Skip to content
Elisabeth C. Miller Library

Archives: Plant Answer Line Q & A

Plant Answer Line questions and answers.

Hydrangea pruning

It’s January, and my hydrangeas look bedraggled and terrible. When should I prune them?

 

The answer will depend on which species of hydrangea you are growing. According to the American Horticultural Society Pruning & Training edited by Christopher Brickell (DK Publishing, 1996), Hydrangea macrophylla (bigleaf or mophead hydrangea) should be pruned after flowering in warm climates but in colder climates it is best to leave the old flower heads on the plant over the winter, and prune in spring. This rule also applies to Hydrangea serrata. If your hydrangea is blue-flowered, it is probably H. macrophylla. In her Guide to Pruning (Sasquatch Books, 2006), local pruning expert Cass Turnbull of Plant Amnesty says this plant should not need much pruning, but if you want to remove the faded blooms, you can do this in February by looking for four or five pairs of plump buds below the old flowers, and cutting just above the lowest or second lowest set of buds.

Hydrangea paniculata should be pruned in early spring, before active growth begins.

Hydrangea arborescens needs little pruning, and any pruning at all should be done in early spring.

Oakleaf hydrangea (Hydrangea quercifolia) also needs little pruning, but may be pruned in spring.

The U.S. National Arboretum offered a good introduction, no longer available on their website, but excerpted here:
“Established bigleaf, panicle, oakleaf and smooth hydrangea plants can often benefit from regular pruning. Removing about one-third of the oldest stems each year will result in a fuller, healthier plant. This type of pruning is easiest to do in winter, since the absence of leaves makes it easier to see and reach inside plants.
Gardeners may also want to prune to control height or to remove old flower heads. The best time for this type of pruning differs between species. Bigleaf and oakleaf hydrangea, which flower on previous year’s growth, should be pruned shortly after flowering is complete. Panicle and smooth hydrangea flower on current year’s growth and can be pruned anytime from late summer until early spring. If pruning these two species in the spring, try to prune before leaves appear. Plants of H. arborescens ‘Annabelle’ have been known to produce a second flush of flowers if pruned lightly after the first flowering.
Stems of bigleaf hydrangea that have been damaged by cold should be pruned as soon as it is determined that they are dead. Watch for new growth at the base of the plant. If your plant has basal shoots that are 6 to 8 inches in length, but the upper parts of the stems are still bare, then the bare stems need to be removed. For bigleaf hydrangea plants that are subject to frequent weather-related dieback, other than removing the dead stems, you probably won’t ever need to do any other pruning–Mother Nature has been doing the work for you.”

Kitsap County Master Gardener Peg Tillery, in an article formerly available on the WSU Extension Kitsap County web site, recommends waiting until March to prune hydrangeas: “In our climate we need to wait until early March to prune roses and summer blooming hydrangeas. This way the tender new growth won’t be harmed by frosts.”

The Royal Horticultural Society provides general pruning recommendations.

Here is an excerpt from a Seattle area gardener’s response to a question about hydrangea pruning on Garden Banter, a now-defunct British gardening forum:

“Different species of hydrangeas have different criteria for pruning. Some need very little pruning at all, other than to shape, as with H. quercifolia and H. anomala petiolaris. H. arborescens does well with the dramatic pruning you describe. Pruning of H. paniculata would depend on if you were training it to be upright like a tree or as a broad shrub, and need not be dramatic pruning, just barely enough to induce new growth on which flowers occur, though in your zone a more dramatic pruning might be needed because of winter damage. Most hydrangeas prefer late winter pruning, but H. macrophylla is better done in late summer when flowers are getting scruffy and new shoots are developing.”

,

Fragrant Hydrangea

I want to add a hydrangea to my garden but I would prefer one that is also fragrant. Are there varieties that have a noticeable pleasant scent?

 

There are some species and cultivated varieties of Hydrangea that are reported to be fragrant. Bear in mind that everyone’s sense of smell is different. I recommend seeking out examples when they are in flower and doing a sniff test in nurseries, gardens, or large parks and arboreta with a good selection. The ones that have a fragrant reputation are:

  • Hydrangea quercifolia: the smell is a rich honey-vanilla to my nose. This shrub is also a wonderful magnet for honeybees, bumblebees, pollen wasps, and syrphid flies. Its inner flowers are fertile, while the more dramatic outer sepals are sterile. Cultivated hydrangeas have been bred to emphasize the sterile florets, while wild hydrangeas tend to have fewer of these and are more useful for pollinators. In my garden, all the pollinator activity is humming along on the fertile inner flowers beneath those sterile four-petaled parts of the inflorescence. The showy parts of a hydrangea so prized by humans for their beauty are not what interests the pollinators .
  • Hydrangea angustipetala and its cultivar (‘Golden Crane,’ also called ‘MonLongShou’): said to smell strongly like jasmine or sweet alyssum; of the species, Dan Hinkley says: “The deeply scalloped sepals of the infertile florets surround a disk of striking chartreuse fertile flowers while emitting a faint but beguiling fragrance.” [Horticulture, Jun/Jul2009, Vol. 106, Issue 5]
  • Hydrangea scandens: Dan Hinkley says: “As its name implies, it can be a sprawling shrub but far from what would be considered disheveled. The branches possess a pleasing burgundy-brown color and the lacy cream-colored flowers pack a powerful fragrance during March and April. Hydrangea scandens ‘Fragrant Splash’ adds a bonus of variegated foliage.” [Ibid.]
  • Some hybrids of Hydrangea macrophylla x Hydrangea angustipetala
  • Hydrangea macrophylla ‘Ayesha’: “one of the only Hydrangeas to have a delicate fragrance in bloom” [Great Plant Picks profile]
  • Hydrangea anomala ssp. petiolaris (a climbing hydrangea)
  • Hydrangea paniculata: “slight floral scent” or “mild fruity fragrance”

Close relations in hydrangea family:

  • Pileostegia viburnoides: “In late summer, frilly cymes of heavily-scented flowers erupt amidst its foliage, filling the air of our woodland drive with a delicious aroma of honey. Not surprisingly, honey bees are highly attracted to the flowers that rely entirely on scent.” [Heronswood blog, August 29, 2018] However, not all noses smell alike. An article in Arnoldia [June 2, 1964] says “The floral odor is described as ‘fragrant’ or ‘ill-smelling.'”
  • Decumaria barbara (woodvamp): a climber in the Hydrangea family, native to swampy areas of the southeastern U.S., with fertile flowers that are slightly fragrant or fragrant, depending on the source.

Based on the research above, Hydrangea quercifolia and Hydrangea angustipetala cultivars seem like the best choices.

,

Encouraging blooms on Hydrangeas

Help! We live in North Bend and have several Hydrangeas. I have a large Annabelle that
has never bloomed properly. Is there some special care or
fertilizing that I can do to encourage normal blooms on these
plants?

 

Here is some information on how to improve flowering, from the U.S. National Arboretum:

Excerpt:

“There are three possibilities for lack of flowering among the hydrangea
species. The first two too much shade and improper pruning apply to
all hydrangeas, while the other weather-related damage to flower buds applies primarily to the bigleaf hydrangea.

“While most Hydrangea species benefit from some shade, too much shade can
reduce flowering. This is particularly true of panicle hydrangea, which
is the one Hydrangea species that grows well in full sun. If you have a
hydrangea that used to bloom well but now flowers only sparsely, evaluate
whether the growth of nearby trees has reduced the amount of light that
reaches the hydrangea. If so, you may want to consider moving the
hydrangea to a sunnier location.

“Improper pruning can also reduce flowering in Hydrangea. Since bigleaf
and oakleaf hydrangeas flower on previous year’ s growth, potential
flowers buds would be removed if the plants were pruned in fall, winter
or spring. Panicle and smooth hydrangea flower on this year’s growth, so
pruning them in early summer would reduce or eliminate flowering for that
year.

“The most common reason for lack of flowering in the bigleaf hydrangea is
unfavorable weather. Most H. macrophylla cultivars flower primarily on
previous year’s growth. Weather conditions that damage aboveground parts
of the plant can reduce flowering. Damaging weather conditions include
early fall freezes that occur before the plant is completely dormant,
extremely low winter temperatures, and late spring freezes that occur
after the plant has broken dormancy. In USDA Cold Hardiness zone 6 and
warmer, which is the recommended growing area for H. macrophylla, the
most common of these unfavorable weather events is late spring freezes
that damage tender new growth. This is particularly true in the
southeastern U.S., where ‘see-saw’ temperatures are very common in the
spring.

“Bigleaf hydrangea responds quickly to warm temperatures in late winter
and early spring by breaking dormancy and producing new leaves.
Unfortunately, these spells of warm weather are often followed by periods
in which temperatures reach well below freezing. The severity of the
damage caused by these freezes depends on how many of the buds had broken
dormancy. If a substantial portion of the buds on a stem were actively
growing, the whole branch may die. For some cultivars, the loss of the
aboveground part of the plant will completely eliminate flowering the
following summer. The plant will produce new buds from the base of the
stems, but stems produced from these buds will not flower in these
cultivars.”

,

Plants for privacy screens

Could you recommend some plants for a privacy screen that are also narrow? These would be planted in front of a fence in our backyard.

 

Here is some general information on plants for creating a screen.

Trees for Problem Landscape Sites — Screening from Virginia Cooperative Extension

Bet on Hedges by local garden writer Valerie Easton.

Landscaping for Privacy: Innovative Ways to Turn Your Outdoor Space into a Peaceful Retreat by PNW author Marty Wingate.

Here is a list of narrow plants for a screen from local garden designer Chris
Pfeiffer: “Fastigiate shrubs for naturally narrow hedges.” Compiled by Chris
Pfeiffer. 2005.

Zones 5-6:

American arborvitae ‘Rheingold’ (Thuja occidentalis ‘Rheingold’) 5’h x
3’w

Barberry ‘Helmond Pillar’ (Berberis thunbergii f. atropurpurea
‘Helmond Pillar’) 6’h x 2’w

Boxwood ‘Graham Blandy’ (Buxus sempervirens ‘Graham Blandy’) 8’h x 1-1/2′
w

English yew ‘Standishii’ (Taxus baccata ‘Standishii’) 4’h x 1-1/2′ w

Irish yew (Taxus baccata ‘Fastigiata’) 20′ h x 4′ w

Japanese holly Jersey pinnacle (Ilex crenata ‘Jersey Pinnacle’) 6′ h x
4′ w

Japanese holly Mariesii (Ilex crenata ‘Mariesii’) 3′ h x 1-1/2′ w

Zones 7-9, in addition to the above:

Dwarf yeddo rhaphiolepis (Rhaphiolepis umbellata Gulf GreenTM) 3-4′ h x
2′ w

Heavenly bamboo ‘Gulf Stream’ (Nandina domestica ‘Gulf Stream’) 4′ h x 2′ w

Japanese euonymus ‘Green Spire’ (Euonymus japonicus ‘Green Spire’) 15′ h x
6′ w

 

You might also consider installing a trellis to increase the height of
the fence, and then growing an evergreen vine such as Clematis armandii,
evergreen hydrangea (Hydrangea seemanii), or star jasmine
(Trachelospermum jasminoides).

This link is also helpful (scroll down to
“Evergreen Vines” and look for appropriate height and light requirements).

You could grow bamboo, but I would recommend growing it in a container,
or a series of containers, as you do not want the roots to spread. I have
seen an effective bamboo screen between two houses growing in a long
rectangular lined wooden trough (lined with bamboo barrier). Some species
of bamboo are more tolerant of partial shade than others. Look for a
clumping, rather than a running, bamboo (like Fargesia) to be on the safe
side.

Growing Bamboo
in Georgia

Running and Clumping Bamboos

Bamboos for hedges or
tall privacy screens

, , , , , , , , , , , ,

Distinguishing Hydrangea petiolaris from Schizophragma hydrangeoides

There’s a climbing vine in my garden which has hydrangea-like flowers. I have lost the tag, and don’t know if it is Hydrangea petiolaris, or Schizophragma hydrangeoides–how can I distinguish one from the other?

There is an article in Arnoldia (the journal of the Arnold Arboretum at Harvard University) from July 24, 1933 which explains that the two plants are related, but the flower ‘petals’ (which are actually sepals) surrounding the center are distinct:
“In Hydrangea petiolaris this encircling tiara is composed of greenish white flowers, each one made up of four rounded sepals. In Schizophragma hydrangeoides these showy sepals are a purer white and they are borne singly rather than in fours.”

University of Arkansas Extension also says that Schizophragma “differs in flowering time (after climbing Hydrangea) and in the fact that it protrudes less from structures.”

There is another less commonly grown climbing Hydrangea, Hydrangea integrifolia, an evergreen that grows to 40 feet tall and 20 feet wide. Its flower buds look almost like those of peonies, and its leaves are elongated and glossy.

,

Hydrangeas won’t bloom

I have two Hydrangeas growing up the side of my house in a northeastern exposure. This will be their 4th year. Leaf growth is robust… flower growth almost non-existent (on one of the shrubs, one bloom last year; one forming this year). What can I do to encourage bloom or should I start over?

According to the Plant Addicts website, there could be several reasons why yours are not blooming well. Check out their page, “Why Won’t My Hydrangeas Bloom?”

There is another useful resource that may be of help. Try Why Plants Fail to Bloom, by Leonard P. Perry, a professor at the University of Vermont Extension. Perry suggests there are five possible reasons: Age, Temperature, Alternate Flowering, Light, Nutrition and Pruning.

In addition, I consulted two books on hydrangeas. Both mentioned that Hydrangea anomala subsp. petiolaris can take time to bloom. According to Michael A. Dirr’s Hydrangeas for American Gardens, Time is [the climbing hydrangea’s] biggest ally. That is, once it gets established, there is no stopping it.

Michael A Dirr. Hydrangeas for American Gardens. Timber Press, 2004. p. 24.

Toni Lawson-Hall’s Hydrangeas: A Gardener’s Guide also says that Hydrangea anomala subsp. petiolaris “grows well on north-facing walls but takes a while to get established.”

Toni Lawson-Hall. Hydrangeas: A Gardener’s Guide. 1995. p. 81.

You are probably wondering how long a while is. Alas, I was unable to locate a specific timeframe for when you might expect those gorgeous blooms to start, but from what I can gather, time may help.

,

Moving and dividing hydrangeas

I have a very large hydrangea that has been in the ground at least 15 years. I’d like to move it, and have heard that it can be divided into several bushes. Are there any special details I should consider when performing this task?

I found a reference to the technique you describe in Hydrangeas: A Gardener’s Guide by Toni Lawson-Hall and Brian Rothera: “DIVISION. Sometimes, when moving a large H. macrophylla cultivar, the plant falls apart during the operation. It has been found that, provided each section has good roots, planting the separate pieces is totally successful.” This is similar to the process of layering, where branches are nicked and then pinned down into the soil to allow roots to form, and then severed from the parent plant with a sharp shovel six months to a year later. The small plants will be genetically identical to the original plant.

Hydrangea care

I have a few Hydrangeas that have not been doing their best. I think I have the exposure down, and their colors. I need advice on what fertilizer is best for them and when, how much and how often to apply it. I try to stay as organic as possible.

According to Hydrangeas: A Gardeners’ Guide by Toni Lawson-Hall and Brian Rothera (Timber Press, 1995), hydrangeas do not generally require special feeding. If you wish, you can apply a general fertilizer twice a year. “As important as feeding, and in fact another method of supplying nutrients, is mulching.” The authors suggest using mature compost or leaf-mould. Mulching in the spring to a depth of about 3 inches will help protect the roots of your hydrangeas from drying out (but never put mulch directly up against the base of your shrubs). Mulching also suppresses weeds. Since you garden organically, mulch may be your best bet for supplying a slow and gentle dose of nutrients. If the plants continue to fare poorly, you may want to do a soil test to see if there is some kind of nutrient imbalance that needs correcting.

planting hydrangeas

I would like to plant hydrangeas along a south-facing shed. The site gets some morning sun and quite a lot of afternoon sun from over the roof of the house. This area currently has moss growing on it and has not been previously used for planting. Your reply will help me to decide whether to plant there or not.

I have amended the area where I have planted the hydrangeas with chicken manure and compost. I dug holes about 14 inches deep and and about 14 inches square. I also put a spade full

Most hydrangeas will do well in sun to part shade although full sun in a hot climate would be too much. You also want to bear in mind that hydrangeas, especially Hydrangea macrophylla, prefer adequate moisture. According to Michael Dirr’s Hydrangeas for American Gardens (Timber Press, 2004), an inch of water once or twice a week should be sufficient as long as the plant’s soil needs are met (consistently moist, well-drained, acidic soil which is rich in organic matter such as leaves, compost, well-aged manure). Some species of Hydrangea tolerate heat better than others, according to Dirr. Hydrangea macrophylla, H. serrata, and H. umbellata do not fare as well as H. paniculata. Some species, like H. quercifolia and H. aspera, prefer shadier spots. Dirr recommends using drip irrigation for plantings of Hydrangea, specifically using drip tubing, extender lines, and emitters attached to a garden hose, possibly with a timer.

The Michael Dirr book says that good soil preparation (not just of the planting hole) ahead of time is the best thing for hydrangeas. He does not subscribe to the rule sometimes put forth, that the hole must be 3 times as wide as the root ball. I also consulted Hydrangeas: A Gardener’s Guide by Toni Lawson-Hall and Brian Rothera (Timber Press, 1995) which says the hole should be 2 times the rootball’s width and depth. Be sure to check the state of your plants’ roots. You want to make sure they are not coiled in a spiral or restricted in any way. You may need to tease out or prune the roots a bit before planting.

Professor Linda Chalker-Scott discusses planting procedures in her book, The Informed Gardener (University of Washington Press, 2008), advising that the planting hole only needs to be the depth of the root system, but twice the width. She also recommends against amending the planting hole in any way. Backfill the hole with native soil, not a soil amendment. The idea is not to ‘spoil’ the plant by putting rich compost just in the hole, which will deter the roots from spreading out into the surrounding area. Here is more of her writing on this subject.

You have already amended the surrounding soil, so the addition of gravel to the hole is not necessary, and is possibly not a good idea, according to general planting information from University of Minnesota Extension. Here is an excerpt: “If soil drainage is inadequate, species that are tolerant of poorly drained soils may be planted, or soil drainage may be improved. This can be done in two ways. If a hard pan is present (a compacted, impermeable layer of soil) with an underlying layer of well-drained soil, a hole can be dug down to the permeable layer to provide drainage for the planting hole. If the soil is poorly drained and there is no well-drained layer below, a tile system can be laid. This, however, is expensive and requires the assistance of a professional for proper design. Simply adding gravel to the bottom of the planting hole will further decrease oxygen availability to the root system.”

of gravel at the bottom of the hole to improve drainage as I suspect that the base soil is mostly clayey.

browning of leaves in oakleaf Hydrangea

Why are the leaves of my oakleaf Hydrangea turning brown around the edges and falling off?

We do not diagnose plant problems, especially without a sample. It might be wise to bring a sample to your local Master Gardener clinic. You can locate a Master Gardener Clinic within Washington State here.

However, based on my personal experience with my own oakleaf Hydrangea (Hydrangea quercifolia), it is a semi-deciduous shrub that will hold on to its leaves through winter, only to replace them with fresh growth in the spring. All my Hydrangea’s old leaves have turned reddish-brown and look very ratty. Once new growth resumes in spring, I cut off most of the tattered leaves. (Don’t do it too early, in case there is a late frost.)

If you have new growth, do not worry about your shrub, but if you do not have new growth or it is the new growth that is turning brown then you should take a sample leaf into one of the Master Gardener clinics (linked above).