Iberis sempervirens 'Whiteout'
Profuse flowers on evergreen foliage are lovely cascading over stones, planted at garden's edge, or along walkways. Blooms attract butterflies. Makes a striking companion to spring-blooming perennials and bulbs. Shear after blooming for best appearance.
Early to late spring
Grows 6-8" (15-20 cm) tall.
12-14" (31-36 cm) apart.
Tips and Uses
Prefers well-drained soil; check often until established
Shear back lightly after bloom to maintain neat appearance
Attracts butterflies, evergreen leaves, showy ground cover
› General Plant Info
- Perennials are long-term residents of the garden, hardy plants that go dormant during the climate’s most difficult season and reemerge when conditions improve. In regions with cold, harsh winters, plants go dormant with the first killing frost and return with the spring thaw. In warm, arid regions, plants go dormant during the most hot, dry periods of the season.
- The average lifespan of perennials varies widely by type. Garden classics such as peonies are exceptionally long-lived and can be passed along through generations in a family. Hybrids with heavy and continued flowering may be shorter-lived, but are well worth growing for their showy display.
- Perennials are dynamic plants in the garden and landscape, providing constant change as they cycle in and out of bloom throughout the season. Some varieties provide a consistent presence with attractive foliage or long bloom, while others may spring up only briefly, explode into bloom, and quietly disappear. One of the key aspects of perennial gardening is to plan for the seasonal changes.
- Perennials have a place in nearly any home landscape.
- In containers:
- Use a single plant to create a bold statement.
- Combine with annuals as a foliage or seasonal component, with plants that have complementary or contrasting colors and textures.
- Use in a mixed perennial garden, designed to a specific theme or style, or create something all your own.
- Plant in sweeping masses for blankets of seasonal color.
- Mix with annuals, which can help provide constant color as the seasonal perennial display comes and goes.
- Use as a part of larger landscape plantings to complement an overall design.
- Design and site considerations:
- Bloom time - In a small garden, you may want all of your plants to bloom at the same time. More commonly, gardeners want to grow an assortment of perennials that will ensure flowers throughout the growing season.
- Color - Choose plants that will carry your color theme through the season, or shift over its course. Don’t forget to consider foliage color too, as this can be a more constant part of the garden than flowers on some plants.
- Height - To make sure all your plants get good lighting and none get lost among the other plants, start with tallest varieties at the center or back of a planting, and place gradually shorter plants as you move toward garden’s edge. It’s fine to put the occasional tall, open plant closer to the edge as a see-through plant to keep things interesting.
- Remember that height and bloom time will work together, and change over the season. Delphiniums, for example, will provide garden height in early summer but not in mid- or late season.
- Spacing - Recommendations are based on the normal width of a mature plant. You may space plants farther apart or closer together, depending on how quickly you want the planting to fill in, keeping in mind that you will need to thin or divide plants if they get too close and crowd each other out.
- Light preference - Always match your plants to the light conditions where they will be growing. Most plants are put into one of three basic categories:
- Full sun - 6 or more hours of direct sun
- Part sun - 4-6 hours of direct sun. Also includes bright areas with dappled light
- Shade - Less than 4 hours of direct sun
- Watering needs - All the plants should have similar watering needs so they will be happy together.
- Special features - Look for qualities that suit your gardening objectives or theme, such as low maintenance, fragrance, attractive to butterflies, resistance to deer damage, etc.
- You’ll enjoy your plants more if you visit them regularly, and they’ll always look fresh if you take the opportunity to remove any faded blooms or trim stray growth. You’ll also notice any problems early and be able to take preventive action.
› Water Tips
- New transplants will take a few weeks to establish as their roots grow out into their new soil. Initially, you’ll need to frequently check the soil at the plant’s base, and water if it feels dry to the touch. Observe plants closely for the first few weeks to check for signs of flagging, and make sure to water before plants wilt.
- As plants become established, they will need less water, depending on rainfall. A rain gauge is a helpful tool to understand how much water plants receive, and determine any additional irrigation needed.
- Conditions that increase water needs:
- Dry, windy weather
- Periods of drought
- Sandy, light soils that do not hold moisture well
- Containers or plants in hot, sunny spaces
- Methods and timing:
- For new transplants, make sure the water soaks the original root ball as well as the surrounding soil.
- Slow, deep watering is most effective; frequent, shallow watering encourages shallow root systems that are less drought tolerant.
- Apply water at the base of the plants to minimize evaporation and keep foliage dry, which also helps prevent disease.
- When overhead sprinklers are used, water early in the day; morning is best. Foliage will dry quickly as the day warms.
- If plants are wilted, water right away.
- Check daily, and water thoroughly when soil feels dry an inch below the surface.
- If plants appear to be wilting, check soil to make sure it is dry — extreme overwatering can also cause wilting.
- Drip irrigation or soaker hoses can be put on a timer and help make vacation watering a breeze.
› Planting Tips
- Most soils benefit from the addition of organic matter such as compost, peat, or composted manure. This helps poor, sandy soils to better retain moisture, and breaks up heavy clay soil to help air, water, and plant roots move more freely.
- For new planting beds, prepare the soil by tilling and work in organic matter following product recommendations.
- To add new plants to an established bed or use as a specimen in the lawn, dig a hole twice as wide and about the same depth as the root ball of your new plant, slightly more shallow if soil is heavy clay. Mix organic matter with the native soil.
- Gently remove plant from its container, taking care not to pull on the main stem as this can cause damage by breaking away roots.
- Loosen roots gently if they are packed tightly into the shape of the container.
- Plant at the same depth as it was in its container. For larger plants, make a low ring of soil around the plants, just wider than the root ball. This will help funnel the water to the plant’s roots during the establishment period.
- For peat pots, break away the lip of the container so that it is just lower than the soil, then plant as above. For other plant-in-the-pot products, follow package directions.
- Water carefully with a gentle flow, soaking the prepared soil and the transplants.
- Mulch may be applied between the plants, keeping a few inches away from the plant stems. This will help prevent weeds and retain moisture.
- Choose a container with drainage holes. There is no need to add pot shards or stones to the bottom of the pot.
- Large planter boxes and containers are best if you want the plants to stay in the container for more than one season. Plants have more room to grow, and are less likely to dry out or be damaged by freezing in larger containers.
- Use a lightweight professional potting mix for small containers, or a high-quality organic garden soil for larger, open-bottomed planters.
- Remove plants from the nursery pots.
- Loosen roots gently if they are packed tightly into the shape of the container.
- Plant at the same depth as the root ball.
- Water thoroughly with a gentle flow.
- Note that plants grown in containers are more vulnerable to winter extremes. In cold winter climates it is recommended to transplant perennials into the ground in fall, unless they are hardy to two full zones colder than your region.
- As a rule, perennials will not require a lot of fertilization. Replenishing soil annually with a top dressing of compost, and maintaining a 2-3” layer of organic mulch will help build and retain good soil fertility.
- Sites with poor or lean, sandy soil may need some help with supplemental fertilizers.
- Time-release fertilizers can be used as a soil dressing, and are applied only a few times during the growing season, depending on the product. Each time the plant is watered, a small amount of fertilizer is released.
- Water-soluble fertilizers can be applied with a hose end sprayer or mixed in a watering can or bucket. This gives plants an immediate boost, but will need to be repeated throughout the season.
- Choose a product formulated for perennials and follow package instructions carefully.
- If you are uncertain about your soil fertility, your local extension service can usually recommend testing services.
- New transplants will not require fertilization until after they have become established, unless you are using a specially formulated starter product.
- Perennials grown in containers may need more fertilization, especially if grown in a soil-less potting mix. If you choose a soil with fertilizer mixed in, apply supplemental feeding only as recommended for that soil product.
- If your plants are dry and wilting, water immediately with clear water. Fertilizing at this time will cause leaf burn. Wait until plants have perked back up before feeding.
› Pruning Tips
- Fall-blooming perennials such as mums and tall asters benefit from early season pruning. Cut plants back by half when they reach 6-8 inches in height. This causes plants to branch out, producing more flower buds and a shorter, bushier form.
- Extend perennial bloom season by removing spent flowers before they set seed. This also keeps plants looking tidy and prevents unwanted reseeding. You may wish to allow seed to form toward the end of the plant’s natural bloom period, if winter interest or reseeding is desirable.
- Early-season bloomers may need a midseason shearing to encourage new growth and improve the plant’s appearance. Some plants, such as delphinium and perennial salvias, often respond by reblooming in the fall.
- Perennial maintenance:
- Tall perennials may need staking in windy sites to prevent damage. Choose natural-colored materials that will not be noticeable, and tie plants gently at a natural angle.
- Some perennials have a naturally tumbling habit and look best when they are supported by other, sturdier perennial companions rather than stakes. Common examples include baby’s breath (gypsophila), tall blanket flowers (gaillardia) and catmint (nepeta).
- Keep garden plots and containers weed-free to prevent competition and reduce pest and disease problems.
- If established perennials begin to open or lay flat in the center, it is time to divide them. Consult a reference book or experienced gardener for the best time and method for the specific type of perennial.
- Take notice of any signs of disease or insect damage so that you can take prompt preventive action.