Followup: Disease Resistant Roses

We all love to visit rose gardens in the spring, when every bush is in its glory.  But a trip in the dog days of summer can be very enlightening.  Summertime in Houston is when roses really show us what they're made of.

June, July and August are hard times for roses.  It's so hot that it can be difficult to provide enough water.  Disease and insect pressure are peaking.  And even good roses often look a little ragged.  Flowers are smaller, leaves look a little burned on the tips and plants sometimes drop leaves from stress.  So a rose that looks presentable in the Houston summer is likely to be a good performer for your garden.

Recently I visited the  Hermann Park - Houston Garden Center at 1500 Hermann Boulevard, across from Miller Outdoor Theater.  Of course, the Knockout series of roses looked great.  They almost always do.  But three others caught me eye, too.  Here they are:

This is Easy Does It, the 2010 AARS Winner.  This beautiful floribunda appeared healthy and strong, and was covered with mango-colored flowers.  I saw these roses at about 10:00 in the morning and the fragrance was barely detectable, but they are likely much more fragrant on an early spring morning or evening.  Wonderful disease resistance and free-flowering habit make this one a keeper.

I was also taken with Crimson Bouquet, an AARS 2000 winner.  This is a grandiflora rose, featuring a rich rosy red color and great disease resistance.  Crimson Bouquet has more petals than Knockout, and I liked the color better.  (Although I like the Knockout's red foliage, too!)

And finally, a rose that I wasn't familiar with.  This is Eutin, another floribunda.  These little shrubs featured masses of cherry-red simple flowers that were thriving in our hot, humid summer.  This one gets about 4-5 feet tall, so may be more manageable than the larger Knockout red.

The downside to viewing roses in late June (aside from the sweating) is that it can be hard to find them in nurseries this time of year.  Write down the names of the ones you like and ask your nursery to get them for you in the spring.  The best selection for roses is usually February through April.

Black Spot: Live With It

This is black spot, a fungal disease particularly common to roses, caused by the organism Diplocarpon rosae.  It's a summer problem:  the fungus needs at least 7 hours of temperatures above 74° and moisture.  Does that sound like Houston or what?  I think we are in for another 3 months of temperatures above 74° and the humidity here is enough to cause fungal problems on inanimate objects.

Black spot is a problem for roses because eventually these leaves all turn yellow and drop.  A bad infection can cause defoliation of the entire plant.  The disease is most pronounced on new leaves, and young plants can be killed outright. 

Fighting black spot requires good cultural habits.  Try not to get water on the leaves of the roses, and space them far enough apart to ensure good air circulation. Remove diseased leaves from the plants and keep the fallen ones raked up and out of the compost pile.  The organism that causes black spot can overwinter in garden debris and jump right back up when temperatures warm up.  Select resistant varieties if you can -- more and more rose breeding is focused on disease resistance.

There are conventional fungicides that can be applied to control black spot:  Synthetic protectant-type fungicides can be used in small quantities and don't appear to contribute to the problem of resistant disease organisms.  Systemic fungicides tend to promote resistant diseases and often negatively affect soil micro-organisms.  The most frequently recommended organic solution is sulfur, which has problems of its own.  Yes, it's organic and yes, it's been used for thousands of years.  But it only works while a film of sulfur remains on the leaf and thus must be frequently reapplied.  Sulfur also tends to build up in the soil, and can burn plants if applied when the weather is hot (like it is when black spot is a problem).  It can also cause problems with non-pest and beneficial insects.  And sulfur mining's not the most environmentally friendly thing you can do either!

So we're left with disease-resistant roses.  Bear in mind that disease resistance doesn't necessarily mean disease-proof.  I've seen black spot on Knockout roses and every other kind of resistant rose.  The strong ones fight it off, but if you're looking for picture-perfect foliage, skip the roses.  In our climate, you'll be resorting to some pretty heavy chemical weaponry to maintain perfection.

Here are some great pictures of sulfur mining and here are the rose cultivars that have been awarded Earth-Kind status, in part because they are resistant (not immune) to black spot.