Boxwood blight

The black streak on the stem is a unique feature of boxwood blight, an incurable viral disease.

Where do you start when talking about boxwood? I am going to start with what seems to be the most misunderstood part of caring for these hardy shrubs found all throughout Southern Virginia, mulching.

Properly mulching boxwoods is key to healthy long living boxwoods. If you are going to mulch at all, it is best for the plant to do it right.

Boxwoods are a shallow-rooted shrub so some mulching, and I emphasis some mulch, can help retain moisture and keep roots at an even temperature. Experts recommend no more than 2- to 3-inch layer of mulch. The mulch should extend about 1 foot beyond the dripline of the boxwood plant.

What mulch to use? It helps to know what you have available locally, but some softwood mulches work as well as some hardwood mulches. Some hardwood mulch chips lessen the growth of fungi, and it has been found that cypress and cedar chips discourage termites. Yes, you will probably pay more for these mulch materials, but consider the outcome and advantages or each type of mulch.

Keep in mind that boxwoods like a slightly acidic soil, 6.8 pH, so if you use softwoods (pine bark chips or pine needles), the soil will naturally become more acidic over time as the vegetative material breaks down. You may need to come back and sweeten the soil with a little lime at a later date, based on a soil test.

Established boxwoods, if mulched, should have no more than a 2- to 3-inch layer of mulch and as previously stated, extend at least 12 inches past the foliage or dripline. As with all shrub mulching, trunks (or the crown of the plant) should never be covered. Leave at least a hand-width distance between the trunk and the mulch. This will help with air flow and help keep debris buildup from occurring. Dampness increases the opportunity for disease and insects. Annually, you should clear out all debris that falls into the crown of the plant so the plant does not develop numerous aerial roots and promote early leaf drop and harbor some fungal issues. You can rake out the mulch and use a hose with your finger over the end of the hose to gently wash the debris out, then rake again. Come back the next day and replace your mulch less all the debris.

Aside from watering and mulching, growing boxwood is a low maintenance task, unless you wish to keep them as a sheared hedge. Pruning is the most time-consuming part of boxwood care when they are grown as a hedge, but you will be rewarded with a healthy, long-lasting hedge. Older boxwood care will include thinning limbs to allow sunshine to reach the inner foliage which has naturally become shaded by the outer growth and triggers leaf drop deep within the plant. Letting light in will rouse the plant up to new deep growth which helps the plant provide more food/chlorophyll which improves the plants overall health.

We don’t have many insects that are damaging to boxwoods outside of the boxwood leaf miner. The leaf miner is very damaging to the leaf on the boxwood and it takes a sharp eye and dissection of the leaf to discover this little larva nestled in between the upper epidermis and the lower epidermis of the leaf. If foliage begins to yellow, give the extension office a call so we can make sure of the pathogen or insect that may be affecting your plant.

Over the past few years we’ve had a variety of winter-to-spring transitions, some with way too much rainfall, then following dry periods that can bring on other root fungi concerns. This is especially true if the ground has remained saturated for long periods of time. Again, get in touch with the extension office and we’ll see what we can do to help. During COVID-19, we’ve had to find different ways to diagnose plant issues, but we are up to it.

Yearly soil tests can determine if the soil pH for the boxwood is correct. Soil pH should be between 6.5 and 7. Most boxwood experts prefer the perfect 6.8 pH, but we know that isn’t what everyone has. At least do the soil test once every three years so that acidity doesn’t get too low, or possibly too high if you’ve added lime annually.

Alkalinity is usually not a problem east of the Mississippi River. I’ve never seen a problem with boxwoods growing in a pH greater than 7 in Halifax County. It is best to test the soil before planting new boxwoods. You will get recommendations from the soil lab when you receive the results of your soil tests, if you get it done through the VT lab. If you need help interpreting the results, just give your local extension office a call for help understanding the recommendations. We have the ability to access your Virginia Tech Lab reports and go over it with you.

As a slow-growing landscape plant, boxwoods are valuable, and consequently they are expensive. Take time to choose where to plant boxwood carefully. Remember to water and mulch properly for a long-lived, vigorous specimen. Too much moisture is as bad as too little moisture. Once established the plants usually do not require any additional fertilizer.

With the introduction of the boxwood blight across the southern tier of Virginia, be mindful of what varieties/cultivars of boxwood you introduce to your landscape. All import of boxwoods has been stopped now for over two years. Boxwood blight (also known as box blight), is caused by the fungus Calonectria pseudonaviculata. It is a serious fungal disease of boxwood that results in defoliation and decline of susceptible boxwood. here are new Boxwood Blight resistant varieties already available now from in-state Virginia nurseries that are available to you.

There are other plants that carry the pathogen C. pseudonaviculata such as Pachysandra terminalis (Japanese spurge) and Pach¬ysandra procumbens (Allegheny spurge). Sar¬cococca species (sweetbox), which are in the same family (Buxaceae) as boxwood also carry the pathogen. If they carry the pathogen, they could introduce the disease to a landscape of your favorite boxwoods.

Early symptoms of the disease are brown leaf spots. As research continues, new host plants may be identified as researchers learn more about this disease, but hosts will likely be limited to members of the Buxaceae family. It is important to note, that when boxwoods get this disease, complete removal of roots and exposed parts are the only option or the disease will spread to other boxwoods.

If you think you have infected boxwoods, get in touch with extension ASAP. To learn more about boxwood blight, download the Virginia Cooperative Extension publication “Best Management Practices for Boxwood Blight in the Virginia Home Landscape.”

While we are all still practicing social distancing due to COVID-19, and all Halifax County buildings are closed to the public, should you have gardening questions, you can best reach an extension master gardener or extension staff member by sending an email to wmccaleb@vt.edu or ask@ssmga.org. If you can’t email, you can call and leave a message at the Extension Master Gardener Help Desk at 434-830-3383, giving us your name, telephone number and nature of the call. The help desk phone is checked timely and someone will get back to you, although it may be from a different telephone number. Keep washing your hands, wear your mask and enjoy the beauty that is all around us in our landscapes.

McCaleb is the Southside Master Gardener Association coordinator for the Virginia Cooperative Extension.