Plant Performance Evaluation Criteria
All H. macrophylla introductions are evaluated on several parameters – winter hardiness of plant & bud, bloom count & quality, reblooming capability, sun tolerance, general growth characteristics, and disease & pest attack.
Winter Hardiness – Plant & Bud
We believe cold and wind are equal threats to bud survival. Much of Cape Cod is a very windy place in winter and the desiccating effect of these often very dry and very cold winter winds – particularly from the northwest – can easily kill H. macrophylla buds. Exposed tip buds typically fail in the adversely cold and windy conditions so common to our winters. Stem bud survival is generally better, and those cultivars that bloom well off their lower lateral stem buds, can deliver quite good blooming come the following summer.
While H. macrophylla plants and buds prefer a mild, narrow temperature range, they can handle consistent cold, but NOT sudden or fluctuating low temperature events such as we have experienced on numerous occasions over the past 6 winters.
The other related and negative H. macrophylla characteristic is their often too rapid response to warming periods during the winter months. Just a few days of 50 degrees F in February – with increasing day length – can get the buds ‘thinking spring’. Surely a cold snap follows, and bud loss can be significant.
Bloom Count & Quality
Normal Cape Cod summer weather allows plants to be grown in more direct sun given the typically moderate temperatures. Plants are somewhat reduced in size (less internode stretching) but flower bud count increases resulting in higher bloom count. On the downside, the increased sun does tend to scorch the blooms, particularly the paler colors. Cultivars susceptible to leaf spotting, particularly cercospora, also suffer from excessive sun.
This is totally about how well and how quickly a plant develops new flower buds. ‘Twist n Shout’ is a very strong rebloomer, perhaps the best we’ve observed to date. ‘Endless Summer-The Original’ generally does well too. Other newer introductions, advertised as ‘rebloomers’, have shown mixed results to date.
Some older cultivars are good rebloomers as well and this fact has been known for some time. As previously mentioned, cultivars like ‘Penny Mac’, ‘David Ramsey’, ‘Decatur Blue’, even ‘Nikko Blue’, show this characteristic when grown well.
The duration of ‘summer’ is relevant to reblooming as well. Summers on Cape Cod are comparatively short. ‘True’ spring weather seldom arrives before mid-May and even then, nighttime temperatures remain in the mid-fifties. The real growing season is just 4 months long – June through September. Autumn can be lovely but day length is decreasing rapidly by mid-September. This ‘short’ season may not provide enough favorable growing time for some ‘rebloomers’ to fully express their secondary flowers.
Nevertheless, the ‘reblooming’ mantra has been very well marketed and well documented with selected cultivars. It is now almost a pre-requisite for a new Hydrangea macrophylla introduction. And why not – the ability to bloom at all following the recent damaging winters on the Cape strongly supports the benefit of this capability.
Plants having medium green, matte finish leaves commonly wilt in the afternoon sun. This group of H. macrophyllas includes ‘Nikko Blue’, ‘Penny Mac’, ‘Endless Summer-The Original’, and several other well planted cultivars. As we have already noted, these are the same varieties that possess the best winter hardiness and reblooming characteristics.
Watering does not cure the wilting problem; once the sun passes and the plants are in shade, wilting subsides quite quickly. Siteing these cultivars in a location providing afternoon shade is much preferred.
H. macrophylla plants having shiny dark green foliage – and dark bloom pigment – tolerate the same sun conditions much better. These attributes are common to several ‘old-time’ cultivars – ‘Mathilda Gutges’, ‘Marechal Foch’, ‘Alpengluhen’, ‘Merritt Supreme’. Unfortunately these cultivars do not rebloom. Perhaps a great breeding opportunity might be awaiting.
Autumn on the Cape – with nighttime temperatures falling back into the 50’s – triggers an ‘antique’ color change for most H. macrophyllas. Rich ‘washed’ tones of green, pink, mauve and even reds, develop in the predominantly blue sepals of a maturing blossom’s sterile florets – a condition that is particularly striking on mophead flowers. Best ‘antiquing’ occurs on mature blooms that have had favorable shade conditions during August and September coupled with adequate moisture. Even a modest ‘blue’ can develop into a splendid complex mix of colors.
General Growth Characteristics
Ultimate plant maturity – plant size – may take more growth seasons than were trialed initially by the breeders /developers. And plant size is important especially in current times as gardens are getting smaller and so must the plants.
Stem count in a mature plant is a characteristic that can be quite variable. Modest stem density can make for a loose ‘open’ plant – Twist N Shout would be a good example as it develops long unbranched stems. On the other side, dense stem development may appear favorable but often creates expansive basal crown growth, making for more difficult pruning, and poorer total plant form. Pruning is a true best practice for virtually all hydrangea species and proper application on the H. macrophyllas will definitely promote improved plant form and increased bloom numbers.
Disease & Pest Attack
Two fungal and one insect problems persist with H. macrophylla plants at Heritage. Leaf spotting – Cercospora – is typically a late season condition from late August onward. Overhead watering worsens the situation and for this reason all hydrangeas at Heritage receive surface irrigation. Powdery mildew is a lesser aliment in our climate but does develop later in the summer – late August, and especially into early September. And we have noted strong correlation with certain cultivars.
Chili Thrips have become more of a problem since their identification a few years ago – especially during our warmer and dryer stretches in late summer. These thrips are identified as ‘raspers’ and attack the tender uppermost stem tip leaves, sucking them dry. In the most severe cases, the leaves become embrittled, easily crumbling into pieces. Mid-August into September is the peak attack period and again we see some cultivar specificity – particularly on the fleshier, shineyleaved varieties. It is not fatal but does create a poor looking plant specimen with sad and disfigured foliage by early autumn.
The photos below are good examples of these maladies. It should be noted that fungicides and insecticides are not used at Heritage.