Leggy Chokeberry

My four-year-old red chokeberry bushes are lanky and leggy. Is this just their nature or can I prune them somehow to make them fill out?

Unfortunately, it is the nature of red chokeberry (Aronia arbutifolia) to be lanky. Most of the leaves are borne on the top third of the stems. This trait can be more evident in plants that get too little sun, since chokeberries grow naturally in wet, sunny meadows. The popular cultivar called ‘Brilliantisima’ is somewhat fuller and bears heavier, redder fruit; but is still too lanky for formal or foundation plantings. Chokeberries look better in naturalistic groupings. You may try cutting them back to about six inches in the spring to force new and bushier growth. Incidentally, birds do not readily eat chokeberries because they are very astringent. This makes them less than ideal for wildlife plantings but insures that we humans will get to enjoy the decorative quality of their bright red fruit well into the winter.

Avoiding powdery mildew

What can I do to avoid getting powdery mildew on my lilacs and zinnias?

There are a number of plants that are susceptible to powdery mildew, and you have named the top two. Others include phlox, monarda (beebalm) and roses. The three components necessary for plant diseases to flourish are a pathogen, a susceptible plant and ideal environmental conditions. To solve the problem, we must remove one or more of these components. Luckily, you can do all three in this case. There are a number of resistant varieties of lilac and zinnia. You can learn more about mildew resistant plants online at sites like www.gardenweb.com and by reading plant catalogues, which often indicate disease resistance. Spring spraying of a ten percent baking soda solution, beginning at bud break and every ten days, is a simple way to control (we said control, not eliminate) the fungus that causes mildew on lilacs. Removing old leaves in the fall helps too. The last consideration is choosing a nice sunny sight that gets plenty of air circulation for any susceptible plant. A word of caution: Disease resistance can be somewhat subjective to a geographic area. Plants that have been touted for their resistance in one test area of the country may prove to be less resistant in our summer humidity. A little experimentation may be necessary.

Non-blooming iris

Why won’t my Japanese iris bloom? They get lush green foliage but no flowers. I have them planted on the west side of my deck.

Japanese iris (Iris ensata) grow naturally along stream banks and in wet meadows. Plants that grow in these conditions are a challenge for gardeners since wet meadows provide full sun, constant moisture and lots of oxygen in the soil, aerated by moving water. You may want to try them in a water garden (container or pond) where they should be placed with crowns above water and roots submerged. In the ground, give them plenty of water and good loamy soil and lower the pH with an acidic plant food. Newer cultivars of Siberian iris (Iris siberica) have the more dramatic flowers with wider, horizontal falls that people love in Japanese iris, but they are much easier to grow. Incidentally, all iris can be stubborn about blooming if planted too deeply. The rhizome should be barely covered.

Dealing with shallow beds

What can I do to improve a bed of shallow soil over rock? Raising the bed is not an option since it is next to our foundation.

A shallow foundation bed sounds like it would be dry and alkaline (from the concrete leaching) as well as poor nutritionally. You can improve the soil by working in generous amounts of organic matter like leaves and grass clippings or compost. You can also amend soil from the top down by simply adding a layer of organic matter to the soil surface every year and letting worms and microorganisms process it into the soil. Beyond that, it is probably best to choose plants that grow naturally in such soils. Junipers and arborvitae, ubiquitous as they may be, do well under such conditions. For more interest try herbs such as catmint, sage, showy oregano, thyme and lamb’s ears. If the bed is on the east or north side of the house then boxwood, hosta, epimedium and lamium may be your best bets. Be sure to see that the overhang and house are not blocking rain to the site. If they are, then irrigation will be necessary to grow anything.

Replacing forsythias

My Lynwood forsythias are taking over my shrub border. They root everywhere they touch the ground. How can I remove them and what would be a good, spring-flowering replacement?

A well-grown forsythia that has never been given a haircut is a sight to behold. The arching branches of these fountain-like shrubs bear loads of lemon yellow flowers in very early spring. Unfortunately, they do become quite large and folks tend to cut them a lot, marring their natural shape and sacrificing the flowers that bloom on old wood. It is not uncommon to see forsythia pruned into a stiff oval sporting a fringe of flowers only around the base.

‘Lynwood’, an old cultivar discovered in a Northern Ireland garden, does sucker freely. Remove them by digging them out. Any suckers or rooted portions that you can’t dig can be killed by applying Roundup or woody plant killer. Cut the stem after bud break in the spring and immediately apply a coat of Roundup to the cut surface. Roundup is easily applied to stems by pouring an ounce or so into a baby food jar and then painting it onto the stem surface with a small brush.

Digging out old unwanted shrubs gives you a better opportunity to increase organic matter in the soil before replanting.

Two underused shrubs that offer spring bloom are the corneliancherry (Cornus mas) and a cultivar of common pearlbush (Exochorda x macrantha) called ‘The Bride’.

Corneliancherry is offered as a single stemmed small tree, or a multi-stemmed shrub. It bears small, yellow flowers very early in spring, beautiful, shiny green leaves and glossy red berries that are relished by birds.

Pearlbush (‘The Bride’) is a slow growing shrub that develops into a 4 foot by 8 foot mound. The flowers appear like large pearls along the stems in April with lower ones opening up like apple blossoms. Pearlbush looks like any green shrub for the remainder of the year, but the fresh white flowers are quite stunning in spring, especially with a clump or two of purple tulips at the base.