The Ornamental Garden in July
July 1 is the average midpoint for our frost-free growing season of 182 days in Virginia. I lay rather low as the temperatures climb during this hottest month of the year. According to US Climate Data, July is also the wettest month in Virginia. Rainfall in Charlottesville averages 5.3”, a little higher than the state average of 4.7”. This combination of heat and moisture causes a burst of vegetative growth. The perennial garden is abloom with color: Rudbeckia, coneflower (Echinacea), butterfly weed (Asclepias tuberosa), Agastache, bee balm (Monarda didyma and Monarda fistulosa), tickseed (Coreposis), catmint (Nepeta), Culver’s root (Veronicastrum virginicum) and so much more. The weeds like heat and moisture too! What tasks are required to keep our July gardens looking fresh and happy?
- The “wettest” month of the year is based on average rainfall, but reality may differ from statistics. One inch of rain or water per week is a good guideline for established plants. Keep an eye on the rain gauge and prepare to provide supplemental water as the temperatures climb.
- July is a month when gardeners are especially rewarded for having successfully grouped plants with similar moisture needs. My full-sun garden includes ornamental grasses and bee balm (both Monarda didyma and Mondarda fistulosa). Grasses like it hot and dry. Monarda didyma likes it sunny but moist. This means I need to supplement this species with additional watering. Fortunately, this garden is close to the house and to a hose. Monarda fistulosa, also called bee balm or wild bergamot, can take somewhat drier conditions as compared to Monarda didyma. Butterfly weed (Asclepias turberosa), also planted in the full-sun garden, doesn’t require supplemental watering, and is a good companion for ornamental grasses. When assessing your garden’s strengths and weaknesses, keep moisture compatibility at the top of the list. At the least, consider whether you are willing to put in some extra effort when plants have different moisture needs.
- Use soaker hoses rather than overhead sprayers. Make sure to water at the base of the plant rather than wetting the foliage. This approach will help reduce problems such as powdery mildew.
- Check container plantings for moisture needs on a daily basis, depending on the types of plants and plant groupings.
- Continue to weed garden beds; weeds compete with plants for moisture and nutrients.
- Edge garden beds for a neat appearance.
- Mulch depth should be about 2-3” deep; refresh when necessary to maintain moisture, help condition the soil, and suppress weed growth. Avoid overly heavy mulching which could cause root rot. Keep mulch away from the crown of perennials.
- Deadhead to encourage repeat blooming. See the June “Tasks and Tips” article for guidelines on how to deadhead and how to cut back summer bloomers.
- Mid-July is the cut-off point for pinching back plants such as asters and chrysanthemums to promote bushier habits. Cutting back later in the summer might sacrifice blooms this fall.
- Perennials that can be divided in July/August include: bearded iris, oriental poppy (Papaver orientale), and daylily (Hemerocallis).
IDENTIFY PLANTS THAT ARE RESISTANT TO POWDERY MILDEW
At midsummer, powdery mildew often appears on the foliage of many plants. Both Monarda didyma and Monarda fistulosa, for example, are subject to fungal problems. The June “Tasks and Tips” article provides a good description with photos of both powdery mildew and downy mildew. One way to deal with fungal diseases is to select plants that are more resistant to these diseases.
The Mt. Cuba Center has conducted experimental trials on Monarda didyma and Monarda fistulosa to determine which cultivars are more resistant to fungal diseases. See this article for a list of the most successful cultivars. Additional work is being conducted at the Mt. Cuba Center on the attractiveness of these monarda cultivars to pollinators. I haven’t yet seen the published results of these studies; interested readers can periodically check the Mt. Cuba research trials for results, and to see the results of other plant trials as well.
The Mt. Cuba Center also conducted trials on species and cultivars of phlox, another plant that is very susceptible to powdery mildew. Phlox paniculata (garden phlox) is the most popular of the phlox and represents the bulk of the trial. See, “Phlox for Sun”, for trial results and recommendations for particular cultivars.
The Chicago Botanic Garden also has a large plant evaluation program that evaluates herbaceous and woody plants in comparative trials, ultimately recommending the top performers. See this list of plants that have been comparatively studied. It is a great source to check when you’re considering whether to add a particular perennial or shrub to your garden. These Chicago trials have also included monarda and phlox.
See this article from Clemson University Extension for recommendations on how to prevent and treat powdery mildew.
CAN YOU PRUNE TREES AND SHRUBS IN JULY?
For recommended times for pruning particular trees and shrubs, see the Pruning Calendar published by the Virginia Cooperative Extension, which you’ll find in The Garden Shed, “When to Prune” The general rule is that flowering shrubs that bloom on old wood should be pruned right after flowering. The later they bloom, the later into summer they can be pruned. July is the latest month for pruning many of these spring-to-summer blooming shrubs without removing buds for next year’s bloom. Examples include: both deciduous and evergreen azalea, cherry laurel (Prunus laurocerasus), mountain laurel (Kalmia latifolia), Harry Lauder’s walking stick (‘Corylus avellana ‘Contorta’), spring-blooming hydrangea (Hydrangea macrophylla), lilac, rhododendron, Carolina allspice or sweetshrub (Calycanthus floridus), and deciduous viburnum.
PRUNING EVERGREEN SHRUBS
Evergreen shrubs, such as juniper and yew, are best pruned in late March or early April before new growth begins. Light pruning, however, may also be done in mid-summer. Avoid pruning evergreen shrubs in the fall. Fall-pruned evergreens are more susceptible to winter injury. For a helpful guide on different types of evergreens and when to prune, see this article from the University of Delaware, “Pruning Evergreens.”
HELPING NEW TREES AND SHRUBS SURVIVE JULY
How much do you water new trees and shrubs, especially as we head into the months of July and August? I have found guidelines from the University of Minnesota Extension to be very helpful:
Newly planted trees or shrubs require more frequent watering than established trees and shrubs. They should be watered at planting time and at these intervals:
- 1-2 weeks after planting, water daily.
- 3-12 weeks after planting, water every 2 to 3 days.
- After 12 weeks, water weekly until roots are established.
Newly planted shrubs are considered established when their root spread equals the spread of the above-ground canopy. Establishment times increase with tree size at time of planting. Refer to the charts included in this article for guidelines on estimated establishment times based on trunk caliper size, as well as watering guidelines based on caliper size. (Caliper size is the diameter of the tree measured at 6” above the ground for trunks 6” or less in diameter, and 12” above the ground for trunk diameters greater than 6”.)
July is a peak month for Japanese beetles. The majority of adults emerge in July, when (especially on warm, sunny days) adults are easily seen flying around, feeding on fruit and foliage, and mating. Females then lay white spherical eggs 2-6” deep in the soil. Eggs hatch after 8-9 days, and immature grubs then develop in the soil and feed on roots until the weather begins to cool down in autumn. High aggregations of beetles tend to attract other beetles from afar. For this reason, homeowners are NOT advised to erect commercially available Japanese beetle “traps,” as they often attract more beetles than they capture. Traps should be used for monitoring; they will not reduce Japanese beetle abundance or damage to plants. The beetles can feed on the foliage, flowers, and fruit of over 275 different plant species. Some of their favorite ornamental landscape plants are roses, crape myrtle, linden, hibiscus, crab apple, and elm. The beetles “skeletonize” leaves by feeding on the upper leaf surface and eating tissue between leaf veins. This gives leaves a lacy appearance. Since beetles are more attracted to each other than to particular plants, homeowners can shake plants to dislodge beetles each morning. Without beetles already on a plant, it is less likely that beetles will aggregate there later in the day. You can also shake the beetles into a tub of soapy water. Removing the buds from plants such as roses can also reduce damage to plants, although this preemptive step may be a bit too drastic for rose lovers. In some settings, flowers or plants can be protected with cheesecloth or other fine mesh.
Reducing breeding sites is the best long-term control for mosquitoes. Eliminate breeding sites in your yard by regularly tipping out any container that holds water. Wash out birdbaths and pet saucers daily; clean out gutters so that they do not hold water; and store buckets and wheelbarrows under shelters where they cannot fill with water. Getting rid of old tires and other debris in your neighborhood will help reduce mosquito populations community wide. Treat pools of water you cannot empty, such as rain barrels, with mosquito dunks. These doughnut-shaped wafers contain a naturally occurring bacterium (Bacillus thuringiensis or BT) that kills mosquito larvae before they mature. Mosquito dunks containing BT are effective for around 30 days and are not harmful to fish, birds, mammals, or other wildlife. Pyrethroid-based mosquito sprays or fogging programs only kill the adult mosquitoes present at the time, and may potentially kill bees, beneficial insects, and pollinators also in the area.
As Doug Tallamy points out in his book, Nature’s Best Hope (2019, p.210): “Targeting adult mosquitoes is the worst and by far the most expensive approach to mosquito control, because mosquitos are best controlled in the larval stage.”
SUMMER SPOTLIGHT ON TREATING INVASIVES: NON-NATIVE INVASIVE TREES
As reported by Blue Ridge PRISM, now is the perfect time of year to go after non-native invasive trees on your land. At this time of year, trees are fully leafed out which means that nutrients move from the treetops into the root system. (During spring, nutrients move upward from roots into the upper branches.) This downward movement transports the herbicide along with the nutrient flow throughout the root system to thoroughly kill the tree and prevent resprouting. You can use three treatment methods to kill invasive trees: basal bark treatment for slender trunks, cut-stump treatment for manageable-sized trees, and hack & squirt treatment for large trees. See the PRISM factsheet titled How To Control Invasive Plants: Manual, Mechanical, and Biological Methods for detailed, how-to instructions on these three methods. Each one requires the application of a concentrated herbicide that will kill the root system.
The worst non-native invasive trees to be concerned about in our region are:
- Callery pear (Pyrus calleryana) and its cultivars, including the Bradford pear (Pyrus calleryana ‘Bradford’), are now considered to be invasive in 29 states. See the PRISM factsheet on ornamental pear trees.
- Princess tree (Paulownia tomentosa), sometimes called empresstree or paulownia after its botanical name. See the USDA factsheet on princess tree.
- Tree-of-heaven (Ailanthus altissima), sometimes called paradise tree or ailanthus, after its botanical name. See the PRISM factsheet on Tree-of-heaven.
INVASIVE JAPANESE STILTGRASS (Microstegium vimenium)
Late summer is also the time to attack Japanese stiltgrass. According to the Blue Ridge PRISM Factsheet on Japanese stiltgrass, flowering begins any time from July into October, and seeds ripen and drop to the ground from August to December. It is best to treat small infestations of stiltgrass by manually pulling or cutting. These two methods are most effective when you wait until later in the summer, and finish before the seedheads emerge. Mowing and weed-whacking can greatly reduce seed formation, but only if done correctly. Cut stiltgrass as low as possible, scalping the ground, to remove all flowers. Mowing once in late summer can be as effective as frequent mowing in reducing seed production. Japanese stiltgrass is easily killed with low concentrations of herbicides. Researchers at Virginia Tech showed that a grass-selective herbicide is the most effective control method. When a grass-selective herbicide is used, more native plants return than when a non-selective type of herbicide is used. The recommended time for spraying is from July into early September, and before a particular area of stilt-grass flowers and sets seed. PRISM refers to the herbicide chart published by the Virginia Department of Forestry for treating non-native invasive species. The PennState Extension website and the NC State Extension are also good sources of information for identifying and treating stiltgrass.
Nature’s Best Hope (Tallamy, Douglas W., 2019)
“Monthly Garden Tips,” Piedmont Master Gardeners, https://piedmontmastergardeners.org/gardening-questions/monthly-gardening-tips/#July
“Dividing Perennials,” University of Minnesota Extension, https://drive.google.com/file/d/1S6yqog4CnN_zqwG02uiOo28HT-is_LlP/view
“Dividing Irises,” PennState Extension, https://extension.psu.edu/dividing-irises
“Powdery Mildew,” Clemson Cooperative Extension, https://hgic.clemson.edu/factsheet/powdery-mildew/
“Proper Time to Prune Trees and Shrubs,” Iowa State University Extension and Outreach,” https://hortnews.extension.iastate.edu/2015/03-13/pruning.html
“Pruning Ornamental Trees and Shrubs,” Purdue Extension, https://www.extension.purdue.edu/extmedia/HO/HO-4-W.pdf
“Pruning Evergreens,” University of Delaware, https://www.udel.edu/academics/colleges/canr/cooperative-extension/fact-sheets/pruning-evergreens/
“When to Prune,” The Garden Shed, (March 2017), https://piedmontmastergardeners.org/article/when-to-prune/
“Monthly Weather Forecast and Climate, Virginia, USA,” Weather-US.com.
“Monarda,” Mt. Cuba Center, https://mtcubacenter.org/trials/monarda/
“Trial Garden,” Mt. Cuba Center, https://mtcubacenter.org/research/trial-garden/
“Plant Evaluation,” Chicago Botanic Garden, https://www.chicagobotanic.org/collections/ornamental_plant_research/plant_evaluation
“Watering Newly Planted Trees and Shrubs,” University of Minnesota Extension, https://extension.umn.edu/planting-and-growing-guides/watering-newly-planted-trees-and-shrubs
“Japanese Beetles on Ornamental Landscape Plants,” NC State Extension, https://content.ces.ncsu.edu/japanese-beetle-1
“Battling Mosquitoes,” NC State Extension, https://chatham.ces.ncsu.edu/2014/08/battling-mosquitoes-2/
“Japanese Stiltgrass Identification and Management,” NC State Extension, https://content.ces.ncsu.edu/japanese-stiltgrass-identification-and-management
“Insidious and Formidable Japanese Stiltgrass,” Blue Ridge PRISM (Partnership for Regional Invasive Species Management), https://brprismlive.wpengine.com/wp-content/uploads/2017/06/Japanese-Stiltgrass-SAR-5-27-17-VDOF-w-BOX-FINAL.pdf
“Controlling Japanese Stiltgrass in Your Garden,” PennState Extension, https://extension.psu.edu/controlling-japanese-stiltgrass-in-your-garden
“Non-Native Invasive Plant Species Control Treatments,” Virginia Department of Forestry, https://brprismlive.wpengine.com/wp-content/uploads/2019/06/2018_Nonnative-Invasive-Plant-Species-Control-Treatments_pub.pdf
Feature Photo: July Garden, Mondarda didyma, Monarda fistulosa, and little blue stem grass (Schizachyrium scoparium) Photo: Susan Martin