October 21, 2017

Salty Roses: Identifying Rosebush Problems

Southwest Yard & Garden | Marisa Thompson

Question:
Can you tell what seems to be plaguing the various rosebushes in my backyard? 
-          L. Peters, Sandoval County, NM

Compound leaf of rosebush with browned margins on leaflets. Compound leaf of rosebush with blackened margins on leaflets.Compound leaf of rosebush with dark margins and white build-up on leaflets.


Answer:
Thanks for sending such great photos with your rosebush question. These photos depict rose foliage with varying degrees of browning edges on the leaflets and some white crusty buildup on leaflet surfaces, especially at the margins. Some of the leaflets have a burned look with black edges.


First off, the leaf margin damage looked to me like salt burn, and I asked Dr. Curtis Smith, retired NMSU horticulture specialist, to weigh in. Dr. Smith agreed that the tissue necrosis at the leaflet tips and margins might be due to salt burn. Salt accumulation at these points is caused when dissolved salts are brought up to the leaves from the roots during transpiration. This could be due to salts in the soil, in the water, or just accumulated after a long, hot, windy summer. As pure water transpires from tiny pores in the leaves, the salts are left behind.  As they build up there at the tips, they can cause that burned appearance, as well as the crusty, white build up, especially on the older leaves.

These symptoms may be alleviated by watering more deeply, but with longer intervals between irrigations. In the chapter on roses in the 1967 edition of her book, “Southwest Gardening”, Rosalie Doolittle recommends watering deeply at five- to seven-day intervals during the hottest months and then less frequently in the winter. She warns, “More failures in rose growing in this area result from overwatering than any other cause… Light and frequent irrigation causes feeder roots to grow close to the surface and they are easily injured from heat and wind.” Salt build-up is also more likely to occur in the top few inches of the soil profile with light waterings. Contact your county NMSU extension agent to find out how to get your soil tested.

Both Dr. Smith and Ms. Doolittle recommend mulching to conserve moisture and moderate temperature in the soil. Dr. Smith also noted some wind damage, especially on the stems, and the possibility that these rosebushes are getting a little too much shade, based on shadows in the photos. Powdery mildew may also be present on the leaf surfaces as evidenced not as much by the white film of spores spreading across leaf tissue, but by the residual angular darkening on some leaves. Refer back to a September Southwest Yard and Garden column for more on powdery mildew identification and control.

Finally, Dr. Smith suggested deadheading, which is the act of cutting spent flower stalks down far enough to reach the leaves with five leaflets during the summer. Major pruning can wait until later in the winter. Depending on your growing zone, that will likely be late February to late March (or later!), when the buds begin to swell. Rule of thumb: Prune your roses when the forsythia are in bloom. Idea for next week’s column: “How to identify a forsythia plant.” Perhaps I will save that one for February.

Roses offer bright doses of color and perfume to our desert gardens. My mom bought me a 5-gallon hybrid tea rose when I first moved to Las Cruces, and it has performed spectacularly with very little help from me (even after I tortured it for several years by keeping it in the container while moving from place to place until finally planting it next to a superhot cinderblock wall). By “very little help,” I am referring to the fun garden activities we covered: pruning, deadheading, and thorough, but not too frequent, waterings. Of course, there is way more to discuss on rose care. Check back in the spring for tips on fertilizing and the joys of spraying aphids off rosebuds with a garden hose.

Here are some great rose resources for New Mexican growers:



And check out these mini-videos from the Southwest Yard & Garden series with Curtis Smith and John White:


How To Choose The Right Rose To Grow In New Mexico


How To Prune Roses In The Summer


How to Fertilize & Deadhead Roses

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