Our area is great for growing roses, even though we usually have to supplement natural water. Southwestern Idaho is steppe country, only a bit moister than desert areas. Our annual precipitation is usually about 12 inches, nearly all of it arriving in winter and early spring. Thus roses and other susceptible plants do not suffer diseases common to areas with higher humidity and rainfall.
We very seldom see problems of black spot on roses, for example, but it’s preferable to irrigate roses rather than using overhead sprinklers. If you do have a water system that wets the foliage, do that early in the morning so that foliage can dry out before cool evening temperatures arrive. Midsummer temperatures usually range from 100-degree days to low 60s-degree nights. This also means that if a plant puts out pollen in temperatures above 90, for instance, the pollen may be killed by heat. But we do have tomato fruits set on those days – the pollen presumably released during rising or cooling temperatures.
When you buy a rose shrub, pay attention to its growth habit (shrub, miniature, columnar, sprawling, etc.) and its roots. Locally owned nurseries in this area are stocking more and more own-root roses (that is, not grafted onto hardy rootstock). Our winters are not usually severely cold any longer, so many of us don’t winterize rose shrubs. If it does turn that cold, the own-root rose will recover. The surviving roots of the grafted rose will regrow whatever variety the rose shrub’s root was, the grafted portion dead.
We usually wait until forsythia blooms before cutting back on roses, removing dead or crossing branches, setting the stage for blossoms on long canes and checking for disease. We do have a disease afflicting roses that popped up here a few decades ago, an undetermined pathovar of Pseudomonas syringae. The common term for this disease is bacterial cane blight. It can be controlled with spraying Phyton 27 in late summer after you see dark water-soaked blotches on canes. In spring, use a thumbnail to scrape the outer coat off blackened canes. If you reveal dry black areas, prune as far below that winter-killed area as you can, and see if you’re into living tissue. If you reveal moist reddish tissue, it’s a sign of the bacterial cane blight, and you should plan to spray and remove the cane below affected tissue. Be sure to sterilize your pruners after any cut such as this.
Fertilize roses when they begin to bloom in spring, then once a month until August (some in the Boise North End stop fertilizing in September). The object is to let the shrub set hips and prepare itself for winter dormancy. Use a fertilizer formulated for roses or any other with an NPK ratio of the neighborhood of 5-10-5. You don’t want a high nitrogen content because that will produce foliage at the expense of blossoms. The phosphorus (middle number, 10) will encourage more blossoms.
Our soil is naturally deficient in magnesium, so some Epsom salts may contribute to a thriving rose shrub. I use it dissolved, about 2 tablespoons per gallon of water, and water roses and tomatoes with that mixture. I wouldn’t load up on it though year after year, lest you unintentionally block uptake of other needed minerals. It’s easy to oversupply plants, especially with trace minerals. Neem products usually take care of powdery mildew, mites and insects, and it’s safe for beneficial insects and pets. Always read the product label.
Veteran rose growers claim roses are subject to “replant disease,” just as apple trees are in orchards. (Roses and apples are both in the Rosa family) That is, if you remove a rose and plant another rose in its place, it will fail to thrive. This doesn’t always happen, and for that reason I think professional plant pathologists say it’s not a real threat. Owners of apple orchards know it does happen in their orchards, so they take definite steps to avoid that problem when they have to replace a tree. They may grow a grain in that area for a year or two prior to replanting an apple tree, for example. If you have to replant a rose shrub in the area that other roses had been growing, just dig the hole larger, set the excavated soil aside, and fill in around the new shrub with different soil.
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