The Winter of '72: California Coast
Hadley Osborn, El Cerrito. California
The arctic air mass that caused record low temperatures in the Pacific Northwest in early December swept down to coastal California unimpeded and created the worst freeze ever recorded in the San Francisco Bay area. Not only did temperatures reach record lows in most gardens, but the freeze lasted much longer (a full week) then the renowned one of 1932 and closed, in the east Bay at least, with a withering wind.
Old friends will doubtless consider coastal California hopelessly soft since the record lows ranged merely from 14° to 22° F (just shirt-sleeve weather in my native state of Michigan); but we are talking here about gardens that rarely record temperatures lower than the high twenties and that are planted accordingly. Most rhododendrons other than Maddeniis and Malesians can stand such temperatures, but many of the Bay Area's most characteristic other plants were destroyed. The University of California Botanical Garden with a minimum of 14° F suffered dreadful losses and in the East Bay hills an estimated two million Eucalyptus trees were killed. I doubt that we will be that lucky and expect most of the damned things to sprout back, but there is ample reason for the amounting alarm over this coming summer's fire threat due to massive accumulation of dead leaves and wood.
Rhododendron losses were generally restricted to known tender plants, but since our climate simply does not harden plants properly, many flower buds and occasional tip growth were lost on varieties that normally can endure much colder weather In fact, transplanted easterners and mid-westerners like myself who instinctively try to harden rhododendrons in the fall suffered the most. Plants forced into at least partial dormancy turned out to be even less likely to resist the sap-quickening blandishments of warm November rains. On the other hand, native Californians who have been outgrowing us for years by fertilizing with nitrogen in the fall and even winter finally got the comeuppance that had been predicted for the last 40 years in vain. Thus few home gardeners lost more than a flower bud or two on their Indica azaleas, but the firm Victor Gatti & Son in San Francisco lost roughly 80% of their forcing crop to the tune of an estimated $293,500. Their problem again was not just their minimum temperatures of 22°, but the duration of the freeze.
A large number of private growers as well as public gardens have kindly furnished me with extensive information which is the basis of the summaries that follow. None should feel their list was neglected because some of their losses or their surprising survivors don't appear. The erratic pattern of plant hardening caused some erratic losses, and plants in highly sheltered positions or under wind-protected lath can hardly be called hardy because they survived. At Strybing Arboretum in the very protected outdoor site where the first Vireya Section hybrids were risked outside, even the extremely tender 'Sir George Holford' not only survived but is in full flower at this writing. Thus no plant is listed among the missing if only one report of its loss was received, and none appear hardier than anticipated unless they survived in normal garden positions.
Killed or cut to the ground (with some already sprouting back), except for a few dwarfish species from 3000 meters (about 10,000 feet) or above in New Guinea. These include R. saxifragoides, R. commonae, R. womersleyi and many yet to flower and being grown only under number. Of outlying species (the old Vaccinoides Series), only Taiwan's R. kawakamii is widely enough grown to report. Mr. Wada has found it disappointingly tender under his conditions (see Rhododendrons 1972, p. 34), but it came through these temperatures well, losing only very soft growth in exposed positions.
Without a green house, I had prepared an emergency plan of action in case such a freeze threatened, but not being very bright had not foreseen that I'd be out of town when one finally struck. Irreplaceable things were thus lost, but in larger seedling batches whole populations would be wiped out with one of two miraculous survivors. Prior to the freeze these unscathed or but slightly damaged plants were indistinguishable from their brethren and were in the same state of growth and in the same flats. So it appears that the rapidly evolving Vireyas are going to vary in hardiness from plant to plant, and these apparently hardier forms of very tender things are going to be propagated and tested further. In the meantime: all large-flowered species and hybrids should be considered as hardy only to plus 25° F.
Maddenii Subseries: Generally escaped with only superficial damage, though an extended period of 15° at UCBG cut back several of their "crassum affinity" plants. Varying forms of species behaved quite differently, though. A magnificent plant distributed as the Bowman form of R. odoriferum was frequently found with bud loss and scorched leaves, whereas a Species Foundation form (RSF 364) was always reported undamaged. Those (including myself) who feel that recognized species in this Subseries might better be considered as but slightly and inconsistently variable forms of a single polymorphic species were taught a little humility, since the R. H. S. statement that R. polyandrum was the hardiest of the Subseries did apply. Plants we grow under this name may actually have a varying number of stamens and of ovary cells from plant to plant and even from flower to flower, but they all did turn out to be a touch hardier.
Megacalyx Subseries: These survived with just superficial damage except in the coldest gardens (minimum below 15° F), where R. lindleyi was twice reported killed or cut back and young seedlings of others were destroyed. The ARS only rates R. nuttallii and R. rhabdotum and rates them at 30, yet no reports of damage in the Subseries other than some bud loss were received from gardens whose minimum was 20° or higher.
Ciliicalyx Subseries: The finest of the large flowered species fared badly. R. veitchianum confirmed its reputation as one of the tenderest, and Thailand's magnificent R. ludwigianum and its more pedestrian compatriot R. lyi succumbed except under lath or in the most protected sites. The superb Cox introduction of R. inaequale (C & H 301) is now but a memory, and R. dendricola, R. taronense, R. parryae, and the huge-flowered R. cuffeanum KW 21909 were frequently reported killed or cut back. Most forms of R. cubitti lost some of their buds even in mild gardens and fared worse elsewhere. Though the late flowering R. scottianum survived generally, the magnificent UCBG specimen was badly hit and perhaps killed.
In sum: The men who decided where the Tropic of Cancer should be were wise. Any Ciliicalyx Subseries species occurring south of this classic border of the tropics were very tender, as were any species growing below 6000 feet in areas slightly north of it. R. taronense and R. scottianum disappointingly confirmed their British rating of H-1. The former is reported to occur up to 11,000 feet in Yunnan, but the forms we grow (or grew) must have come from much lower. R. scottianum also made us wonder how it can survive at 8000 feet in Yunnan. Only six of these species are rated by the ARS, and these ratings were verified except that plants we grow as R. johnstoneanum, including Species Foundation selections, proved hardier than those we grow as R. burmanicum. Both are rated at plus 15. KW 21921 fared the best of our burmanicums and is the most authentic. The Brodick form has been suspected of being a hybrid (see Curtis's Botanical Magazine, May 1969), but why should R. valentinianum blood make it more tender? A very similar plant grown here as the Cox form proved equally bud tender (though plant hardy). It is perhaps useless to urge all gardeners to take pains to keep collection numbers or correct form names with their plants. Many better men have tried with little effect - but it is still important. Rhododendron species do vary, and different forms of a species vary significantly in hardiness.
For Ciliicalyx Subseries species that were previously unrated (see Rhododendron Information, pp. 115-123) but are reasonably widely grown the freeze suggests the following ratings:
Plus 25: R. cuffeanum, R. dendricola, R. ludwigianum, R. lyi, R. parryae, R. veitchianum.
Plus 20: R. carneum, R. cubitti, R. formosum, R. inaequale, R. scopulorum, R. scottianum.
Maddenii & Edgeworthii Series Hybrids:
'Reine Long' confirmed itself as one of the tenderest and 'Countess of Haddington' as one of the hardiest of the large flowered varieties. R. ciliatum hybrids again revealed why they were first created. They are hardier. R. edgeworthii (particularly forms grown under the now submerged name of R. bullatum) survived magnificently, but many an old specimen of 'Forsterianum' (its hybrid with R. veitchianum) took a fearful beating. In my mild yard 'Forsterianum' merely lost all buds while 'My Lady' ('Forsterianum' selfed) lost but a few and is now in fine flower. Even at UCBG both 'Else Frye' and 'Fragrantissimum' survived, and my plant of 'Else Frye' didn't lose a bud while 'Fragrantissimum' lost only a few. Interestingly, in hot summer gardens where the latter variety is preferred it was reported hardier. This might lead to the bizarre conclusion that hardiness like beauty is in the eye of the beholder - if it were not that 'Fragrantissimum' also hardens its woods better in these gardens.
Arboreum Series: Many forms of R. arboreum and its hybrids are now in flower, though a low elevation selection of the species from near Katmandu was reported bark split in two colder gardens. The unique, dwarfish KW 21976 from Mt. Victoria in southwestern Burma was only once reported damaged and is now in magnificent bloom elsewhere around the area.
Boothii Series: Came through well, though at least partial bud loss occurred on R. boothii and R. chrysodoron and their hybrids. The almost total loss of buds on 'Lemon Mist' (R. xanthostephanum x R. leucaspis) was a blow to admirers of this choice little shrub.
Falconeri and Grande Series: Young seedlings of R. protistum and R. giganteum killed at 15-20°F., but UCBG's patriarchs survived and flowered. Seedlings of Tashi's Sikkim selection of R. falconeri were badly damaged.
Irroratum Series: Untouched except for the sparingly distributed Malesian R. wrayi, which was eliminated from our gardens. Even though many Parishii Subseries plants, particularly UCBG's R. facetum, were still very soft at the time of the freeze they were untouched.
The Freeze Next Time
Observers of human nature will not he surprised that the freeze merely confirmed gardeners in their prior prejudices. Foes of winter blooming Indica Azaleas duly noted the loss of buds and blossoms, while friends of the Indicas rejoiced that the plants themselves were little damaged. Those who had not planted Vireyas or the tenderer Maddeniis were convinced of their wisdom, while those who had the largest collections and who had most enjoyed the unique beauty of these tropicals are already eagerly replanting. Gardeners who noted that plants which had been fertilized late in the year were more likely to be damaged and that tender plants fared much better in wind-protected sites may modify their cultural practices, but it would be unwise to count on it. It has been 40 years between such freezes, and 40 years is a long time.