Too much pruning leaves tree open to sunburn

Q: Attached are pictures of a Palo Verde which has some unusual black growths on the upper sides of the limbs. I'm wondering if it has something to do with pruning, since it appears to be only in areas that have been pruned, although other pruned areas are unaffected.

A: My first reaction to the pictures was sunburn damage. But I would have to see if this damage was sunken, like a canker. These are now posted on my blog at Xtremehorticulture on blogspot.

Next, I would take a sterilized knife and cut around the edge of the damage and through the damage to see if the wood below was dead or not. If it was dead below the damage and the damage was sunken, then it is most likely sunburn . The pictures you sent had damage all on the upper side of the limbs, which is consistent with sunburn damage.

You wondered if it might be associated with pruning, which it is. The big mistake people make on Palo Verde is pruning so much that too much light gets inside the canopy and burns its photosynthetic limbs. Even though they are designed to absorb light there is such a thing as too much light.

These trees don't make a lot of shade and when we remove too much canopy we will get sunburn on the limbs. Once we get these limbs sunburned, then we have to look for possible borer damage. They go hand in hand . The adults focus on damaged trees for egg laying.

So cut into the damage and look for football-shaped exit holes about three-eighths to one-half inch long and one-quarter inch wide under the damaged, sunburned bark. If this is the case, don't prune so heavily next time and leave enough cover to prevent sunburn.

Q: This past spring I cleared a large "L" shaped area in my yard and planted eight various rose bushes. They have sun from about 9 a.m. to about 5 p.m. They are watered in the evening and fed regularly. They are well-planted with planting medium in large holes, and then mulched. All flowered during the spring and early summer.

Now, however, they are very spindly looking. The leaves left on them are brown and the stems are turning brown. There are no bugs that I can see. I tugged on them and they are still firm in the ground. Have they died and should I replace them, or wait until spring and see what comes back? Should I cut them back and, if so, how much? I do not want to cut too much off. I always had rose bushes at my other homes both in New Jersey and here and never saw this problem .

A: Roses will grow best in an eastern exposure with protection from late afternoon sun. They can grow in full sun but they grow best in the exposure I mentioned.

It is important to use a good compost mixed in the planting soil at the time of planting as well as a starter fertilizer high in phosphorus. If they are bare root, make sure your plants are planted the same depth as they were in the container or in the nursery. Make sure the bud union or dog leg on the stem is above ground and not buried.

Roses do well on drip irrigation. Use two emitters per plant or you can use drip pipe with inline emitters . If on drip, it is important to note the amount of water the emitters deliver per hour and adjust your time so that each plant gets about 2-3 gallons every time it is irrigated. Irrigations should be about two to possibly three times per week this time of year. Irrigate less often as it gets cooler, but the length of time or hours should remain the same.

It is best to use wood mulch rather than rock mulch. All mulches must be pulled away from the rose stems at least 6 inches or the roses can get crown rot on the stem and die.

Fertilize roses once in January with a rose fertilizer plus a soil-applied iron chelate containing EDDHA. Follow label directions. You can fertilize again lightly after the heat of the summer has passed to help stimulate fall rose production .

My guess is that they may have been planted too deep, the mulch touching the rose stems may have caused collar rot or irrigations may have been inadequate, either too often or not enough water. It's just a hunch.

Bob Morris is a horticulture expert living in Las Vegas. Visit his blog at xtremehorticulture.blogspot.com.