Hebe Landscape Evaluation

The landscape evaluation of Hebe at the North Willamette Research and Extension Center (NWREC) was established in 2000 and was removed in 2009. The purpose of the planting was to compare performance of a range of Hebe species and cultivars under typical western Oregon conditions and gather information on their landscape performance. One of the main goals of this trial was to develop comparative data on hardiness of Hebe cultivars and species and identify cultivars that were capable of tolerating typical cold events in a Pacific Northwest winter. In addition to assessing hardiness, other goals were to record flowering and growth information on the various cultivars and species, and also any pest or disease problems.

• Data Collection
• Cold Hardiness of Hebe Cultivars and Species 
• Cold Hardiness Results 
• Pests and Diseases
• Drought Tolerance 
• Books and Websites

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The first 48 selections of Hebe were planted in 2000. Over the following 4 years, plants were received from cooperators, usually as unrooted cuttings. These were rooted, grown on to 4” or 1 gallon-sized plants, and added to the evaluation each April. In 2001, 88 selections were added to the planting, and in 2002, an additional 45 were added. In 2003, 80 were added, and a final 33 selections were planted in 2004. Primarily because of plant losses to cold and to some extent disease, the evaluation consisted of 201 clones in November, 2006.

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The planting consisted of a 0.4-acre plot, with 16 double rows, with individual plants spaced at 3’ by 3’ within each double row. Three plants of each clone were planted to allow statistical evaluation of data, although because of the sequential planting over several years, the plants are not randomly distributed in the evaluation, but planted in groups of three. Planting occurred once per year, in April or May, to allow for good plant establishment before winter. Following planting, each row was mulched with bark dust.

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Each established plant was fertilized with 2 tbs of 13-13-13, while each new plant received ½ tbs of the same fertilizer. A 5’ wide grass strip separated each row for access. A micro sprinkler irrigation system was installed with the assistance of a grant from the Hardy Plant Society of Oregon.

Meteorological data, including daily maximum and minimum temperature, rainfall and wind speed, are available from the US Bureau of Reclamation Agrimet weather station installed in 1998, located immediately adjacent to the plot.

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Plants came from a variety of sources in the western United States and Canada, including nurseries in Oregon, the University of California (Santa Cruz) Arboretum, Strybing Arboretum in San Francisco, Washington Park Arboretum in Seattle and the University of British Columbia Botanical Garden in Vancouver, B.C. However, the majority of the collection was provided by cooperators the New Zealand and the United Kingdom, including Lincoln Botanical, Landcare Research and Christchurch Botanic Garden (New Zealand) and individuals within The Hebe Society (U.K.).

Data Collection

Plant size was recorded at planting time and again in October at the end of the growing season. Plant size was recorded by measuring the height, and two width measurements at right angles to one another, allowing a plant size index to be calculated.

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Data on flowering was collected once per month throughout the year, as flowering on various selections continues almost year-round. Flowering information included a rating of flowers on a 0-5 scale. Cold hardiness information was collected following cold events and again in early April. Plants were rated for damage on a 0-5 scale, with 0 indicating no damage and 5 indicating plant death. Intermediate ratings indicate varying levels of damage to leaves and shoot dieback. Information on insect pests and disease problems was collected on a casual basis. The main disease problems proved to be Phytophthora root rot; leaf spot, caused by Septoria exotica, and downy mildew, caused by Peronospora grisea. See Pests and Diseases for more information. A more recent evaluation involves assessing foliage quality and form of the plants, again on a 0-5 scale. The goal was to give some indication of the appearance of the plant in general.

Cold hardiness of Hebe cultivars and species

In New Zealand, Hebe species can be found growing in a wide range of habitats, from sea level to alpine regions, so it is no surprise that cold hardiness of the species, and the cultivars derived from them, varies widely as well. There is truth to the old saying that hardiness of Hebe is related to leaf size. As one goes up in elevation from sea level to alpine areas in New Zealand, the leaf size of the Hebes tends to decrease, and overall plant size decreases as well (Kristensen, 1989). Other characteristics like coriaceous and/or glacous leaves and white flowers are also typical of alpine Hebe species (Wardle, 1978). So, generally speaking, you could say that the larger the leaf of the Hebe, the less cold hardy it tends to be. As with all living things, the rule is not perfect, but the most tender Hebes are usually the largest-leaved, and the hardiest are those with the smallest leaves.

The most extensive study of cold hardiness of Hebe was undertaken by Warrington and Southward (1995), who assessed summer and winter hardiness of 35 species and cultivars. This study showed that large differences in hardiness existed among the various selections. Not surprisingly, the hardiest of those tested were two whipcords, H. cupressoides and H. propinqua, both of which are typically found in alpine or subalpine regions and have very tiny leaves. More important than altitude however, this study observed significant differences in hardiness of species from northern or southern parts of New Zealand, with species of southern origin exhibiting greater overall hardiness.

Some of the research on hardiness of Hebe has utilized excised shoots as sample material for laboratory studies. Bannister (1986) found that detached shoots of Hebe albicans, a South Island species often found in subalpine scrub above 1000m, withstood mid-winter temperatures of -10°C, one of the hardier of the native species studied. Other Hebe species were not tested. In a more extensive study, Bannister (1990) found that mid-winter freezing resistance of foliage of H. buchananii, a diminutive shrub of alpine areas in Canterbury, was as low as -11°C. Freezing resistance of foliage of H. rakaiensis and H. salicifolia, both of which are found at lower elevations on the South Island, was found to be -5.2°C and -6°C respectively. Buds of H. rakaiensis were found to be hardy to -11°C, significantly hardier than leaves. Testing of both H. speciosa, a tender species from seacoast areas of the North Island, and H. odora, a widely distributed shrub of subalpine scrub, found early-winter hardiness of -6°C and -9°C, respectively. Sakai and Wardle (1978) tested excised stems of a wide variety of New Zealand trees and shrubs in mid-winter for hardiness. H. brachysiphon was rated as one of the hardiest species tested, having leaf and bud hardiness of -10°C and -13°C, respectively.

Probably the most extensive anecdotal evidence for hardiness of a wide range of Hebe species and cultivars is provided by Trees and Shrubs hardy in the British Isles (Bean and Clarke, 1991). This monumental work, in 5 volumes, lists descriptions and basic hardiness information on over 100 Hebe species and cultivars. The most extensive field trial of Hebes which included cold hardiness results was the trial of RHS Garden Wisley between 1980 and 1982. In addition to selecting 33 cultivars or species for awards, the trial also indicated which of the trialed plants failed to survive the winter of 1981. Harris and Decourtye (1995) evaluated cold hardiness of many New Zealand plants in field trials over several years in Angers, France. This trial indicated that H. dieffenbachii was tolerant of prolonged cold weather with temperatures as low as -12°C. H. pauciramosa, H. pinguifolia and H. amplexicaulis showed some foliar damage from these conditions. Later data from this trial (Harris et al. 2000) showed that H. albicans and H. subalpina were not injured or suffered only slight injury from prolonged winter cold snaps with temperatures as low as -15°C.

In North America, Hebes are grown primarily as a landscape plant. Because of the intolerance of most Hebes for excessively hot or cold weather, cultivation of Hebes in North America is almost entirely limited to west of the Cascade or Sierra Nevada Mountains in California and the Pacific Northwest. Elsewhere in North America, the climate is generally too cold or hot, or both, to allow for outdoor cultivation, although dedicated enthusiasts have been successful with some varieties in many other areas.

Even in the Pacific Northwest, Hebes are sometimes thought of as too tender for general landscape use, a reputation which is primarily the result of experience with a few popular cultivars which are not particularly cold hardy. Plantings of ‘Amy’, ‘Tricolor’ or ‘Patty’s Purple’ have been severely damaged in cold events on a regular basis, which has unfortunately given the entire genus this reputation. The key to growing these tender cultivars is to provide a protected location near a house or nearby plant.

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Despite this, many other commonly-available Hebe cultivars like ‘Emerald Gem’, H. carnosula, or H. cupressoides ‘Boughton Dome’, never suffer winter damage.

The level of injury to cold will of course vary depending on the temperature experienced. The hardiest Hebes-the whipcord types and other small-leaved plants-have not typically shown any reaction to winter temperatures in western Oregon over the course of this trial. Since 2000, the minimum temperatures have been no lower than 19°F (-7°C). Even tender Hebes will tolerate temperatures of 25°F without showing signs of stress, especially if these temperatures occur in mid-winter. Abnormally cold temperatures in the fall or early spring are often responsible for damage to these plants and that has been the case in this trial.

The mildest form of damage is leaf discoloration at the shoot tips. More severe cold damage will cause browning and leaf loss on shoot tips.

Major cold damage will cause browning of most of the leaves on the canopy, followed by dieback. Sometimes, plants will recover over a 2-3 year period from this damage if subsequent winters are mild. Very severe, sudden cold often turns the entire plant brown and sensitive cultivars do not recover from this damage and require replacement.

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References

Bannister, P. 1986. Winter frost resistance of leaves of some plants growing in Dunedin, New Zealand, in winter 1985. New Zealand Journal of Botany. 24:505-507.

Bannister, P. 1990. Frost resistance of leaves of some plants growing in Dunedin, New Zealand, in winter 1987 and late autumn 1989. New Zealand Journal of Botany. 28:359-362.

Bean, W.J. and D.L. Clarke. 1991. Trees and shrubs hardy in the British Isles. 5 Volumes. John Murray.

Harris, W., A. Cadic and L. DeCourtye. 2000. The acclimatization and selection of New Zealand plants for ornamental use in Europe. Proceedings of the 19^th International Symposium on Improvement of Ornamental Plants. Acta Horticulturae 508:191-196.

Harris, W. and L. Decourtye. 1995. Observations on cold damage to New Zealand plants grown at Angers, France. Horticulture in New Zealand. 6(1):9-19.

Kristensen, L. 1989. The Genus Hebe-A botanical report. Danish research Service for Plant and Soil Science. Report #S-2034.

Sakai, A. and P. Wardle. 1978. Freezing resistance of New Zealand trees and shrubs. New Zealand Journal of Ecology. 1:51-61.

Wardle, P. 1978. Origin of the New Zealand mountain flora, with special reference to trans-Tasman relationships. New Zealand Journal of Botany. 16:535-550.

Warrington, L.J. and R.C. Southward. 1995. Seasonal frost tolerance of Hebe species and cultivars. New Zealand Journal of Crop and Horticultural Science. 23:437-445.

Cold Hardiness results

Winter temperatures over the course of the trial varied significantly, as might be expected. Temperature data for each period from late October through mid-March, from 2000-2006, are shown in the graphs below. Generally speaking, only temperatures of 27°F or below are of significance, as in this trial, even the most tender Hebe seemed not to suffer injury above this temperature, regardless of when it occurred. More than the actual minimum temperature, the date on which a low temperature was recorded has had the most profound impact on Hebe survival and damage at this site.

The most serious cold damage which occurred during the evaluation followed cold temperatures in early November, 2002, when temperatures dropped suddenly to 22°F. This was a particularly early freeze that caught many Hebes growing and in full bloom, and did extensive damage to many selections, some of which failed to recover. A similar, though less severe freeze, occurred on November 1, 2003, when the temperature again dropped quickly from mild lows to 24.6°F. Again, because of the relatively severe temperature early in the winter, significant damage was done to a wide range of plants. Another early freeze occurred in the first winter of the trial, when the temperature dropped to 23°F on November 18^th. The fourth severe freezing event occurred in mid-February, 2006, when the temperature dropped to 20°F on February 20. Much like the early November freezes, this was an uncharacteristically low temperature for the time of year, and occurred following a period of relatively mild temperatures. By late February, many plants, including Hebes, are beginning to grow, and are more susceptible to cold injury. These four events accounted for virtually all of the observable damage to Hebes over the duration of this trial.

In contrast to the injury caused by these early, or late, freezes, similar temperatures in mid-winter have caused no identifiable damage. Temperatures in the low 20’s°F have occurred in January 2004 (20°F, January 6), January 2005 (23°F, January 5) and December 2005 (19.5°F, December 16). In two of the winters of this evaluation (2001-02 and 2004-05), winter temperatures were so mild that no damage which could be attributed to cold weather could be observed.

The accompanying chart shows cold damage to Hebes in the trial by year.

Note that the names of the plants appear in the chart as they were received. Some of these names are invalid or were incorrect when the variety was obtained and have since been corrected. Others were assigned to distinguish unidentified varieties obtained from a source. Contact the author for information about the identity of individual plants.

Flowering

Flowering times of the Hebes in this trial were assessed monthly. Newly-planted Hebes are not assessed in the year of planting to allow for establishment. Flowering was rated on a 0-5 scale, with 0 meaning no flowers present and a rating of 5 given to those plants which are completely covered with blooms. The following chart shows flowering of the various clones in the trial. In many cases, flower data collection has been interrupted, or stopped, by cold weather affecting the plants.