There is a rich tradition of European and American alpine and rock gardening using containers, raised beds, alpine houses, and open gardens. “Trough” gardening, or the use of stone troughs for miniature gardens, is the most popular, yet only one of several alpine gardening forms. This article explores growing alpine plants in containers in general.
Alpine container gardening incorporates many styles and forms, as specimens or collections, miniature gardens, living art, bonsai-like creations, and even tiny, picturesque landscapes. As single specimens or in groupings, containers accentuate the beauty of certain species in their unusual forms and rich blooms. Combinations of alpines can create finely-detailed, miniature gardens or artistic collages with striking contrasts in color, texture, foliage, and shape. As focal points or as part of a miniature landscape, dwarf trees and shrubs can add further detail and contrast with the other elements.
What are alpine and rock garden plants?
Alpines refer to plants that grow above the tree-line, generally from higher elevations. Alpines are exposed to various conditions including extreme moisture levels, low nutrient levels, wide temperature ranges, shifting substrates, grazing by animals, exposure to wind, and high light levels. Adaptations include small size, shapes that minimize exposure to the harsh wind and cold, dormancy under snow, mineral encrustation and crystal growths, foliage coloration, extensive roots to cling to mountain sides or seek water and nutrients, hairs for condensation, or spiny growths to deter grazing. It is important to note that there are many permutations of alpine and subalpine climates. Alpines thrive in specific conditions, but often can not compete or survive under certain lowland or even differing alpine conditions.
Rock garden plants, which include alpine plants, generally refer to any plants suitable for rock gardens, based on size, foliage, compatibility with other plants, appearance, growth rate, and other factors. The categories of rock garden and alpine are interchanged fairly freely.
Which plants are good for containers?
Hundreds of fascinating alpine and rock garden plants are suitable for container gardening. Depending on the desired effect, plants come in various forms including mats, cushions, and mounds. Grey, “silver”, and green mat plants include the raoulia, arenaria, saponaria and azorella. Popular “cushion” plants or dense, hard mounds include scleranthus, silene, armeria, and alpine dianthus. There are moss-like “green rocks” of androsace and draba, as well as alpines that feel like rocks in the calcium-encrusted saxifraga. Alpines also include the needle-sharp pincushions of the acantholimon, dianthus, and arenaria. Suitable rock garden plants include numerous heathers, small shrubs, woodland plants, dwarf or miniature conifers, and small bulb plants.
Given the wide range of alpine and rock garden plants, there are practical and aesthetic considerations to guide your selection. Practical factors include suitability for your climate, size at maturity, nutrient needs, moisture requirements, and light requirements. The bottom line is the plants need to thrive and survive in your climate and particular conditions. Aesthetic factors include color, bloom, texture, comparative size, shape, and foliage. Books, alpine society journals, and the Internet are good places to find descriptions and specific needs. What you choose should be based on your particular tastes and the combined effect you have in mind.
Where can you find alpine plants?
Many local nurseries carry rock garden plants, but usually a limited number of alpines. Some botanical gardens and arboretums, such as Strybing Arboretum Society in San Francisco, propagate a number of alpine plants for regular fund raising sales. A few specialist nurseries throughout the country also carry alpines. A wide range of plants can be obtained through mail order companies. Seeds can be obtained through mail order and in alpine/rock gardening society seed-exchanges.
Plants should be purchased carefully to match your planting scheme. Plants may be incorrectly identified as rock garden or alpine plants and can include overly invasive plants or plants that will grow too large. Also, one should be careful with alpines planted in regular soil and with too much fertilization. Many of these plants thrive in the rich soil in summer, but will die from the same conditions in fall and winter if there is too much moisture. If you do like a particular plant, you may try to remove the soil from the roots and transplant into a more suitable medium.
It helps to see or read descriptions about the actual species in gardens, magazines, or books. Many plant genera have a number of species, both alpine and non-alpine, that vary greatly in form. For instance, a Gypsophila aretoides is a dense, flat, green mat, while Gypsophila paniculata is the Baby’s Breath commonly used in flower arrangements. Similarly, while most gardeners associate Dianthus with carnations and border pinks, alpine Dianthus are appreciated more as tight mounds or domes with various foliage color and texture.
What conditions do alpine plants need?
With numerous exceptions, alpine plants generally need excellent drainage, bright light, protection from winter moisture, and low nutrients. Many alpine plants are adapted to dry or fast-draining conditions with fresh water. Consequently, the plants may be susceptible to root rot when exposed to overly wet conditions such as standing, stagnant water. With some exceptions, alpines grow in high light due to direct sun exposure and lack of competition from other plants. Many alpines become dormant to survive winter. Some are protected under snow cover that keeps the plants relatively dry. Consequently, many will die if exposed to excessive moisture while dormant. Lastly, most alpines grow in low nutrient substrates. Excessive nutrients or organic matter may cause irregular plant growth.
Certain plants require alpine houses or cool green houses to survive the fall and winter. It is helpful to talk with experienced growers to find out what plants do best in your area. You can also observe which alpines thrive in local botanical gardens.
What containers are suitable for alpine plants?
Alpine container gardening has several advantages. Alpine containers allow elegant gardening in unusual settings including patios, decks, balconies, sidewalks, and rooftops. Elevated containers allow a closer view of the plants. Planting mix and location can be controlled to match the plants’ needs. The appearance improves with each growing season as plants fill in and form mounds. Given advantages to using containers, there are both practical and aesthetic factors in selecting a container.
Practical factors for container selection involve drainage, depth, volume, weight, and durability. For drainage, containers need holes in the bottom to allow excess water to drain away. Saucers should be avoided, where containers sit in water. Container depth is vital where plants will be exposed to significant rainfall, particularly in the fall and winter. Deeper pots create better drainage, promote root growth, and allow for greater low-capacity water retention. While alpines can run roots deep in search of water, many will suffer if they are only few inches from standing water at the bottom of a pot. Greater volume allows more room for root development. However, increased volume means more weight when it comes to moving pots around or placement on a weak structure. Durability is another factor as alpines can live in containers for years. Stone, clay, plastic, and solid hypertufa containers may last for decades, while wooden boxes, wine barrels, and the popular hypertufa-coated styrofoam container can deteriorate over time.
Aesthetic factors of containers depend on your personal tastes. These include the size, color, patterns, style, shape, and texture of the container, as well as the scale compared to the plants. The characteristics should match the your planting as well as its surroundings.
Can you make your own trough or container?
Troughs can be great containers for alpines, given sufficient depth and volume. Troughs come in various sizes and forms, and offer different options for garden structure and formality. For those who want the classic look of stone trough planting, but can not find or afford a trough, there are many people who create their own. You can cast a hypertufa trough or coat a styrofoam container or a sink with hypertufa mix to create the appearance of a stone trough. While this article focuses on the planting of containers, rather than trough building, one recipe for hypertufa is a mix of one part portland cement, two parts perlite, pumice, or course sand, and two parts of peat moss. There are many articles and books that can provide additional recipes and detailed instructions on building your own trough.
What kind of potting mix should be used?
There is a range of potting mixes suitable for alpines. For mixed plantings, mediums must be developed to suit the needs of various plants. As a general rule, the planting mediums must have high drainage, good aeration for the roots, and adequate nutrients. Most mediums are a mix of inorganic and organic materials.
Inorganic or drainage materials provide the base for the mix. Many growers use from 50% to 90% inorganic materials, including gravel (1/4″ or less), pumice, very coarse sand, perlite, and other materials. Some reputable growers use 100% inorganic materials. Fine sand (builders and beach sand) and vermiculite are generally not recommended for containers as they do not contribute to aeration. Many of these ingredients are available at nurseries, and landscape and building suppliers. While there is a wide range of recommended mixes, I tend to use very lean mixes and rely on very dilute fertilization for nutrients.
Inorganic amendments can be added to provide mineral elements. Small amounts of green sand (glauconite), rock phosphate (not super phosphate), red lava rock, and stone meal can provide nutrients over time. Red lava rock should be used sparingly as it can melt into a clay-like material under wet conditions. Some nurseries use slow release fertilizers to increase growth for sale, but these should be used in moderation.
For organic materials, many growers use various soil-less ingredients such as peat moss, leaf mold, compost, bark, and commercial soil-less potting mixes. I use combinations that last in the medium and will not impair drainage. Garden soil is not recommended for alpines in containers due to the potential for weeds and other problems. Regarding the core ingredients, you can combine materials in different ways as long as a suitable balance is maintained between inorganic and organic materials.
Certain plants will require or grow better with specific or greater proportions of organic material. Based on plant needs and growing conditions, there is a delicate balance between the amounts of organic material and drainage materials. Often plants with a good organic base and poor drainage may thrive in the summer, only to die in winter due to excess moisture. As a general rule, the drier your winter, the more organic materials you can use.
How do you set up the container?
Set up includes creating the potting mix, preparing, planting the alpines, hardening off, placing the container, and care until plants are established.
Create the potting mix using the guidelines in the section on potting mixes. You can use a wide screen to cover the drainage holes to keep in the materials. Fill the pot partially to the level at which you will place the plants. One suggestion is to have a leaner gradation of the planting medium toward the bottom of the pot. Tamp the mix down lightly and avoid creating air pockets.
Plants may require minor preparation while planting. Carefully pick out any weeds, moss, or liverwort around the plant. For plants that are root bound in their growing pots, it is often necessary to tease out the roots to ensure new root growth. If the plants are from mail order, it may be necessary to gently loosen the root ball before planting. Some growers suggest bare-rooting the plants to encourage plants to extend roots in to the potting medium, although care should be exercised to minimize root damage. Examine the root system and original planting medium to determine the conditions under which the plant was raised. You may be able to determine the level of watering as well as the richness of the medium. If plants were grown in regular soil, remove as much soil as you can without seriously damaging the root system.
Plant the alpines at the soil level in which they were grown. A layer of top dressing, such as gravel or pumice, or thin sheets of rock may be used to lift the plant from direct contact with the planting mix. Mound the material so the plants and soil level is just above the container rim. This will promote the mounding effect and will compensate for some settling of material over time.
The plants may need to be hardened off before and/or after the planting by leaving plants outside in partial shade. If the plants were raised in a colder climate, plants may need to acclimate to the new light and temperature range. If the plants have limited root growth such as young cuttings or suffer root damage in the transition, the roots need to be established before exposure to strong sunlight. For example, many dwarf conifers can tolerate strong sun, but only with an extensive root system.
Place the container in a location that meets the needs of the plants. Most alpines and rock garden plants prefer a sunny or well-lit location, although there are some that do well or better in partial shade or full shade. If heavy, containers should be planted at or close to its selected site. Containers with alpines can be quite heavy due the planting depth required, the container used, and the type of growing medium. Care should be exercised in moving the pots, both to protect the container and your back. Due to light and air circulation requirements, alpines should be grown outside or in alpine houses. Cool, well-ventilated, green houses or alpine houses are ideal for alpines that can’t survive winter wet or excessive rain.
Provide care for plants after the planting. Alpines may need sun protection and increased watering for a short period after planting until the roots extend into the container. Pick out any weeds that surface. As the medium settles, add extra top dressing if roots become exposed.
Care and Maintenance
Once set up, alpine containers are relatively low maintenance. Regular care includes watering, fertilizing, weeding, controlling aggressive or out-of-scale plants, protection from pests, and winter protection.
Watering should be regular, but not excessive. Generally, the growing medium should be damp, but not soggy. With established roots, many alpines can grow in fairly dry conditions, although some alpines thrive with frequent watering in the growing season. Given the lean potting mixture, the plant should not be allowed to dry out completely. During winter dormancy, plants should not be watered from overhead and should be kept relatively dry.
Remove aggressive or oversized plants that don’t match the scale of the design. There are also a number of weeds, including tiny alpine weeds, which thrive in rock garden conditions. Remove weeds carefully so as not to damage the roots of desirable plants. Mosses and liverworts should also be removed as these compete with alpines.
Light fertilizing is advisable during the growing season given the lean planting mix. Nutrient needs vary according to the plant and time of year, but a general rule is to dilute the fertilizer to quarter-normal strength or less. With the right potting mix, fertilizing may not even be necessary for some plants. Keep in minds that growth is slow and many containers only fill out after a complete growing season. Even if the planting appears sparse, resist the temptation to over-fertilize as you may create abnormal growth that can die in winter.
Certain pests can harm alpines. Like other plants, some alpines are susceptible to aphids, slugs, and snails. These can be removed or prevented by picking them out, through the careful use of sprays and pellets, or through organic means. Earthworms are generally discouraged in containers as they may deposit too much organic material around the roots. Birds can also be problematic as they occasionally pick apart cushions, pull out seedlings, and stash their own seeds in the container. Barriers such as covers or stakes can be used to discourage birds.
Winter is often the test for alpine survival. Winter protection is required for certain plants that are normally dormant under snow cover or live in otherwise dry winter climates. It may be necessary to move the container into a greenhouse or alpine house, or to provide direct protection under a pane of glass or similar cover. For sensitive plants, watering should be conducted from the side or bottom, rather than overhead. An antifungal/mold treatment may be necessary if mold appears. For beginning alpine gardeners, it is probably easier to start with alpines known to survive outdoors without protection in winters in your area. Winter also shifts sunlight patterns and may place plants that need light into shade.
Miniature Garden Design
Aesthetic factors have been mentioned throughout this article. Alpine and rock gardening allows a wide range of options to suit your tastes. While not restricting your own creativity and preferences, here are six general recommendations.
First, combine and space plants with their ultimate size in mind. Avoid plants that will outgrow the container or grow out of scale with the other plants. If plants grow too large, they can be removed. Slower growing plants may need a head start or extra space to develop away from more aggressive plants. You can use more vigorous plants to compete with each other and also start with larger specimens of slower plants.
Second, take advantage of contrasts in color, foliage, and texture. Alpines come in a range of colors, with various shades of greens, grays, “silvers”, and browns. The fine leaves can create fine detail. In texture, many alpines are surprisingly dense and rough to the touch.
Third, consider a formulaic approach including using one or more dwarf conifers, one or more mounding plants, and one or more matting plants. This high/medium/low combination can create interest with contrasts in height and color as well as create the appearance of a miniature garden or landscape.
Fourth, move plants around to suit your design. Many plants can tolerate the occasional move during the growing season if you minimize root disturbance, particular in leaner mixtures. Your first planting attempts may not create the effect you have in mind. As you learn more about the plants, you will develop a sense of how the plants will grow and combine.
Fifth, patience helps greatly in rock gardening. Many of the plants grow slowly and take one or two growing seasons to spread or fill out. Some plants may need to have an established root system to really flourish.
Lastly, draw techniques and ideas from other gardening areas such as bonsai, stonework, moss gardening, and Japanese and other forms of Asian rock gardening. Plants, particularly dwarf conifers, can be shaped for various effects. You can use effective positioning of plants and features to create a sense of tension, motion, balance, or impact.
Word of encouragement and where to find more information
Alpine container gardening is well worth exploring. There is a wonderful array of plants virtually unknown in the regular nursery trade. You can create tiny gardens for small spaces or projects on a grand scale. You may enjoy learning which plants thrive under your particular conditions or what mixes best suit your climate.
You can also get a head start by doing a little research. There are many good books and journals on alpine gardening. The library can give you a good introduction to the various forms of alpine gardening and the range of alpine plants available. Look for books on alpine gardening, alpine plants, and specific plant genera as well as the North American Rock Garden Society (NARGS) and Alpine Garden Society (AGS) quarterly journals.
The North American Rock Garden Society (NARGS) is another good source. There are local chapters throughout the country. Many members have substantial experience in growing and propagating alpine plants.
The Internet has many sources on alpine and trough gardening, including instructional web sites with pictures and articles. There is also the Alpine-L, an on-line alpine garden journal, where numerous enthusiasts share information.
Rick Lupp, Proprietor