PREPPING THE SITE
Consider sending a soil sample to a lab for testing to determine nutrient levels and pH (acidity or alkalinity). Amend the soil according to lab recommendations.
Determine the type of soil – is it:
Heavy with lots of clay that tends to stay wet: amend with lots of good quality compost, mixing some throughout the top 6” and reserving some extra to incorporate into the top 1”-2”. Plan to “plant high” to increase drainage around roots.
Lightly clumping, organic-rich and well-drained: incorporate an inch or two of compost into the top 1”-2” after digging.
Loose and sandy, draining very rapidly so it is overly dry: amend with a large amount of compost or a combination of compost and loose peat to a depth of at least 6”-8”.
Combination of the above: amend according to predominant type or types, digging to about 6” to loosen soil.
PLANTING YOUR NEW GARDEN PERENNIALS
- Dig a planting hole that is as deep as the height of the root ball and at least twice as wide as the container or root ball.
- Add a small amount of starter or slow release fertilizer to the bottom of the planting hole and cover with about an inch of soil. Starter fertilizer generally has a higher proportion of phosphorus, which promotes root growth. By placing the fertilizer in the planting hole the phosphorus will be in the root zone where it can be used. If desired, a small amount of fertilizer may be sprinkled on the surface; just make sure it does not touch the plant stems or leaves.
- Loosen the root ball to expose the roots and remove any loose potting mix. Cut off matted roots at the bottom of the pot, unwind or trim any roots that are circling the root ball and trim any that are broken before placing the plant into the hole. Center plant in hole with crown at soil level.
- Soak the hole and the root ball. Firmly replace soil around the roots. If the plant is large, stop when the hole is half backfilled and water it to settle the soil before adding remaining soil.
- Water well to settle all soil, to minimize air pockets and to moisten the surrounding area. The level of your plant when you are finished should be the same in the garden as it is in the container, or slightly higher, if drainage is poor.
Natural mulches such as pine bark and shredded hardwood are great for reducing weeds, for maintaining soil moisture and even temperature, and will eventually break down to add beneficial organic matter to the soil. Usually 1”-2” is sufficient - do not use more than 3” of mulch. Do not pile mulch on the base of the plant, as it will encourage rot and boring insects.
Some Mediterranean and other dry climate perennials will appreciate gravel mulch that dries rapidly and reflects light and heat so their foliage dries quickly. Approximately an inch of mulch is sufficient.
Water when the root ball is dry. You should check frequently because the root ball typically will dry out before the surrounding soil.
Depending on the plant and the weather you may need to provide supplemental water during the plant’s first year in the garden or until it is well-established.
- Remove broken or damaged portions of plants and cut off old blooms (deadheading) to prolong the beauty and bloom season. It is usually best to trim just above a leaf node (where a leaf joins the stem).
- If plant foliage becomes unattractive during the growing season cut plants back to healthy leaves and stems or to within an inch or two of the soil. Fertilize lightly and water well to promote new growth. Plants that have become too tall can also be cut back in the first half of the season to promote fuller growth. This will delay flowering if it’s done before they have bloomed.
- Remove old growth after plants have gone dormant in late fall or in early spring to make way for new spring growth and to remove winter lodging for pests and diseases. If you are leaving seed heads for birds, etc. just clear away rotting matter and fallen leaves. It is best to allow foliage to remain on evergreen perennials such as lavender, autumn fern, rosemary, etc., until early spring and remove it just before or just after new growth starts. Some plants that are not evergreen will also benefit from waiting until spring for cutting back, including Perovskia, Hypericum, and Agastache.
- Check plants frequently for evidence of disease or insect pests. If found early many problems are treatable with non-toxic controls such as insecticidal soap, horticultural oil, beneficial insects or nematodes, and/or mechanical removal of pests.
- The need for fertilizer depends on the plant, the soil type and the amount of rainfall. It is best to learn the specific needs of a plant –some are “heavy feeders” and respond well to fertilization. Others prefer average or even poor soil and may actually be negatively impacted if unneeded fertilizers are added. For many perennial plants an annual top-dressing of compost is sufficient to meet their nutrient needs. Avoid heavy nitrogen applications to prevent excessive vegetative growth on plants grown for flowers.
- Many perennial plants benefit from division every few years. Bearded irises, daylilies and asters, for example, will normally respond with strong growth and increased blooms. Other perennials, such as hosta, ferns and many groundcover perennials, rarely require division unless you want to make more plants for your garden or to give to your neighbors.
- Refresh mulch. Organic mulch gradually degrades, which can improve the soil, but it will need to be replenished every couple of years, or if it has been significantly disturbed by digging or weeding. Gravel mulches don’t degrade but do tend to work their way into the soil and may need topping off as well.
- Control weeds to minimize competition for nutrients and moisture. Weeds can provide hiding for garden pests or serve as hosts for disease organisms that may spread to your garden plants. A good mulch is the first line of defense against weeds. For areas that cannot be mulched or are under high weed pressure a pre-emergent herbicide may be used to keep many weeds from sprouting. Always read and follow label directions carefully to avoid harming desirable plants. Most post-emergent herbicides will damage desirable plants and are not recommended. Learn to recognize common weeds that seem to sprout overnight, so that they can be removed before they set seed.
CHOOSING THE RIGHT PLANT
Growing perennial plants successfully requires meeting a plant's specific cultural requirements. Before you choose your plants, consider your site’s exposure, climate, soil type, drainage, pH, nutrient availability, and water availability. Some of these, such as pH, drainage, nutrient levels, and water availability can be altered, but others cannot. If you pick plants whose cultural requirements are closely matched to your site, you are more likely to be successful. Exposure and climate are two conditions we can rarely change, so these are very important to consider.
Exposure, or availability of sunlight, is a major consideration for most plants. Determine whether your site has full sun (at least 6-8 hours of direct sun a day), partial sun/partial shade (4-6 hours of direct sun preferably earlier in the day), dappled or light shade (intermittent shade throughout the day and fewer than 4 hours of direct sun) or full shade (less than 2 hours of direct sun, either very early in the morning or late in the day).
It is important to observe your site over time to get a true read on the exposure. The changing of the sun’s angle over the year and the shedding of deciduous trees can significantly change exposure in a given area. Shade that is created by structures is often deeper and more consistent than shade produced by deciduous trees. Many shade perennials actually benefit by taking advantage of the sun before trees unfold their leaves in spring or after trees drop their leaves in fall and may languish in the shade of buildings.
Also consider any slopes. South or west facing slopes can actually be as much as a full hardiness zone warmer than the rest of your site. A north-facing slope or low spot can be much cooler or more frost prone.
Climate includes temperature averages and extremes, humidity levels, general precipitation amounts and so forth. Using the USDA Hardiness Zone maps will help to determine the typical temperature extremes for your area, but they do not incorporate information such as humidity and rainfall. In other words, Zone 7 in Virginia is not the same as Zone 7 in California.
Consider maintenance requirements. No plant is truly “no maintenance” (even plastic plants fade and get ugly) and “low maintenance” is a relative term. Choosing plants adapted to your site will reduce maintenance chores. If you can’t or don’t want to irrigate, choose drought-tolerant plants. If your soil tends to be soggy, opt for plants that don’t mind “wet feet.” If you don’t like cutting off old blooms, choose plants that are self-cleaning or that can simply be cut back once or twice a year.
Pests and diseases can also be an issue. Look for pest and disease resistant varieties and avoid overcrowding to reduce the incidence of diseases. Spraying pesticides is a time-consuming chore that is best avoided if possible. Choose deer and rabbit resistant plants where necessary.
Be careful when planting perennials listed as rapid spreaders, invasive or “very vigorous,” or that readily self-sow, if you do not want to spend time pulling or digging unwanted plants from your garden. Many a new gardeners have been surprised at just how far a violet can seed itself or how fast moneywort can take over a small garden. Look for plants whose ultimate size fits into your space. A vine that gets 20’ long will be much more work to maintain on a trellis or arbor than will an 8’-10’ vine. Some plants, such as bearded irises, require frequent division (every 3 yrs.), which means digging up the plant and splitting it before replanting. Many others are happy when simply left to their own devices.
Some plants, such a Perovskia and Caryopteris, need to be cut almost to the ground each spring to bloom well and remain bushy. Others, like garden salvia, benefit from being cut back hard after they bloom.
Keep garden cleanup in mind. Plants that tend to catch a lot of leaves or debris will require more effort to keep them looking neat.
Consider the size of your planting area. Choose plants that are appropriate to the space available. Although it is tempting to fill every foot of garden space with plants, it is important to allow enough room for plants to grow. Most perennials will not reach their mature size until their third year, so plan your design on the basis of the mature height and width of plants. Putting in plants that are too large for the area (or putting in too many) will mean extra work such as pruning to keep their size in check, removal of excess plants, and/or more treatment to reduce disease problems that are made worse by overcrowding.