Great Late-Summer Color!

Just because it's late summer doesn't mean you can't have fresh plants. We continue to receive regular deliveries of fresh perennials and shrubs for late summer and fall--check out some of the great color available now!

Beware Downy Mildew on Impatiens

I hate to even have to report this, but it appears that an ugly disease that has spent the last couple of years spreading westward across the country has reached the Pacific Northwest. It's called impatiens downy mildew, and it's quickly becoming the worst enemy of gardeners who love to grow impatiens. If you've struggled with growing impatiens this summer--or just want to be aware of what to watch for--here's the scoop on impatiens downy mildew.

First off, the symptoms (courtesy Michelle Grabowski, UMN Extension Educator). Typical symptoms that impatiens display may include the following:

Advanced symptoms of downy mildew.
Courtesy M.A. Hansen

• Leaves first appear light yellow or stippled yellow & green.
• Leaf edges curl downward; leaves appear wilted.
• Fluffy white growth occurs on the lower surface of leaves.
• Blossoms drop first. Next leaves fall off leaving bare green stems. Finally stems collapse and lay flat on the ground.
• Plants infected young are stunted and have small leaves.
• In very wet conditions, infected plant tissue becomes water soaked, soft and mushy.

Now, how does it spread? Downy mildew can spread any number of ways. It may come in on your new plants at the time of purchase. (At Vander Giessen's, we were very proactive this year, inspecting our impatiens 3-4 times per week for any symptoms of the disease. Thankfully, we never saw any symptoms.) It may also spread by spore-like structures through watering or via air currents.

Arguably, the worst aspect of impatiens downy mildew problem is its longevity. Because it's a relatively new disease to the U.S., little is known about how long the disease can last in infected soil. However, a very similar strain of the disease has been shown to last up to 10 years in contaminated soil, so impatiens downy mildew may have similar impacts.

As for treatment, unfortunately at this point there are no products that can cure the disease and little that gardeners can do to prevent it. If you discover downy mildew in your impatiens, remove the diseased plants and any fallen leaves or flowers, seal them in a plastic bag and dispose of them in your garbage. Don't risk spreading the disease by throwing your plants in a compost pile.

To avoid problems in your garden, be as proactive as possible! Impatiens downy mildew tends to be most active in wet, humid conditions, so to prevent making conditions right for the disease to thrive, water impatiens early in the morning and avoid getting the foliage wet if possible. Regularly check your plants for any symptoms of the disease and take quick action to dispose any suspect plants. 

New Guinea impatiens are a great disease-resistant
alternative for shade gardens.

All traditional varieties of impatiens (impatiens walleriana) are susceptible to downy mildew; however, New Guinea impatiens (impatiens hawkeri) have shown themselves to be highly resistant to the disease, so if you're a diehard impatiens lover, consider upgrading to the showier, larger-flowered cousin of this garden favorite. You can also consider other shade-loving and downy mildew-resistant plants like coleus or begonias for your flowerbeds.

Unfortunately, impatiens downy mildew is becoming a reality for gardeners now nationwide. While scientists work to develop a solution to the deadly disease, you can arm yourself with the knowledge of what you may be up against and the steps you can take to deal with it should it strike your garden.

Hydrangeas: When to Prune and How to Change Their Color

Macrophylla hydrangea--the most common type!

It's late summer, and despite all the other fun activities you’re trying to squeeze in before fall, now is the time to prune many varieties of hydrangeas around your yard. If you’ve ever struggled with getting your hydrangeas to bloom right—or even if you want to change the color of your blooms—here are some basic tips that can make your bushes look their best.

Hydrangeas come in many shapes, sizes and varieties, but for the most part, you’ll find four different types in garden centers and around your neighborhood—and knowing which type you have will determine when and how to prune.

First, macrophylla hydrangeas are the most common types. Whether you have the standard mophead (large, round flowers) or lacecap (very small blossoms surrounded by a ring of larger blooms on each flower head), prune them now! Macrophylla hydrangeas typically only bloom on old wood, meaning if you wait until next spring to prune your shrubs back, all the new growth they put out after their haircut will be nothing but leaves. Traditional mopheads and lacecaps should be pruned just as the blooms fade in summer so they have time yet before fall to develop flower buds for next year.

Paniculata hydrangea

The second most common hydrangea species is the paniculata family. You might know some of the common varieties like ‘Limelight,’ ‘Pee Gee’ or the newer ‘Bombshell,’ all of which are easily distinguishable by their cone-shaped flowers. Unlike macrophylla types, paniculatas can be pruned anytime between late summer and mid-spring, and may not even need regular pruning at all.

 A third common hydrangea species for our area is the arborescens type.  The most common of these varieties is ‘Annabelle,’ a white variety with large blooms, although others like ‘Invincibelle Spirit’—a  pink variety—are common, too. Like paniculatas, these hydrangeas can be pruned at any time other than late spring or summer.

The last main variety of hydrangea you’re likely to grow is quercifolia, more commonly known as oakleaf hydrangeas. They have cone-shaped flowers similar to paniculata types, but their leaf shape mimics that of an oak tree. Like macrophylla types, oakleaf hydrangeas should be pruned in late summer in order for them to bloom reliably the following year.

Now that you know the basic types of hydrangeas and when to prune each, let me offer this caveat: many of the new introductions in the last several years can be pruned just about any time of year, regardless of which species they are. So, if you’re in the market for a new hydrangea and want to keep things simple, ask at a garden center for some of the newer varieties! 

Finally, how can you change the color of your hydrangeas? In the Pacific Northwest, the acidity of our soil naturally turns macrophylla types blue over time. If you want to turn a hydrangea pink—or keep one that way—treat it with Bailey’s Color Me Pink™, a soil additive available at Vander Giessen's. Likewise, if you wish to intensify the shade of blue in your hydrangea, use Bailey’s Color Me Blue™.  Depending on your conditions and desired color, multiple applications of either product may be needed. For better or worse, the color of white varieties cannot be changed.

Hydrangeas are beautiful shrubs and easy to grow. With a little attention to what type you have in your yard, you can enjoy loads of flowers each year while also maintaining the size of your plants. And with a little extra care, you can even change the color of your blooms to suit your taste!