This excerpt from California Native Plants for the Garden, a new book from Cachuma Press, provides simple guidelines for preparing for and planting California’s native plants in the garden, activities best scheduled for the autumn months. Though focused on California’s natives, the advice is relevant for other regions along the West Coast with wet winters and dry summers, as well as for other plants from mediterranean climates.
The optimal time for planting most California natives is autumn, except in the mountains, high deserts, and other cold-winter areas of the state. Planting during this period—at the beginning of the rainy season—gives natives many months to establish root systems that will be much better prepared to survive California’s long, warm, dry summers. Plants installed in fall also produce more new growth and flowers the following spring or summer than those that are planted later.
For the native plants of our mediterranean climate, California’s fall months are comparable to early spring in the rest of temperate North America. At this time, soils are still warm but are not too hot for tender new roots. Capillary movement of water is upward, toward the soil surface. Nights are cool, promoting less shoot growth and more root growth; days are also relatively cool, leading to less stress on new plantings. These cooler conditions are less favorable to many soil-borne pathogens that are apt to infect root systems that have been broken or damaged during planting. California natives can be successfully planted outside this time period, but they will initially require much more care and attention. The exceptions to this general rule are those species that come from year-round moist environments, such as coast redwood, white alder, and giant chain fern. Although it is best to plant these water-loving species during fall and winter, you can be more flexible as to when you install them, but realize that they need adequate irrigation throughout their lives.
Site Preparation and Soil Management
The celebrated diversity of California’s native flora is due in part to the state’s tremendously varied geology and geography. Some of our most reliable plants for horticulture, such as coast live oak, coffeeberry, and toyon, are widespread and found on a variety of soil types. Another group of natives is distinguished by its demand for well-drained soils and intolerance of waterlogged situations. Plants that occupy steep slopes typical of chaparral plant communities fall into this category; proper site preparation and soil management can be crucial to their success in the garden. Still other species naturally occur only on soils that are characterized by a particular mix of nutrients and physical properties. Two examples are leather oak, which is restricted to serpentine soils, and saffron buckwheat, which is limited to volcanic outcrops. However, these two species, and many like them, can grow in soil types not found in their natural habitats and are easily cultivated in most home gardens.
If your goal is to create a landscape that is well adapted to your site, it is best to select plants that are compatible with your garden’s soil conditions. Unless the soil has been significantly modified physically or chemically, such as by heavy compaction or over-fertilization, there is usually no need to add amendments; this once-routine practice is unnecessary in most cases. However, if your landscape plan includes natives that are highly incompatible with your soil, you will need to alter the soil structure. Plants that require excellent drainage often fail in heavy, slow-draining clay soil; plants that thrive in rich, moisture-retentive soils may not succeed in nutrient-poor, fast-draining sandy soil. One solution that works in both cases is to amend the soil with organic matter; this adds nutrients and beneficial microorganisms and also improves soil structure. As organic matter becomes embedded in the pore spaces between large sand particles, it acts like a sponge and enhances the fertility and moisture-holding capacity of sandy soils. In heavy soils, it increases porosity and drainage by binding the fine clay particles together into larger aggregates.
When adding organic matter to the soil, work it into the entire garden area, not just the planting holes. A one-time application of organic matter will not provide long-term results, since this material breaks down rapidly in our mediterranean climate, leaving the parent soil with most of its original physical and chemical properties. To succeed with this technique requires an ongoing program of soil amending and mulching. If you are primarily cultivating chaparral or desert plants, use organic matter sparingly, if at all, since this can lead to excessively rich soil for species that are better adapted to the nutrient-poor soils of their native habitats.
For gardeners with heavy soils who wish to grow plants that require good drainage, consider mounding the native soil into berms. Mix gravel into the berm or mound if your plants need especially good drainage; sprinkle the soil surface with some of the gravelly substrate as well. Strive to create mounds that visually blend with the overall topography of the site—otherwise they will look contrived and unnatural. A few well-placed boulders can help create fast-draining pockets and add visual interest. Another approach is to build raised beds and amend the soil as desired. Before you create mounds, berms, or raised beds, it is a good idea to break up any shallow, preexisting hardpan layer.
When planting in adobe clay, it helps to pre-moisten the garden area to a workable consistency before the autumn rains begin. This gives you a chance to cultivate the soil when its moisture level is neither too dry nor too wet. Once rains have saturated a heavy clay soil, planting activity is inadvisable, unless there has been an extended warm spell that has dried the ground. Cultivating wet clay soils promotes the formation of unworkable clods and simultaneously increases compaction.
Most garden soils, even in newly cultivated areas, will contain mycorrhizal fungi appropriate for the native plants that would naturally be found in the vicinity. Mycorrhizal fungi are beneficial organisms that inhabit roots of nearly all plants. These fungi assist their host plants with nutrient absorption from the soil. Some nurseries are inoculating their container-grown stock with these fungi, though the practice is not without controversy. On heavily graded sites where no topsoil remains, new gardens will almost certainly benefit from planting specimens that have been inoculated with mycorrhizal fungi. Another technique is to add these organisms directly to the soil when planting.
Obtaining Plants and Seeds
Buying native plants is an adventure. It may require a bit of research, because some California plants are not widely grown. Your quest for a particular plant will often lead to specialty nurseries and smaller growers. It may take time to obtain all of the plants that you want for your garden, as some natives are grown in small quantities and may only be available in one season of the year.
When buying seeds, it is a good idea to inquire about their freshness, since seed viability varies with species, age, and storage method. Always purchase your native plants and seeds from reputable nurseries and suppliers. Among other attributes, such businesses accurately label their plants, sell only properly permitted plants, pay royalties on patented and trademarked plants, and have a knowledgeable staff.
Collecting plants, seeds, divisions, or cuttings from public lands is illegal unless appropriate local, state, and federal permits are first obtained. A number of California’s horticultural treasures are endangered in their wild habitats due to over-collecting, and numerous others may be pushed to the brink of extinction if illegal collecting activities are not stopped. Do not hesitate to ask vendors questions
about their sources of native plants, especially when endangered, rare, or large specimens of slow-growing species are offered for sale.
When selecting plants at the nursery, keep in mind these guidelines:
- The plant with a balanced “root to shoot” ratio is the best buy. Do not automatically choose the largest or the smallest plant; the largest plant is likely older and root-bound (or it may have been over-fertilized), while the smaller plant may be too young and have a fragile root ball.
- Ask the sales staff if they will allow you to inspect the root system. The best plants will have numerous healthy white roots. (There are exceptions, such as barberries, which have yellow roots.) The roots should knit the rootball together but should not form a solid mass. Root inspection is not always possible, and it is not advisable for plants with fragile root systems, such as bush poppy, Matilija poppy, and some manzanitas.
- The lushest plant in the block may have received too much fertilizer and may not have a fully developed root system that is able to support all that foliage. Such plants are often prone to pests and diseases.
- Check the crown (where the stem meets the root) of the plant. For the majority of California native trees and shrubs, it is important that the crown is at, or slightly above, the soil level. If the crown is buried, the plant may be susceptible to crown rot. If the crown is too high, the plant may not be able to hold itself upright after it has been placed in the garden or a new container.
- Smell the soil at the drainage hole of the pot. Do not buy the plant if the soil has a foul stench; this indicates that the plant’s root system is rotting.
Take extra time and care when setting a new plant in the ground, since poor planting techniques can compromise its success. The hole for each plant should be twice as wide as the size of the rootball. Avoid digging a hole that is perfectly round and smooth, as this can discourage root penetration into the surrounding soil. Make sure the rootball is moist before removing the plant from the container. Gently loosen the edges of any rootball that has formed a tight mat while in the nursery pot. Where gophers are a serious problem, consider enclosing the entire rootball with one-inch or half-inch aviary wire, leaving a small opening at the bottom. Fill in around the plant with the same soil you removed to make the hole, and gently tamp the soil to eliminate air pockets. Water thoroughly to settle the plant into the ground. When you are finished, the plant’s crown should be less than one inch above the original ground level.
It is often necessary to create an irrigation basin after installing a new plant. This is particularly important if you plan to water by hand or when you are planting on a slope. To form the basin, mound the soil into a berm that surrounds the plant, and be sure the diameter of the basin is wider than the original hole. When the basin is filled with water, the water level should always be below the crown of the plant to prevent crown rot. Remove the berm before the rainy season because it will trap excess rainwater, which may cause crown or root rots.
These specialized storage structures rest during the dry summer months and begin to stir underground when the cool temperatures and first rains of autumn trigger their new growth cycle. If all goes well—and there’s no guarantee—the adventurous gardener will be amply rewarded with endearing, beautiful flowers. Flowering is highly dependent upon rainfall; bulbs may produce only foliage in lean years. Some species bloom profusely following wildfires, suggesting an affinity for the potassium released from wood ashes and increased amounts of ground-level sunlight.
When cultivating bulbs, careful attention to watering is essential. Start by determining whether the bulbs you want to grow occur naturally in habitats that become dry in summer. Most California bulbs benefit from regular water during their growth phase (desert species excepted), followed by a dry period after flowering. Yellowing leaves are your visual cue to curtail watering or risk rotting the bulbs. To maximize transfer of nutrients back to the bulb or corm, avoid the temptation to remove the withering leaves.
Be prepared to guard against a host of predators. The juicy new shoots are a delicacy for snails, slugs, rabbits, mice, and birds. Gophers, other rodents, and birds eat the bulb itself, while deer nibble lily buds just when you think you have earned the flowers. Aphids and other sucking insects stress bulbs and simultaneously spread harmful viruses.
Planting native bulbs in containers or raised beds makes pest and water management much easier. As the bulbs come into bloom, tuck the pots into the garden to brighten a border or complement other containers, then whisk them away when flowering ends.
Most native bulbs are surprisingly tolerant of heavy soils, as long as water is withheld during the dormant period. It is safer, however, to plant them in well-drained soils, whether in the open ground or pots. A one-time dose of low-nitrogen fertilizer is acceptable within the first month of active growth but is not necessary.
Excerpted and adapted, with permission, from California Native Plants for the Garden, a beautiful and inspiring new book by three authors familiar to readers of Pacific Horticulture. Published by Cachuma Press, the book is due out in early 2006.