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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 a harsh environment.

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 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 35.6-42.8°F hotter than the surrounding countryside.” Your rock garden, pavers or concrete can do even worse to plants in summer. A 107.6°F day can have a heat effect on the plants that are touching the rock, equivalent to 122°F. 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 that's why choosing Aussie plants is essential in a rock garden, they a suited to this situation. Some of the best can even be planted in summer in these rock or hard surface gardens. Please note that you should check whether these plants are suited to your area.

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), Purple Lea (taller) and Pennstripe (variegated foliage) 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). The new compact Imparata cylindrica Yalba is a great native with red foliage colour in autumn and winter, good for mass planting and roadsides.

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 longifolia Breeze has been proven to work on hot roadsides better than any other plant. Katrinus deluxe is another tough Lomandra longifolia, 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 Dianella's that work well in rock gardens are Little Rev with it striking architectural foliage and Big Rev with its masses of blue flowers. Little Becca also works well and has an amazing number of deep purple flowers, 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. Some natives to America and exotics that will work well in rock gardens are listed below. Please note that you should check whether these plants are suited to your area.

Ground Covers
Gazania rigens is extremely capable of doing well in heated rock or conrete areas. Other good plants include Convolvulus mauritanicus, Erigeron karvinskianus, Trachylospermum jasminoides.

Shrubs
Nandina domestica Blush and Flirt, 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 Just Right, it 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 Bermuda grasses near rock gardens, paths or roadsides, that have vigorous above and underground runners as they invade quickly. In warm regions avoid Fescues and Rye grass as they struggle with the heat, and have very poor drought tolerance. The best lawns to use are St Augustine varieties (provided there is occasional watering) and Empire Zoysia for dryer areas. 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 St Augustine.

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.