Rock Gardening in Northern Climates
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Rock Gardening in Northern Climates Adapted from ROCK GARDENS & ROCK GARDEN PLANTS FOR THE INLAND NORTHWEST by WSU Cooperative Extension in Spokane DESIGN TYPES OF ROCK GARDENS: Classic The classic rock garden rises naturally from a low level to a higher one and appears to be a normal part of the surroundings. A background planting of low shrubs and small trees will help counteract any impression that a man made structure is artificial. Many home and commercial grounds in Alaska have natural rock formations that can be enhanced with rock garden plants and alpines. Walk about beds A "walk about" rock garden is usually a slightly raised bed of soil that is edged with timbers, rock or brick. Large walk about beds should have paths through the center for access to view and care for the plantings. Each section could have the soil amended for different types of rock garden plants. BANKS, WALLS AND LEDGES Banks By definition banks may have few rocks in their structure. To prevent erosion a bank should not be too high or steep. A basic planting of low growing shrubs such as junipers or cotoneasters interplanted with the larger and wider spreading rock garden plants is effective as a groundcover on large banks. Walls Dry walls are built with a minimum of cement or mortar, leaving crevices for plants. Walls can be constructed with rock, stone, brick or pieces of broken cement. Large or very high walls should probably be professionally built both for esthetics and safety . Several of the recommended Rock Garden Books have construction plans for walls. Ledges Ledges are low walls, usually not more than 2 1/2 feet in height. Choose the more compact growing plants for a ledge. WOODLAND ROCK GARDENS Many homes in South Central Alaska have large stands of Spruce, Willow, Birch and Poplar. Consider constructing a path of woodchips through these or other groupings of trees. Plant with shade tolerant rock garden plants and ferns in the shadiest area. Place sun loving plants toward the edge of the openings. TERRACES Terrace paving that is constructed with flagstone, brick, patio blocks or pavers can have openings left for low growing rock garden plants. Examples of rock garden plants that will tolerate some traffic are the Thymus serpyllum varieties,Veronica repens, Achillea tormentosa and Armeria maritima. Many of the more delicate plants are low growing and can be placed in areas where there is less or no traffic. RAISED BEDS AND CONTAINERS A raised bed might be built from timbers, rock or blocks at a convenient height. The large whiskey barrels are an alternative. Drill drainage hole in the barrels and fill with a fast draining soil. Add extra peatmoss for acid loving plants such as dwarf rhododendrons, heathers and some ferns. This type of garden can be a showcase for many small rock garden plants and the smaller conifers. To protect plant roots from winter damage build the raised beds no higher than two feet tall. Whiskey barrels can be sunk a foot or two below ground. Hill up soil and pine needles, leaves or sawdust against the sides of raised beds or containers in late fall for extra protection. Carefully lay conifer boughs over the top of the containers to help prevent desiccation. CHOOSING A SITE Drainage: Good drainage is the most important factor to consider for a successful rock garden. Although there are rock garden plants that thrive in bogs or along streambeds, the majority of plants require excellent drainage. If the soil is not porous, plant crowns and roots may rot during wet weather. First, select a site where standing water does not collect even during a long rainy period. Slopes are great. Then dig a hole one foot deep and fill it with water. The water should drain away within 1/2 hour. If your soil is less then ideal see Soil Preparation. Existing Trees and Shrubs: Avoid a site directly under the canopy of mature trees with dense foliage and/or fibrous surface growing roots, such as maple, horse chestnut, pin oak and spruces and firs. Only a few plant species tolerate heavy shade and even fewer successfully compete with tree roots for soil moisture. In the fall, large thick soggy leaves can smother plants and cause crown rot. Plants with small thin leaves such as birches and honey locust are fine. SOIL PREPARATION Soil pH: Soil tests from locations throughout Alaska have shown that our soil pH is very variable, leaning toward acid ranging from 5 to 6.5 on the pH scale. This meets the requirements of the many rock garden perennials that prefer acid soil . Areas in which to grow plants that prefer a neutral soil require special preparation by the addition of lime.. Soil Types: Garden soil: Any type of existing unamended soil with good drainage. Rich soil: Soil is high in organic content. For this requirement, spread 3 inches of compost, peat moss or bagged steer manure on the area and mix into the top 8 to 12 inches. Lean, gritty or sandy soil: Some plants grow well only in fast draining soil. If your soil is not already mostly coarse sand or gravel, spread a 3 inch layer of pea gravel or coarse builders sand and mix into top 8 to 12 inches. Containers or raised beds: Fill with a mix of 1 part sandy loam, 1 part pea gravel or perlite and 1 part peat moss. Most 3-way mixes from a soil yard can be considered a sandy loam. Watering: Rock garden plants or alpines need adequate moisture. Though most rock garden plants do not like wet roots, they originated in high altitudes or areas where their foliage received moisture from mist or fog through out much of the year, or the water in the scree is constantly percolating. Stagnant moisture can be fatal to these plants. Plants that prefer dry soil should be watered when the soil is dry down to one or two inches. If plants are going to be on an automatic watering system that is also used to keep lawns green, select plants described as needing moist soil. MAINTENANCE Spring: Remove winter mulches over a period of several days. Frost can damage the unprotected foliage. Remove dead foliage, trim away broken pieces. Push plants that have been heaved out of the ground by frosts back into the soil. Fertilize with any 5-1010 fertilizer following label directions for perennials. Once a year in the spring is enough. Most rock garden plants require only moderate fertility. Top dress with 1/2 inch of equal parts of compost or bagged steer manure and coarse sharp sand or 3/8 inch gravel. Top dressing renews soil that may have washed away and encourages plants that tend to die out in the center to fill back in. Be sure to use rhododendron and azalea type fertilizer for acid loving plants. Top dress these plants each year with 1/2 inch of peat moss or finely shredded oak leaves. Remove or trim back plants that are becoming invasive. Clean out grass and weeds. Summer: Groom plants by cutting off flowering stalks as they finish blooming. Refer to chart for plants that should be trimmed back 1/3 after blooming. Winterize by protecting with a layer of Spruce tree branches or a light layer of Spruce needles. When used as a mulch, Birch, Poplar or other large leaves tend to become soggy and smother out small shallow rooted rock garden plants. Some rock garden plants must be protected from too much winter wet by covering the crown of the plant with a piece of broken pottery. The best protection is a good layer of snow cover. ADDITIONAL REFERENCES Books: Rock Gardening. H.L. Foster. Timber Press, 1982 Rock Garden Plants of North America. An anthology from the Bulletin of the North American Rock Garden Society. Timber Press, 1996. Bernard Harkness Seedlist Handbook. Timber Press 1993 All about Rock Gardens and Plants, by Walter A. Kolaga (Doubleday, 1986) Collector's Alpines, by Royton E Heath (Hamlyn Publishing Group 1981) Gardening With The New Small Plants, by Oliver Allen (Houghton Mifflin Company 1987) Rock Garden Plants, by Doretta Klaber (Henry Holt 1959) Rock Gardens, by Wilhelm Schacht (Universe Books, 1981) Plant Sources: Companies listed here are mail order and have catalogs but many local garden centers have rock garden plants during the growing season. Alplains 32315 Pine Crest Court Kiowa, CO 80117 (303) 621-2247 Catalogue: $1.00 High Altitude Gardens Box 419 Ketchum, ID 83340 (800) 874-7333 Catalogue: $3.00 Mt. Tahoma Nursery 28111 - 112th Avenue East Graham WA 98338 (206) 847-9827 Catalogue: $1.00 Siskiyou Rare Plant Nursery 2825 Cumming Road Medford OR 07501 (503) 772-6846 Catalogue: $2.00 Rocky Mountain Rare Plants Box 20483 Denver, CO 80220 Catalogue: $1.00 J. L. Hudson, Seedsman Box 1058 Redwood City, CA 94064 Catalogue: $1.00 Recommended Rock Garden Plants Rock Garden Plant Blooming Season Flower Color Alyssum saxitile ‘Basket of Gold' spring yellow Antennaria spp. - Pussytoes early summer whitish or pinkish Arabis spp. - Rockcress spring white or pink Armeria maritima - Thrift early summer pink or white Artemisia schmidtiana ‘Silver Mound' all season (foliage) silver foliage Aquilegia flabellata. - Dwarf Columbine late spring mostly blue Aster alpinus - Alpine Aster early summer Aster dumosus - Dwarf Michaelmas Daisy Aubretia deltoidea - Rockcress early fall Campanula spp. - Dwarf Harebells spring/early summer mid-summer blue, purple, pink or white pink, red, purple or lavender-blue purple, magenta or lavender-blue blue or white Daphne cneorum - Rose Daphne late spring pink Dianthus spp.- Pinks mid-summer Euphorbia myrsinites - Spurge late spring- early summer late spring/early summer late spring combinations of pink and white yellow Genista pilosa "Vancouver Gold" Gentiana acaulis - Trumpet Gentian Gentiana septemfida - Fall Gentian yellow deep blue deep blue Geranium spp. - Dwarf Hardy Geranium late summer-early fall all summer Iris pumila - Dwarf Bearded Iris late spring variety of colours Iberis sempervirens - Evergreen Candytuft Leontopodium alpinum - Edelweiss late spring white early summer white Papaver alpinum - Alpine Poppy late spring through summer early summer white, yellow, orange or pink pink, purple, blue or white pink, blue, red or white yellow Penstemon spp. - Dwarf Beardtongue Phlox subulata - Moss or Creeping Phlox Potentilla spp. - Dwarf Cinquefoil Primula spp. - Dwarf Primrose late spring/early summer late spring/early summer spring purple, pink or blue pink, purple or white Pulsatilla vulgaris - Pasque-flower spring Saponaria ocymoides - Soapwort early summer purple, pink, red or white pink Saxifraga paniculata - Encrusted Saxifrage Saxifraga X arendsii - Mossy Saxifrage early summer mostly white spring white, pink or red Sedum spp. - Stonecrop summer Sempervivum spp. - Hens and Chicks all season (foliage) yellow, pink, red or white white, pink or red Thymus spp. - Creeping Thyme summer purple, pink or white Veronica spp. - Speedwell late spring/summer blue Viola spp. - Violets blue, purple or white Primula spp. - Dwarf Primrose spring/early summer spring Pulsatilla vulgaris - Pasque-flower spring Saponaria ocymoides - Soapwort early summer purple, pink, red or white pink Saxifraga paniculata - Encrusted Saxifrage Saxifraga X arendsii - Mossy Saxifrage early summer mostly white spring white, pink or red Sedum spp. - Stonecrop summer Sempervivum spp. - Hens and Chicks all season (foliage) yellow, pink, red or white white, pink or red Thymus spp. - Creeping Thyme summer purple, pink or white Veronica spp. - Speedwell late spring/summer blue Viola spp. - Violets spring/early summer blue, purple or white pink, purple or white
