Hebe News – Article 2

HEBE – WATER REQUIREMENTS

Hebes are often referred to as ‘drought-tolerant’ shrubs and may be sold by retailers as such. However, even in the Pacific Northwest, summers are warm and dry enough to stress virtually all hebes if regular summer water is not provided. This is particularly true of summer and fall-blooming cultivars, which bloom on new growth, and require regular water to continue growing, without which they will stop flowering.

How hebes came to be regarded as ‘drought-tolerant’ is a bit of a mystery, unless there is a tendency to associate evergreen plants in general with drought tolerance. The term ‘drought-tolerant’ can mean a lot of different things, but if we use the definition, ‘a plant which will survive in the Pacific Northwest entirely on rainfall without added summer irrigation’, then few hebes will perform well under these conditions. The Pacific Northwest is a Mediterranean-type climate, with a pronounced dry period of 3–4 months, and summer temperatures will exceed 90ºF in the Willamette Valley on 14–21 days each summer. Almost all hebes are from New Zealand, and therefore originate in a relatively mild, cool climate with at least some summer rainfall .

Kristensen (1989) classified Hebe species as to their origin in wet or dry habitats. About 1/4 of the listed species evolved in dry habitats. This includes familiar species like Hebe pinguifolia, Hebe carnosula, Hebe rakaiensis, Hebe decumbens, Hebe pimeleoides and Hebe cupressoides. However, even the ‘dry’ habitats received up to 60 cm of rain per year. As a result, most of the available cultivars share this need for at least occasional summer water, depending on conditions. It is almost impossible to say what the water requirement of a particular cultivar will be, because many are of unknown parentage. Instead it will simply have to be planted and evaluated in the conditions characteristic of that site.

Symptoms of drought stress are expressed in a variety of ways. The first signs are typical of most plants and involve drooping of leaves and even shoot tips. This is reversible if water is immediately provided. Repeated events like this will cause the leaf color in many hebes to change from a glossy green to a pale, dull appearance, particularly on the older leaves. Prolonged stress will cause yellowing of the foliage, and brown, necrotic areas will develop on the leaves, especially on the midrib. The older leaves on the bottom and interior parts of the canopy will start die and fall off, in many cases giving the plant a ‘leggy’ appearance. Continued lack of water will cause dieback of portions of the canopy and eventually the entire plant.

The factors which will determine how much water a given hebe will require are many:

Cultivar Some hebes do survive and grow well with limited water. Although evidence is scant, and based primarily on casual observation, in general blue-leaved hebes seem to tolerate dryer garden conditions in the Pacific Northwest than whipcord or large-leaved cultivars. Even these plants will still prefer a good watering every 7–10 days in the warmest part of summer and should not be planted with truly drought-tolerant Mediterranean plants like ceanothus, cistus or rosemary, which are best without summer water.

Site The site will have a major impact on water requirement. Hebes planted in full sun require more water than those in some shade. Plants on the south or west side of buildings will be stressed from the added heat from the building. If protection from mid to late afternoon sun is provided, then water requirement will be reduced. Do not plant in full shade as this will result in poor growth and possibly foliar disease problems.

Soil Soil is another important factor and all hebes prefer a well-amended, well-drained soil for best growth. Heavy, compacted soils do not allow for good root growth of fibrous-rooted plants like hebes and this will increase drought stress. Amendment of these soils with organic matter prior to planting is a good start. Mulching following planting, and avoiding foot traffic on the soil afterward, will help good structure to develop and improve conditions for root growth and water storage in the soil. For more information on soil amendment, see Improving garden soils with organic matter, EC 1561, from OSU Extension publications: http://extension.oregonstate.edu/catalog/.

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

Neil Bell


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