Humble Hydrangea – Dónall Flanagan

Dónall Flanagan, Teagasc Nursery Stock / Ornamentals Specialised Advisor explains how Hydrangea producers are adapting to increasing demand for novelty colours and long flowering periods

Large flowers in subtle and brilliant colours; Hydrangeas have become a must have for landscapers, gardeners and florists. Hydrangea plants have proven to be hugely popular in the last 20 years. A new market has developed for younger

gardeners and Hydrangeas are now one of the most popular plants sold across Ireland, Europe and North America. For example, Hydrangea ‘Endless Summer’ has sold almost 40 million plants since its introduction.

Their future looks positive as they are hardy, suffer few pests and diseases and are innovating at a fast pace. For many years, Hydrangeas were seen as old fashioned plants. Introductions of remontant (repeat flowering) plants such as ‘Endless Summer’, with long flowering seasons and

more tasteful colours in the late 90’s introduced a better product. Landscapers have been loyal to the well established ‘Annabelle’ and ‘Limelight’ but garden centre customers’ demand for novelty and longer flowering plants has pushed innovation. The industry has responded with support, through breeding, trials, conferences and technical improvement in production and marketing. The result today is a demand that witnesses sales of Hydrangea in the tens of millions across Europe each year.

DEFINITION

Hydrangea are deciduous shrubs; H. macrophyla traditionally flower on stem tips on one-year-old wood, H. arborescens and paniculata on new growth. The familiar colour change from pink to blue due to pH level is well recognised. Most coloured varieties are more blue than pink in acid soil. Flowers of H. macrophyla and arborescens are ball shaped and categorised as ‘mop head’ or ‘lacecap’ varieties. H. paniculata has cone-shaped flowers.

PRODUCTION

Young plants can be grown from unrooted cuttings (URC) or plugs. Low costs and ease of rooting have made URC economical and allow access to a wide range of varieties from inside and outside the EU. Potting and production of young plants is highly specialised with specific programmes for pruning, plant growth regulators and pot sizes to deliver plants of consistent quality with exacting branching. If pruning is carried out at the wrong time, the following year’s flower buds will be lost. Each variety will normally benefit from light pruning by pinching of new growth and PGR treatment.

Refrigeration of plants for forcing is begun once flower buds are set. This is to break dormancy and increase the uniformity of flowering. Often plants are stored as early as August, well before natural leaf drop and subsequently, plants are processed to remove dead leaves and prevent rotting.

Forced pot hydrangea are a premium product with high market appeal often produced around Mother’s Day. Production from refrigerated plants can be precisely scheduled. Crops take between six and 10 weeks to flower at 17C. Supplementary light may be required for some varieties but is not essential for all. A staggered flowering period can be achieved with phased potting from cold storage. Garden hydrangea, potted in Spring, should be ready for sale by Summer. Their branching and flower count will depend on their variety and their treatment in the previous year as the correct pinching and hormone treatments are required for each variety to produce well-branched plants.

Growing media should be well-drained and moisture-retentive; in North America, pine bark is used successfully. For white, pink and red Hydrangeas a pH of 6-6.2 is ideal with high phosphorus and low potassium levels to assure pink and red flowers. Blue varieties should be grown between pH 5 and 5.5 with high potassium and low phosphorus to improve colour. Lime induced chlorosis or iron deficiency is often seen where pH is too high.

Disease issues are predictable and can be reduced through a variety choice and some cultivation changes. Rust occurs on many arborescens species, ‘Annabelle’ is especially susceptible. Hydrangea ‘Miyamayae Murasaki’ and ‘Veitchii’ have high levels of resistance to powdery mildew and anthracnose. Research in Tennessee, USA, has shown increasing shading reduces the incidence of Anthracnose

NEW VARIETIES

The dominant forces inbreeding have been Japan and the Netherlands. Since 2010, there have been 328 varieties registered with UPOV; Japan registered the most with 252, USA 32 and Germany with 16. The greater numbers of these new varieties being paniculata rather than macrophyla.

In Europe, the key players are ILVO in Belgium who developed ‘Pinky Winky’ and ‘Bobo’. The Hydrangea Breeders Association (HBA) in the Netherlands is a collaboration between Dutch breeders Agriom and German breeders Kötterheinrich and Heuger. In France, the University Agro Campus Ouest Angers has developed new varieties and partners with local nurseries under the collective of Hydrangea Worldwide (HW2). Their breeding programme and that of ILVO demonstrate the benefits to nurseries of state support for plant breeding. Royalties for these plants generally range from .03 to .35 cent a plant.

RECENT TRENDS

Recent developments in breeding have focused on improved size and shape, disease resistance and flower colours. Repeat flowering has transformed the sector, varieties like ‘Endless Summer’ flower all summer instead of in one flush. There are also numerous varieties that have striking colour change over the season. Kolsters’ ‘Magical Revolution Pink’ starts lime green before changing to light pink, dark pink and then green/purple. Compact plants suitable for tabletop display such as ‘French Bolero’ or ‘Tabletensia’ are ideal for pots and require no pruning. Disease resistant hydrangea have helped build a more robust plant, suited to wider markets.

‘French Bolero’, is a new show stopping hybrid between H. scandens and H. macrophyla that will shake up the sector again. It has pink flowers along its branches as well as at the tips. It has won several awards including best new plant since it was launched at IPM Essen in 2019. With the increasing availability of young plants, this will be highly visible, must have plant, next year.

The new on trend colour of white and lime green will build on the market share of ‘Limelight’. Brand new H. paniculata varieties ‘Panorama’ and ‘Panenka’ and recent introduction ‘Skyfall’, are likely to prove hugely popular in 2021. With an estimated import value of €15 million finished outdoor shrubs and €2 million for liners of trees and shrubs in 2019, there is ample scope for growing production in Ireland. Future breeding may give more climate-resilient plants, better able to tolerate drought and cold ensuring a certain place for the humble hydrangea.

While new varieties are plentiful, better ones are harder to discern. Teagasc will establish a Eurotrials Hydrangea trials programme in Ashtown during the winter of 2020. This will see 140 varieties of H. paniculata planted and assessed in Irish conditions. These will be open for inspection by the trade and interested parties. See www.teagasc.ie/crops/horticulture/ for more details.

CUT FLOWERS AND FOLIAGE

DÓNALL FLANAGAN is a Nursery Stock / Ornamentals Specialised Advisor working with the Teagasc Horticulture Development Department in Ashtown, serving the nursery stock and ornamental sector. He has been working with Teagasc since 2007 and in his current role since 2016.

Dónall Flanagan – Teagasc, Ashtown Food Research Centre Mobile: 087 703 5823; Tel: 076 111 402; Email: donall.flanagan@teagasc.ie