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Monday, November 11, 2013

Rose Leaf Scorch Probably a Soil Problem

Q. I have some roses doing poorly. Any thoughts?
A. If I see it correctly, it looks like rock mulch in the background. No, no, no for roses. This leaf is showing leaf scorch and marginal tip dieback, a sign of fertilizer, soil and water conditions. They are all inter-related.

The first thing we need to consider is soil improvement. In our desert soils they need massive amounts of organic material to bring it up to the bare minimum of 3 to 5 % organic matter for plants like roses. Our soils in Las Vegas are typically around 1/10th of 1% organic matter. Organic matter breaks down over time so to accomplish a 1% change will mean about a 25% volume of compost added to the soil at the time of planting.

Surface mulches made of wood mulch like the kind made from chipping trees will contribute organic matter back to the soil and replenish what is lost over time. Bark mulch will not contribute much of anything. It is decorative only.

Water should be about two to five gallons at each watering. Watering frequency with surface mulch is about three to four times per week, basically every other day. In the winter this might be every ten days.

Fertilizers should be one that promotes flowering, adequate in phosphorus, and balanced with all three numbers present. The first number, nitrogen, should be a relatively high in number. Potassium, the third number would be anywhere from half the value of the first number all the way up to an equivalent number. This would be applied in January or just before or during when new growth commences. Iron chelates should be applied at the same time. Small amounts of fertilizer can be applied every four to six weeks if these are for show. Rosarians suggest the application of magnesium in the form of epsom salt.

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