Plants in Rock and Concrete

By Todd Layt

Plants that perform and look good with rock and hard landscaped areas

Rock can make a landscape, or it can break it. Sure almost always it can be the star attraction, adding style and interest in abundance. It can however have a negative impact on the plants around it. So when planting near rock, and hard surfaces, the correct preparation and maintenance, and appropriate plant selection is vital.

Sandstone, granite, quartz, luck stones, marble, boulders, bush rock, and even pavers all look great, and should be always used if desired, but when they are used keep in mind that the area around them can get very very hot in summer. So to allow for this either leave extra space for the ground and air to cool, meaning a much larger planting space, use mulch around the plant in these larger spaces, or use tough plants that can cope with the heat from the rocks. Many rock garden designs and plant selection lists have come from cooler climates, but they do not work for long in our harsh Australian sun.

Before contemplating the best plant types for a rock garden, the design aspects must first be pondered upon. Sloping terrain can greatly benefit from the use of larger rocks. They not only stabilise the site, they provide a base for smaller rocks to be placed around, and can create the design template for the garden. Outcrops can also be made, by placing a collection of rocks to make higher focal points. Larger rocks, with interesting shapes can capture attention, creating focal points. Series of rock placements, can even act as types of retaining walls, allowing for slopes to be tamed, providing flatter areas for lawns or paths.

Not all rock gardens have to be on slopes. Flat ground can also benefit from strategic placement of rocks. Rocks can be used to provide height to some flat ground gardens. Another favorite is the dry river bed look, where, lucky stones are strategically placed to look like a river bed. A large area can be left through the centre, and can even provide a pebbled driveway. Flat rocks can be used to provide open spaces in a garden design, providing the multi-purpose roll of weed suppression, or a place to step and walk through.

Pavers and even concrete can be used for pathways, and are often used for open spaces. Often gaps are left for plants. These smaller planting areas can get quite hot, as the Australian sun bounces of these hard surfaces. Roadsides can also be hot harsh places for plants to survive. Cities often create what is known as an urban heat island, with the hard surfaces, making the urban area significantly hotter than the surrounding areas. The EPA says: “On hot summer days, urban air can be 2-6°C hotter than the surrounding countryside.” Your rock garden, pavers or concrete can do even worse to plants in summer. A 42°C day can have a heat effect on the plants that are touching the rock, equivalent to 50°C. Last year I stuck a thermometer near plants in a rock garden and found exactly that.

Apart from using plants that can cope with these conditions, there are other things you can do. Firstly, unless the plants are really tough, which there are some; avoid planting rock gardens in summer. Larger holes can be left between the rocks, deeper digging will allow better root establishment, and chunky mulch around the plants will help cool them. Shading is another great tool that will reduce hard surface heat stress. Planting a few shade trees, or mass planting lower growing plants close together will help. The vegetation itself will cool the rock when the gaps are kept to a minimum. Good deep watering will also help, but this is hard to do with water restrictions, so it is better to choose tougher plants.

Each Spring, rock gardens, or heated roadside plantings, should have a wetting agent applied, which will help eliminate one of the most common symptoms of this heated dry environment, namely dry patch. Soils in this environment easily become hydrophobic. Compact soils should also be avoided. Mixing in well composted recycled organics, can help plants survive the extra heat from the adjacent rock and hard surfaces. Make sure the drainage is adequate in places where water will flow from the hard surfaces into the soils in volume, or use plants that can cope with periodic wet feet in those areas; for example; places like gullies next to roads. If you really want to use less robust plants in a rock garden, try using the artificial rocks, which today look very real, and are hard to tell apart from the real thing.

Choice of Plants

Australia is often a sun burnt rocky country, so we have a great choice when it comes to native plants that can cope with this situation. Some of the best can even be planted in summer in these rock or hard surface gardens.

Native Shrubs
These can be used as an excellent source of shade around the rock garden, or near hot concrete areas, with out the problems that larger trees bring, namely, large branches falling off, or roots damaging foundations etc. They can be used densely, or what often looks better, as specimen plants among other very tough ground cover plants, or native grasses or strappy leaf plants. When flowering, many of these tough native shrubs can be the focal part of the garden. Following I have listed many varieties that work well in either rock gardens, or heat island effected plantings such as near roads or paths. Obviously some plants will work better in different regions, so make sure the plants you choose are suited to your region. Most of the following varieties will work in Perth to Adelaide, Melbourne to Brisbane.

Small Shrubs
Dwarf forms of Acacia howitti, Agonis flexuosa and Baeckea virgata. Correa reflexa, Westringia fruticosa and Callistemon Little John, Kunzea ambigua.

Large Shrubs
Callistemon varieties such as Kings Park Special, Hannah Ray and Captain Cook, are standouts when it comes to growing in hot rocky surfaces. The following are other good choices. Grevellia olivacea, Grevillea Flora Mason, Winpara Gem, Hakea laurina and Dodonaea viscosa. All dense mostly upright habits.

Trees
Banksia integrigolia, Callistemon Harkness, Callistemon Dawson River Weeper Melaleuca Revolution Gold, Eucalyptus Summer Red and Summer Beauty. If you can water weekly for the first summer, then even more beautiful evergreen trees, like Luscious, a shinny leaved Tristaniopsis laurina, and Sweeper a weeping Waterhousia floribunda can be used. These are all medium sized trees which do not have large, invasive root systems. Avoid trees with large roots that lift paths, and trees that drop lots of branches should also be avoided.

Ground Covers
Ground covers can look great cascading over rocks, but can lead to more work as they need trimming back next to roads and paths. Some of the best types include; Kunzea ambigua prostrate, Grevillea obtusifolia, Grevillea lanigera prostrate, Myoporum parvifolium forms, Leptospermum Pink Cascade, Scaevola aemula, and Hardenbergia Happy Wanderer. Brachyscome multifida spreads less, is sometimes shorter lived, but it works really well in hot rock gardens. This plant needs trimming every 6 months.

Native Grasses
Mingo, a blue kangaroo grass looks magnificent when it cascades over rocks. Its blue foliage contrasts nicely with any rock surface. The Pennisetum alopecuroidies varieties Nafray (compact) and Purple Lea (taller) are tough native grasses with beautiful plumes, which easily handle the hotter environment. For a more rustic Australian look try common Kangaroo grass (Themeda australis). For Sydney south, try the tough improved forms of tussock grass, like the hardy Poa labillardierii cv Eskdale, or the popular blue Kingsdale.

Strappy Leaf Plants
Of all plant categories, these are probably the best suited to rock gardens, or for planting next to hot paths and roads. All these selections handle summer planting in rock gardens or roadsides. Lomandra Tanika for example, has proven itself, handling hot road median strips better than any other plant. (Best suited from Port Macquarie and South). Katrinus, or Katrinus deluxe are two other tough Lomandra longifolias, well suited to heated rock gardens or roadside situations.

Some rock gardens may have poor drainage, or plants grown near some paths or roads may have a lot of water running off the concrete on to them, or they maybe excessively irrigated. If this is the case, the plants may have to handle long periods of wet feet. Lomandra hystrix is one of the best plants that can tolerate wet feet, or dry conditions, but up until now, they were very large plants. A new compact variety called Tropicbelle, has made this plant far more usable in the general landscape. Another larger type, called Katie Belles, has magnificent yellow flowers.

Many rock gardens or areas adjacent to hot concrete are on slopes, and need the soil stabilised. Newly released King Alfred Dianella caerulea ‘John 316’, which easily handles the heat has recently been shown in scientific tests, to strengthen the soil more than any other tested plant. In the test it strengthen the soil an amazing 752%, compared to common Dianella caerulea which only strengthened the soil 94%. Its blue foliage provides it with highly ornamental qualities ideal for beautiful rock gardens.

Other Dianellas that work well in rock gardens are Little Rev which is very compact and architectural, and Revelation which has masses of beautiful blue flowers. Little Jess is another compact form that has an amazing number of deep purple flowers, and has very tight above ground growing points, that not only are tidy, but allow it to grow and thrive in very trying conditions, including highly diseased soils, extreme tropical conditions, and hot rock gardens.

Exotics
Countires like Spain, parts of South Africa, and South America, or parts of North America have similar conditions to Australia, so many exotic plants can also cope with these heated environments.

Ground Covers
Gazania rigens, although not a native, is extremely capable of doing well in heated rock or conrete areas. Other good exotics include Convolvulus mauritanicus, Erigeron karvinskianus, Trachylospermum jasminoides.

Shrubs
Nandina domestica nana, Abelia grandiflora dwarf, Raphiolepis indica, Strelitzia reginae, Agave attenuata(succulent), Viburnum tinus, Polygala myrtifolia.

Trees
Feijoa sellowiana, Gleditsia triacanthos, Magnolia Little Gem.

Strappy leaf plants
Agapanthus and Dietes are known for there ability to survive in hot baked areas. Another popular plant for these situations, is Liriope Evergreen Giant, although beware of using this plant if you want a uniform planting, as it is now almost impossible to get a true to type evergreen giant. Hundreds of different forms of this plant are now sold, and uniformity is hard to achieve. Some are 400mm high, whilst others are 700mm in height, and some burn in the sun, whilst others work well. There is a new release called Just Right, that burns far less in the sun then the Evergreen giant, and is always uniform, so it would be better to use this new form of Liriope. Mondo Grass, Society garlic, and Day Lilies are other good plants for rock gardens.

Turf Varieties
Avoid using Couch or Kikuyu near rock gardens, paths or roadsides, as their vigorous above and underground runners invade quickly. Avoid Fescues and Rye grass as they struggle with the heat, and have very poor drought tolerance. The best lawns to use are Buffalo varieties, Queensland Blue Couch and Empire Zoysia. These three will be far less invasive, and will cope with the heat. Empire will require less mowing, and is more suitable for large roadside areas, due to its even higher drought tolerance compared to Buffalos. Queensland Blue Couch is mainly used in South East Queensland, and is more prone to weed invasion than the other two, so it is less suitable for roadside use.

When using rock, pavers, or concrete in landscape design and construction, it is important to plan for the consequences of increased heat, and other possible associated problems. With a little effort, only good things will come from using rock or hard surfaces next to plants and turf.


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