April in the Ornamental Garden
April is prime planting season for gardeners in the mid-Atlantic. In fact, this can be our busiest month as we contend with spring cleanup, dividing, transplanting, weeding, and other spring gardening chores. The bright, sunny days and warm spring breezes this month are perfect for working outside and we can be lulled into thinking cold weather is behind us. But it’s important to stay vigilant for sudden dips in night-time temperatures that can result in deadly overnight frosts.
The average last spring frost in Albemarle County (USDA Zone 7a) generally occurs between April 15 and April 25. If a frost is forecast, cover tender new growth to protect it from frost damage. Use a row cover, an old sheet, cardboard, or even layers of newspaper for this purpose. Remove the coverings the next morning so that you don’t inadvertently “cook” your plants as daytime temperatures warm up.
As daffodils and hyacinths finish blooming, cut the flower stalks all the way back to the ground but leave the foliage in place to die back naturally. This allows the plant to focus on storing energy for next year’s blossoms rather than on developing seed heads. The foliage may look a bit messy, but don’t braid or tie it up because this may interfere with photosynthesis, which could affect next year’s blossoms.
If spring-flowering bulbs such as daffodils, snow drops, or crocus have become crowded and didn’t produce as many flowers as in past years, that may mean they need to be divided. The ideal time to divide these bulbs is after the foliage has died back, which may be June or July. Mark the location of the flower clump now while you can still see the green or yellowing foliage. This will help you remember where the clump is located, plus it will remind you not to plant something else in the same spot. Make a note to dig up and separate the bulbs once they are dormant. Either replant them immediately or store them in a cool, dry, well-ventilated place and replant them in the fall.
Top dress established ornamental flower beds with an inch of compost. For new flowerbeds, work compost or aged cow manure into the loosened soil before you start to plant. A slow-release fertilizer and lime may also be added to the soil if a soil test indicates the need for either.
Hairy bittercress. Photo: Leslie J. Mehrhoff, University of Connecticut, Bugwood.org, CC BY 3.0
Remove broadleaf winter weeds before they set seed. These cool-season weeds include chickweed, deadnettle, hairy bittercress, and henbit. They germinate in late summer or early fall, overwinter in the landscape, and produce flowers and seeds in spring. You can suppress their growth in your flower beds by applying a layer of mulch over bare ground or planting a dense ground cover. Weed identification information and photos are available on a number of extension websites such as Virginia Tech (weedid.cals.vt.edu), University of Missouri (weedid.missouri.edu), or the University of Illinois (weeds.cropsci.illinois.edu).
Divide fall-blooming perennials, such as asters (Symphyotrichum species), chrysanthemums (Dendranthema), shasta daisies (Leucanthemum), black-eyed Susan (Rudbeckia), ornamental grasses, sneezeweed (Helenium), false aster (Boltonia), and bee balm (Monarda). Most perennials benefit from being divided every three to five years on average, but if you’re not sure if a plant should be divided, here are a few questions to ask yourself:
- Is the plant not producing as many flowers as in previous years?
- Has it outgrown its assigned space in your landscape and is it crowding other nearby plants?
- Is it alive around the edges of the crown but dead in the center?
- Does it seem less vigorous in general?
- Do the stems in the center of the plant have smaller leaves?
- Are the inner flower stalks weak or flopping over?
If the answer to any of these questions is yes, then the plant probably needs to be divided. Try to complete this task at least six weeks before hot weather sets in so that the divisions have ample opportunity to become well established.
Another reason to divide perennials is to increase air circulation, which helps control fungal diseases. For example, Monarda fistulosa (or wild bergamot) is valued for its highly aromatic flowers that attract pollinator insects such as bees and butterflies, but it is subject to powdery mildew. By dividing this popular native plant every 3 years to improve air flow and providing it with moist, well-drained soil, a sunny site, and destroying all infected foliage, can help keep this common fungal problem under control.
Before digging holes for new plantings, keep in mind the ultimate size of each plant. Also, group plants together according to similar needs for water, nutrients, and sunlight. Remember to update your gardening records indicating the location of your new plantings.
Pinch back chrysanthemum foliage this month when the plants are about 4 inches high. Pinching makes the plant bushier, sturdier, and more wind-resistant later in the season. Tall aster species also benefit from being pinched back for the same reasons. False sunflower (Heliopsis helianthoides) is another plant that benefits from being pinched back in spring to reduce the plant’s height.
At this time of year, garden centers are overflowing with the best selections of landscape plants. Shop for azaleas and rhododendrons while they are in bloom to ensure you like the color and that the color harmonizes with your other landscape choices. This is particularly important if you are adding new plantings to an established landscape. Some pink selections, for example, have an orange or coral undertone that may clash with other spring-blooming species located nearby. Tip: Azaleas generally look best planted as a grouping in part sun or filtered shade and acidic, well drained, organically rich soil with a pH of 5.0 to 6.0.
As you select new plantings for your garden, avoid plant species that are potentially invasive in this area of Virginia. Look for native plants that minimize maintenance, require less water, and increase habitat, particularly for beneficial insects. A number of excellent native plant resources are available, such as the Virginia Native Plant Society’s website at vnps.org, the Albemarle County Recommended Native Plants website at webapps.albemarle.org/nativeplants, or the Virginia Department of Conservation and Recreation’s Natural Heritage Program at dcr.virginia.gov/natural-heritage. Also, explore back issues of The Garden Shed for a number of articles on native plants suitable for our area.
Buy annual bedding plants such as begonias, petunias, pentas, geraniums, or marigolds while selections are plentiful. Choose healthy plants with well-developed root systems that are not too large for their pots. Don’t plant them, however, until the danger of frost is past, night-time temperatures are consistently above 50° F, and soil temperatures are above 60° F. Depending on the weather, that may be toward the end of April or even early May. If you just can’t wait that long, be prepared to protect those tender seedlings from frost if temperatures threaten to turn chilly.
If you prefer to start bedding plants indoors from seed rather than buy transplants from a garden center, you can still sow the seeds during the early part of April if you didn’t get around to it in March. Don’t forget to harden off tender seedlings before planting them outdoors. For the new or inexperienced gardener, Virginia Cooperative Extension (VCE) Publication 426-001, Plant Propagation From Seed, provides good information on sowing seeds.
Plant some everlastings in your ornamental garden this spring. The term “everlasting” refers to a flower, seedpod, or other plant part that can be dried or preserved without the loss of its shape or color. Everlastings are used in dried flower arrangements, wreaths, bridal bouquets, and many craft projects. In addition to strawflower (Helichrysum), baby’s breath (Gypsophila paniculata), and statice (Limonium), all of which are easily preserved, try experimenting with other flowers such as: Bells of Ireland (Molucella laevis), cockscomb (Celosia), or globe amaranth (Gomphrena).
Don’t move your houseplants outside until night-time temperatures consistently stay at 50° F or higher. Depending on the weather, this may not happen until very late April or in May. Place them in a shaded area on a porch, patio, or under a tree or wherever they can gradually acclimate to the increased light levels. Moving a houseplant from indoors directly out into a sunny location can burn the leaves, which will severely damage the plant.
Before you move your houseplants outdoors for the summer, repot any that are rootbound. You can tell a plant is rootbound if:
- The roots are growing through the pot’s drainage hole or can be seen on the surface of the soil.
- The plant is either growing very slowly or has stopped growing even when fertilized.
- The lower leaves are turning yellow, which may be a sign of a nutrient deficiency.
- The potting mix is drying out faster requiring more frequent watering.
- The plant appears to be too large for the pot.
To repot a houseplant, choose a container that is only slightly larger in size. If the pot is too large, the soil can stay moist for too long, which can cause root rot. For additional information on houseplant care, see Virginia Cooperative Extension Publication 426-100, Indoor Plant Culture, or see the University of Maryland Extension’s publication on Potting and repotting Indoor Plants.
Boxwood leafminer damage. Photo: Jim Baker, North Carolina State University, Bugwood.org, CC BY-NC 3.0
With the arrival of spring comes the start of the annual battle with insects, such as leafminers. Leafminers are the larvae of insect species that burrow within a leaf and devour its inner layers, leaving either winding, serpentine tunnels or brownish blotches, depending on the plant and the leafminer insect species. Both the larvae and the damage they cause are generally undetectable until after it is too late. Winding tunnels caused by sawfly insects are commonly found on the foliage of columbine (Aquilegia), hollies (Ilex), and roses. The damage doesn’t actually harm the plant, but it can look unsightly. It’s usually sufficient to snip off the damaged leaves or simply ignore the problem if it’s not too pervasive. Other leafminer insect species cause blotchy or blistery looking damage, which is more destructive to plants such as boxwood. (See University of Maryland Extension publication for a description of and management options for Boxwood Leafminer on Shrubs). To learn more about Leafmining Insects, see Colorado State University Extension Fact Sheet No. 5-548.
Inspect Azaleas for lacebugs (Stephanitis pyrioides), which overwinter as eggs on the underside of infested leaves, then hatch out in spring. They damage foliage by piercing plant cells with their mouthparts and sucking the leaf dry. Look for white or silvery looking stippling on the upper leaf surface. The damage is unsightly, but it won’t kill the plant.
Slugs and snails start making their appearance in spring, particularly if the weather has been cool and wet. One very effective control method is to pick them by hand, or with tweezers if you’re squeamish, and drop them into a pail of soapy water to drown. For more information on how to control slugs and snails, see the University of Maryland extension’s publication on Slugs and Snails on Flowers.
Check emerging Irises for diseases or borer damage. Leaf Spot is one of the more common fungal diseases of irises. For information on symptoms and controls of this disease, see VCE Publication 450-600, Iris Leaf Spot. Iris borers are another common problem. The larvae of this pest feed below the soil level on the rhizomes. Feeding damage is sometimes not apparent until the plant dies or the leaves wilt. Inspect young iris foliage for notches that are cut in the edges of center foliage and slimy frass. This is the point where the borer enters the leaf. If you detect the presence of a borer caterpillar inside the leaf, crush it with your fingers. Once this voracious pest burrows to the rhizome, it will hollow it out and then proceed to other rhizomes. Bacterial soft rot often follows borer damage and can destroy an entire bed of Irises. The best way to control this pest is to burn the foliage or dispose of all dead or damaged leaves in the trash in fall.
And speaking of pests, apply deer repellent as vulnerable plants emerge in spring or take other preemptive measures to discourage deer browse on tender, succulent new plant growth.
Invasive watch: Callery or Bradford (Pyrus calleryana ‘Bradford’) is in bloom this month. This invasive species and other ornamental pears started out as popular landscape trees in the 1960s and are now considered invasive in 29 states. The trees often produce fertile seeds that are easily spread by birds. The seedlings can easily establish in disturbed areas, where they are contributing to the shrinking biodiversity of our urban forests. Control trees less than 6’ tall with a higher-than-usual (3-4%) concentration of foliar spray. Foliar sprays are effective from when leaves emerge in spring until just before they begin to develop fall color. For detailed information on how and when to eradicate this invasive species, including tree trunk methods in the fall, see the Blue Ridge Partnership for Regional Invasive Species Management’s Bradford Pear Fact Sheet.
Garlic mustard should be removed now before it flowers and sets seeds. For more about garlic mustard, see Blue Ridge Prism.org/Garlic Mustard Factsheet.
Feature photo of spring-blooming Phlox divaricata: Pat Chadwick