French Hydrangeas. They can form new flowers over the course of the season, while classic varieties form their flowering buds in the previous year and can no longer flower after a heavy prune in spring. Panicled Hydrangea (Hydrangea paniculata) and Smooth Hydrangea (Hydrangea arborescens) exclusively bloom on new wood.

Propagation

All Hydrangeas can be easily propagated using cuttings in early summer. They generally form roots within two to three weeks. Species that bloom on new wood are also suitable for cutting propagation in late winter.

Diseases and Pests

Powdery mildew, gray mold and leaf spot diseases are the most common fungal diseases. Some viral diseases may also present, but are considerably rarer. Potted plants are vulnerable to scale insects, while bedding plants occasionally fall foul to aphids and Otiorhynchus weevils. Snails can also become a problem when young plants are growing in partially shaded moist soil.

Frequently Asked Questions

When can you prune Hydrangeas?

The best time to prune Hydrangeas is the end of February.

How do you prune Hydrangeas?

Only prune old inflorescences and any shoots that have frozen off on French Hydrangeas. For smooth and Panicle Hydrangeas, trim the old flowering shoots back to one or two pairs of buds.

When can you plant Hydrangeas?

Hydrangeas can be planted from mid March to late August. In colder regions, it’s best to wait until after the last frost.

How often should you water Hydrangeas?

In summer, Hydrangeas should be watered once to twice a day. You should ideally use rainwater or softened drinking water.

How often should you fertilize Hydrangeas?

Hydrangeas should be fertilized from budding in spring until the end of July. When fertilizing your Hydrangeas, follow the recommended dosages on the packaging of your Hydrangea fertilizer.

What should you use to fertilize Hydrangeas?

It’s best to use a special Hydrangea fertilizer to fertilize Hydrangeas. Alternatively, you can also use Rhododendron fertilizer or cattle dung pellets.

How can you turn Hydrangeas blue?

Hydrangea flowers can be turned blue using the mineral alum, which is found in special fertilizers. You will need very acidic humus soil.

Which Hydrangeas can tolerate sun?

In principle, all species prefer off-sun to partially shaded or shady locations. The varieties that can tolerate the most sun are Panicle Hydrangea, Mountain Hydrangea and Smooth Hydrangea.

Which Hydrangeas flower the longest?

Endless Summer Hydrangeas flower from May to October. They are reblooming Hydrangeas, meaning you cut off old flowers so that the Endless Summer Hydrangea will produce new blooms in their place.

What goes well with Hydrangeas?

Which plants work well with Hydrangeas depends on the preferred location of your specific Hydrangea variety. You can use woody plants as well as shrubs. Shade-tolerant shrubs such as hostas, astilbe or masterwort and woody plants such as rhododendron or flowering dogwood are very popular.

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