THE ROCK GARDEN

Galanthus in the rock garden at Gentiana

In the age of mass planting rock gardens have become deeply unfashionable. For those of us with no interest in swathes of Lomandra a rock garden will provide the ideal environment in which to grow a whole suite of plants unlikely to flourish in the open garden. Rocks keep the soil cool allow for drainage and give protection to tiny alpine plants that are likely to be swamped by larger neighbours in other garden settings.

Limestone outcrops at the Waro reserve Hikurangi. Far north New Zealand

Constructing a rock garden is a skill that is borne only with experience. There is however no reason that the average home gardener shouldn’t be able to make one with slow and careful consideration. Always choose the biggest rocks you can manage because they are best sunk into the soil as deeply as possible. The rocks are best packed in as tightly as possible to create crevices, cool the soil and give the plants surfaces to scramble over. The aspect of a rock garden should be determined by the nature of the plants you intend to grow and visa versa. Its very possible for a keen gardener to grow some alpine plants in south eastern Australia but they need to face east, receiving at least 3hrs sun in the morning then protection from hot afternoon sun. Alpine plants would also require some in ground irrigation for the summer months, most of them like a dryish winter. Remember snow is dry until it melts. Our winter rainfall climate does not preclude these plants, it just means they will require extra drainage materials added.

Cyclamen hederifolium in rock crevices at Gentiana.

Irrigation should be installed before planting and set deep into the soil best in the form of a drip system. Overhead irrigation is not ideal for alpine plants and is wasteful of our most precious resource. They prefer to be watered deeply with their crowns being kept dry. Drainage is essential so a sloping sight is ideal and the inclusion of gravel into the soil. A thick mulch of gravel is also a very good way to keep the plants crowns dry.

A crevice garden, part of the rock garden complex at the New York botanic gardens in the Bronx. A crevice garden is made with thin slabs of rock set on their side packed in side by side and mostly buried so the bulk of the rock is below soil level. This form of rock garden is often used by serious enthusiasts to grow alpine plants that require very good drainage. Crevice gardens are generally mulched with a thick layer of gravel.

The range of plants that can be used in a rock garden setting is endless so I will endeavour to break them down into broad groups.

SHRUBS.

These are best situated on the lower slopes of the garden, so the highest parts are reserved for ground hugging plants. The nature of the shrubs used should be determined by the size of the garden. There are plenty of tiny shrubs for small gardens but for a larger one the choice is broader. I have a great fondness for small conifers. Conifers will give your garden bulk and definition. The argument that conifers are boring and unchanging is absurd, a conifer covered in new growth over spring can be as decorative as any flowering plant. Dwarf conifers should be purchased from specialist nurseries and should mostly be grafted. The vast majority of cutting grown conifers will grow larger and can become unsightly with age, there are as always exceptions such as Chamaecypris obtusa (hinoki) selections or Picea mariana nana. Dwarf conifers are dwarf compared to the species from which they derive but may still eventually grow into substantial shrubs with time. They are usually very slow growing. Most labels give height and spread over 10 years, 10 years in the life of a conifer is a very short period of time. I favour pruning them but this requires some knowledge of their growth habits. A good specialist nursery person will be able to provide sound advice on the habits of the plants they sell. There are many dwarf forms in the genus Picea but these plants come from the northern most habitats in which trees can grow and are therefore sensitive to heat. Pines and Cedar are more heat tolerant and some forms are very dwarf and very beautiful. The dwarf selections of Pinus thumbergii (Japanese black pine) are very robust plants as are selections of Cedrus deodara. Pinus strobus also has some lovely and very dwarf forms..

Pinus strobus var. sea urchin. This is the smallest cultivar of the eastern white pine.

Flowering shrubs are essential as well but are mostly not as structured as the conifers. One of my all time favourites is Syringa meyeri pablin the smallest of all the lilacs, it grows to a maximum of 1m x 1m flowers pink in mid spring, has sublime perfume and requires pruning each year after flowering. It requires full sun and once established is very dry tolerant. Helianthemum come in a big range of colours. they are low spreading shrubs that flower heavily in late spring and are also dry tolerant. Helianthemum have fallen from favour because they do not present well in pots at point of sale but this is no reflection on their performance in the garden. Some such as Wisely pink have lovely silver foliage. They all require an annual prune after flowering. Their relatives cistus has a few low growing forms and are very similar in habit. Gevillea and Banksia have some very good dwarf forms but many native shrubs grow too large for all but the biggest rock garden. Azalea are broken up into broad groupings and for the rock garden Satsuki are the best. Satsuki Azalea are mostly slow growing and respond extremely well to clipping. They flower in november long after the other have finished.

Picea mariana nana. One of the very few very good dwarf conifers cultivated by cuttings.

For those of us living in cooler parts the choice is wider. Rhododendron has an amazing array of dwarf species and hybrids, generally the smaller forms like more sun than large growers. They are difficult to track down but I am growing an expanding range of them. Daphne also has some lovely species, these are plants mostly from the mountains around the Mediterranean so require sun and good drainage. As with Rhododendron I am growing an increasing range of Daphne.

There is a vast array of perennials and bulbs suited to rock gardening. Our namesake Gentiana are stunning but difficult even here in Olinda. Some Androsace are much easier and have the added bonus of nice foliage. Pulsatilla I find very easy and their decorative seed heads mean they have a long season of interest. Some Cyclamen (not the florist cyclamen) are perfect candidates for rock gardening but its important to select the correct species as many of them are woodlanders. Hederifolium, Graecum and Persicum would be top of the list. Because Cyclamen tubers tolerate sitting on the surface there is a perception that this is a requirement but in fact I find they perform much better when their tubers are buried quite deep. This is the case for most bulbs. Planting bulbs deep provides them with more consistent soil temperatures particularly over the summer months and gives Australian gardeners their best shot at growing some of the more difficult cool climate bulbs such as Galanthus or Crocus.

Salvia microphylla var. microphylla. Most Salvia are not suited to rock gardens, too big or too floppy. This one however is the perfect candidate, small, compact and long flowering. All this plant requires is to be cut to the ground annually in spring.

Its always better to select plants that are going to thrive within your climate zone. Trying to grow plants that are unhappy can be very deflating.

Here at Gentiana I will give you very honest advice about the type of plants suited to your area, I want the plants to flourish so you think of my nursery as a place where you can buy plants that survive.

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