This is why rose leaves turn yellow and brown

Oct 20, 2023

Rose leaves are shown turning yellow, but there are many factors that could cause this freckling.

Q: Why do rose leaves turn yellow and brown?

A: Looking at the one or two leaves in your photo it appears that droplets of water or some other spray have been on the leaves when the sun was shining brightly. That would account for the tiny brown "freckles."

The most common rose disease is called black spot, and it causes yellowing with large dark brown blotches in the centers of the yellowed leaves. I don't see those classic symptoms in your photo, but I’d suspect it's involved.

It could also be nutritional. The plant may need nitrogen. Many times we forget to give our flowering plants enough nitrogen, thinking instead that phosphorus is more important since it stimulates blooming. But we overlook the fact that phosphorus accumulates in our soils to the point of being excessive.

Finally, I can't discuss rose problems currently without at least mentioning rose rosette virus. I see no evidence of RRV from these two leaves, but one would need a larger sampling. That fatal virus causes very distorted growth: strong "bull" canes, flowerbuds that don't open properly, canes that have incredible numbers of thorns and plants that become weak and lethargic. If you see any symptoms that sound like those do a bit of web searching for photos. Plants infected with the virus must be destroyed, so you want to be sure to identify it accurately.

More Neil Sperry: St. Augustine varieties have different levels of hardiness

Q: I have summer squash in two large pots. They are producing fruit prodigiously. I have two more in the ground that are doing nothing but making more vines. Both get ample sun. Any idea what might be wrong?

A: I assume the plantings are the same age, therefore the plants are at the same stages of maturity. Are they the same variety? And you say that they are "making more vines." Summer squash are most commonly on clumping plants that don't really vine along the ground. That makes me wonder. And finally, the big question for me is always the statement that "both get ample sun." It only takes a slight difference in the amount of sunlight that one plant receives to make a big difference in the amount of fruit it will be able to produce. That's the most common cause of problems that I see in this sort of situation.

Q: I’ve had my gardenia growing in a pot for eight years. It has flourished during that time until last winter when the extreme freeze hurt it badly, even though it was protected in my greenhouse. It's now spindly. It's blooming, but not very heavily. It needs to be cut back. When should I do it, and by how much?

A: Trim it as needed to reshape it so that you can foster new regrowth to help it fill in. Do so as soon as this round of flowering finishes so that the new growth can toughen up before next winter. I can't tell you how much to prune it without seeing it, but it wouldn't surprise me if you needed to prune it by half or two-thirds.

Prostrate sandmat keeps creeping up in the rocks.

Q: Each year, we have had prostrate sandmat pop up in our landscape. We pull it faithfully, but it keeps coming back. Do you have any solution for how we can get rid of it once and for all?

A: In a heartbeat, I would spot-treat it with a glyphosate-only herbicide applied with a 1-gallon sprayer so that I could direct the spray down into the center of each plant. It will make short work of eliminating the plant I call spurge without contaminating the soil in any way.

Turk's cap is shown with crinkled leaves.

Q: Can you tell me what has caused the leaves of my Turk's cap to become crinkled?

A: It looks like herbicide damage, either from drift from somewhere nearby or from residue left in a spray tank. It could also be due to piercing/sucking insects such as thrips or aphids, but the broadleafed weedkiller possibility is more likely.

Grass, such as St. Augustine and bermuda, won't grow in a shaded backyard that doesn't get enough sunlight hours.

Q: I have a problem with my lawn. I had bermuda, but shade from my live oak caused it to thin out and die. I planted St. Augustine, but within a year it was gone, so I replanted with more St. Augustine and it, too, died. I planted zoysia for shade, but it, too, died. I have aerated. The soil is very hard. The lawn looks terrible. What is the problem?

A: You identified the problem yourself. There isn't enough sunlight to sustain the turfgrass. In my 47 years on the radio and 52 years of writing columns, I’ve answered almost one-half million questions, and failure to get grass to grow in the shade is by far the most common problem gardeners have. St. Augustine must have five or six hours of direct sunlight to survive. Bermuda needs seven or eight. Zoysias fall somewhere in between. The frustration you’ve faced is what folks in the Astrodome faced when they had to put shade on their roof to cut the glare when the dome was brand new. And it's why Jerry Jones could never find a natural grass that would survive in the old Texas Stadium. I have a great deal of shade in my yard as well, and I’ve switched over to shade-tolerant groundcovers and low shrubs. Mondograss has become my best friend. I may not be able to play croquet with the grandkids on it, but its texture is similar to turfgrass and it's very low-maintenance. You can do some very creative and lovely landscaping using shade-tolerant plants.

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