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Verticordia in the Garden

Max Hewett

Except for a few tropical species which occur across the top of Australia and which to date have resisted establishment in the southern states, verticordias are endemic to the south-western winter-wet corner of the continent. As might be expected, their introduction to areas with opposing weather patterns is not without problems. However, until comparatively recent times, very few species have been successfully established even in their own climatic region. As with many other examples of Western Australia's outstanding flora, the growing of Verticordia may now be contemplated by those prepared to give that little extra thought and effort.

It has become evident that many parameters are common to both summer and winter-wet areas. Many species are now being grown successfully throughout Australia. With others we have had only partial success but this will serve as a basis for more research into better long-term establishment.

Grafting (see box) has also proved very worthwhile for some species which have shown sensitivity to soil-borne pathogens. More general commercial application will increase the popularity of the genus of these beautiful wildflowers as they could then be grown without provision of special conditions. This article will illustrate the severe restrictions faced in their horticultural application.

Grafting Verticordia
Many of the major problems outlined in this article can be overcome by grafting these showy plants on to hardy root stock.

Experiments carried out by Doug McKenzie indicate that Darwinia citriodora and Geraldton Wax, Chamelaucium uncinatum, are reliable stocks.

Many cultural matters influence plant performance of species from Western Australia when grown in the eastern states such as fertilising, time of planting in various climatic zones, response to dry winter and early spring conditions, especially in summer-wet areas, plant aspect, etc. Early research has been directed predominantly towards identification of the basic major hazards.

Hazards in growing Western Australian plants

Major hazards are recognised as root rot, collar rot and mildew, moulds etc. Chemical treatments may be appropriate at times but research to date has concentrated predominantly on the achievement of control by physical considerations.

Bullet  Root rot of garden shrubs

Root rot is destruction by soil borne pathogens which attack root structures. It results in plant inability to take up moisture and nutrients from the soil. This hazard has confronted gardeners from time immemorial. With specific attention to soil type and drainage this hazard can be controlled reasonably well. It is a particular problem with some Verticordia species in summer-wet areas.

The traditional gardening response has been to improve drainage conditions, and mounding of beds has certainly proved worthwhile. With some Verticordia, particularly in late summer when the soil is warm and moist, inadequate speed of drainage as well as the depth can be a significant factor. Soil compaction over time, particularly with soils in the medium to heavy range, may be a problem. Initial planting procedures can be important and my recommendations are given below. Some species such as Verticordia chrysostachys var. pallida have made very good early growth in finely textured, well mounded loam but slow drainage speed has proved inadequate in late summer. Verticordia picta is reliable in light sand or sand and gravel to a depth of about 30 cm over heavy base, but has been difficult to establish otherwise.

Verticordia pritzelii Verticordia grandiflora  
Verticordia pritzelii (left)
Verticordia grandiflora (right)

Photos: Keith Townsend, Brian Walters
Click for larger image

A reasonable guide to preferred soil type may be assessed from observation of the initial root development of species at the propagation stage. Where first roots emerge predominantly horizontally or at least shallow, it is probable that the species will do well in heavy soils. Varieties of Verticordia huegelii in this category have adapted well in quite heavy clay loams. Where the first root development tends to be predominantly vertical, lighter soils have generally proved more appropriate.

Some Verticordia species may develop a lignotuberous root system and are less troubled by problems of root rot. Verticordia chrysanthella is one such species which has done well in both light and heavy soils.

Bullet  Collar rot of garden plants

Collar rot is attack by soil-borne pathogens to the plant stems, at or near surface level. Destruction of the outer section of the stem known as the cambium layer, results in the inability of the plant to transfer moisture or nutrients between the root system and the foliage. In summer-wet areas such as eastern Australia, this hazard can be a major problem with many garden shrubs, especially many western plants such as Verticordia that have evolved in a summer-dry climate.

Factors effecting collar rot include mulching of the soil surface around the plant, watering methods and for Verticordia, in my opinion, potting mixes and plant dormancy. Of these the surface condition of the soil is the more apparent difference between growing in summer-dry and summer-wet areas. In the west the summer-dry period, as well as extending well into autumn, is generally hotter, and the hazard of collar rot is reduced. This is the establishment period of young plants and vegetative surface mulches created by leaf litter are an advantage.

In summer-wet areas vegetative mulches appear to be a source of collar rot attack. The adoption of more sterile surface conditions using gravel mulches or by permitting undisturbed natural weathering in the case of gravelly soils, has done much to alleviate this collar rot problem.

Many Verticordia flower from late spring well into summer and before flowering they will respond favourably to summer rain or hand watering. Flowering is frequently followed by some degree of dormancy and the humid late-summer conditions which prevail in eastern areas coincides with this rest period. This can place collar rot susceptible species under additional stress.

Verticordia grandis Verticordia chrysostachys Verticordia pennigera  
Verticordia grandis (left)
Verticordia chrysostachys (centre)
Verticordia pennigera (right)

Photos: Brian Walters
Click for larger image

It has been observed with some species that failures have occurred due to the moisture-holding capacity of the potting mix used initially in the nursery stage. It has seemed that this has later contributed to collar rot problems in holding more moisture than desirable during summer-wet conditions, in the region of the plant stems. My solution to this is given in the "Planting Procedure" section below.

To maintain sterile surface conditions around planted specimens it is sometimes necessary to clear a plant's own leaf litter away from plant stems should excessive summer drop occur. Verticordia staminosa ssp. cylindracea var. erecta is one such species - although generally quite hardy and reliable, it can be vulnerable to attack under wet late-summer conditions. Growth response to wet weather throughout the year is good with dense leaf production. It is one of the earlier species to flower in dry conditions in early summer and when flowering has finished, heavy leaf drop can lead to collar rot attack.

Hand watering can increase the risk of collar rot with some species. Verticordia have shown very good tenacity under hot and dry summer conditions and mature specimens, particularly in eastern Australia, have generally proved quite capable of surviving without such attention. An exception can arise where very shallow rooted species have been grown in light sandy soil and the entire root system is in danger of drying out. Watering should be maintained during the establishment stage should dry conditions prevail. In the cooler part of the year hand watering may safely be done throughout the day. When species are in flower it is best to water below the foliage to avoid flower damage. During hot weather water in early mornings so that excessive moisture is not retained around plant stems for long periods. Do not water late or overnight, when most collar rot problems start. It is also important that watering be done on occasions when the weather conditions later in the day could be expected to dry the surface quickly such as from wind, hot temperatures or low humidity. Trickle watering has sometimes been used, however, in light sandy beds it can be difficult to obtain adequate moisture distribution through the soil and spraying may be more appropriate.

Mounding specimens slightly at planting time and forming slight depressions between plants can help throw excess water away from the vulnerable stem areas and thereby effect faster surface drying after rain. The depressions can also be useful in training water deeper into the soil where it may do more good. Such treatment furthermore tends to reduce general surface scour in heavy rain.

Bullet  Mildews, Moulds, etc

Fungal attack on the stems or leaves is a hazard when cultivating some plants, resulting in plant debilitation and, at times, death. This hazard can be more difficult than root and collar rot to control and grafting on to hardier rootstock does not avoid the problem. Attack appears to be related more to seasonal conditions than to geographical division into summer or winter-wet regions. On some plants the effects are relatively minor and may only cause temporary damage even if no counter measures are taken.

Verticordia: The Turner of Hearts
This new publication by Elizabeth George (illustrated by Margaret Pieroni) brings together for the first time available information on all the described taxa of Verticordia (101 species, 13 subspecies and 30 varieties) providing a comprehensive, user-friendly guide to their identification and cultivation.

Species are presented in a systematic sequence that allows for easy comparison and less confusion. The book is illustrated throughout with beautiful watercolours and line drawings and photographs of plants in their natural habitat are also included.

Book Cover
Further information from University of Western Australia Press

For Verticordia, fungal attacks occur more frequently in summer-wet areas and wetter than usual summer conditions in other areas. In summer-wet areas some of the yellow flowered species are particularly susceptible to mildew attack in autumn although when late summer weather patterns are drier than usual the problem is much less evident. Verticordia chrysanthella is such a species and although debilitation may be quite appreciable in some years (with defoliation), recovery may generally be expected later during winter and spring.

This problem appears to be caused, not so much by the wet, late summer conditions to which the plant may have responded with lush new leader growth, but to the effect on such growth a little later in autumn, from widely-contrasting drier weather, and while night temperatures are still moderate. One approach is to do nothing and await natural recovery. Healthy new growth can be expected to return in several weeks with the advent of cooler and wetter conditions. You could lightly prune off the recent lush leader growth, thereby delaying the start of new seasonal development by a month or so. As a last resort chemical treatments with fungicides such as mancozeb or Triforine can assist control.

Relatively minor attacks may occur on a few of the pink-flowered species such as Verticordia plumosa or Verticordia monadelpha but these will grow out later as weather conditions become cooler and wetter.

In near-coastal, summer-wet areas, Verticordia mitchelliana almost invariably suffers debilitation with serious leaf-drop in late summer. In inland districts the problem is rarely evident. The leaf-drop noted is usually rapid and comes at the end of a strong summer growth period during which the leaves have appeared almost succulent. The problem follows a change in weather pattern to cooler, less humid conditions and appears to start with fungal attack at leaf junctions with the stems. The leaf drop is slightly less severe when the seasonal weather change is gradual rather than abrupt. Treatment with fungicides has failed.

Verticordia picta  
Verticordia picta
Photo: Brian Walters
Click for larger image

All three species in Verticordia Section chrysorrhoe (V.nitens, V.aurea and V.patens) are particularly susceptible to foliar fungal attack. For V.nitens it can be extremely debilitating leading to rapid total collapse of the specimen, and such loses are not restricted to summer-wet areas. The natural growth habit is to produce very tall, fast-growing but softwooded flower stems. These stems are very susceptible to attack, the first indication of which is a purplish discoloration of the new growth a few inches below the growing tip. The only counter seems to be early pruning of rapidly growing softwooded stems to induce multiple branching. The plant form is then changed to that of a multi-branched lower shrub, which although still susceptible to fungal attack, is considerably more tenacious. In summer-wet areas the fungal attack is generally still severe enough to present a debilitated appearance. V.aurea and V.patens, although not as badly affected, are still difficult species to maintain, especially in summer-wet areas. The fungal attack occurs predominantly on the new leaves and stems causing severe debilitation with stem purpling.

Planting procedure

Potting mixes are very different in texture, nutrient value and moisture holding capacity compared to the soils that are to receive the plant. Preplanting preparation of Verticordia plants has been used to advantage for location compatability and to provide greater resistance to root and collar rotting pathogens. Harden the plant in a pot of soil similar to that of its intended location. For advanced purchased plants, they are first bare-rooted, any root coiling removed and then lightly pruned. After repotting they should be staged in favourable conditions until new vigorous growth develops.

Some additional measures have been used to advantage. The soil is not pre-cultivated but rather left in the compacted state. This is important for heavy garden conditions. Use a minimum sized hole, water the plant well. Autumn planting is preferred so that subsequent additional watering is not required. The plant is then given a hard prune.

Follow-up treatment is restricted to removal of leggy growth. Verticordia monadelpha, V.attenuata and V.pholidophylla have responded well to this treatment in heavy base soils.

From Australian Plants, journal of the Association of Societies for Growing Australian Plants, December 1995.

Max Hewett is leader of ASGAP's Verticordia Study Group.


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Australian Plants online - June 2003
Association of Societies for Growing Australian Plants