By Mark Gleason
Plant Pathologist
Iowa State University Extension
After a punishing winter, Iowans are eager to see blooming perennials this spring and summer. We’re overdue for a colorful 2009 gardening season.
The one color we’re not planning for is brown, since we’ve had more than enough of that one for the past six months. But diseases can intrude on these plans. How can we banish brown from herbaceous perennial gardens this year? Following a few simple practices will help.
The first principle is spring cleaning. Clearing your perennial beds of last year’s leftovers -- leaves, stems, and flowers -- makes a neater look, but also discourages fungi that lurk in the dead material. By plucking out the debris, you are removing sources of new infections in 2009.
This sanitation strategy is especially helpful for diseases that attack the leaves, such as peony leaf blight and iris leaf spot. It also can pay off in controlling some nasty insect pests, such as iris borer. This bad boy drills holes from iris leaves down to the corms, and a soft rot bacterium follows behind. The result is smelly, slimy, dead plants. Removing last year’s leaves from iris beds can stop this ugly scenario in its tracks, but you’ll need to act now.
Now that you have bags or baskets full of last year’s garden plants, what should you do with all that stuff? If you have a compost pile that is active enough to cook the fungi to death, composting is a wonderfully “green” strategy. But if you compost pile is a more typical one - really just a debris pile that doesn’t generate much heat – it may be better to burn, bury or dispose of the leftovers.
Beyond sanitation, it’s important to look before you buy. If plants at a big box store or garden center have suspicious brown spots (could be a fungal or bacterial disease), mottled or crinkled leaves (could be virus), or yellowing (could be a root rot), it’s best to pass them by. Saving a few cents at a plant clearance sale may backfire if you import stubborn, hard-to-eradicate problems into your garden.
While we’re on the subject of prudent purchasing, it makes good sense to buy disease-resistant plants. Some species and varieties have the ability to fend off diseases, or at least to tolerate them and keep looking attractive. It will take a bit of homework on your part, but resistant plants are worth searching for, since resistance is usually the most cost-effective way to combat diseases.
The most common mistake of new gardeners is to crowd plants too closely. If the label in the pot says to plant 18 inches apart, 12 inches apart is not a better idea -- even if the spacing initially looks ample. Crowded plants compete for resources and dry off slowly, encouraging many diseases.
Another useful tip concerns how to water. Plants take in water through their roots, so it makes sense to supply at ground level. Drip irrigation hoses in a garden will help to keep the foliage dry and thereby discourage diseases. But the biggest payoff is environmental, since drip irrigation conserves water during high-demand periods of the summer. The main trick with drip hoses is to get them in place in your perennial beds during the spring, before you have to fight your way through thickets of vegetation.
There are so many types of herbaceous perennials that it’s hard to keep them straight, let alone fight off their diseases. A brand-new book can help: it’s called Diseases of Herbaceous Perennials. The first one-stop reference on the subject for backyard gardeners and professional landscapers, it has 296 pages and more than 700 color images. The book helps you to recognize and prevent the most important diseases of practically all types of herbaceous perennials. It’s available for $79 at