Heat brings dangerous conditions to rose bushes

Frank Brines

ARS Master Rosarian

I checked the weather projections and learned that temperatures for the next seven to 10 days for the Temecula Valley are for mid- to high-90s along with higher than normal humidity due to warmer ocean water temperatures and "monsoonal moisture." All in all temperatures are trending higher in the last five years.

If you're using drip irrigation, run your system in the early morning or evening to give your roses the opportunity to thoroughly hydrate. If you're using a hose or other non-surface method, do it in the early morning. Assess conditions every morning. It takes only a few days in these temperatures without sufficient water for a bush to be severely damaged or killed. Look for wilted or dry crisping foliage.

Sometimes if you discover dehydration soon enough, dousing the stems and leaves with plenty of water in addition to applying plenty of water to the ground, may save the plant. But be aware that after a hot day, most plants can appear wilted while still receiving sufficient hydration.

Also, inspect your irrigation system to make sure it is delivering enough water, isn't clogged and isn't over watering – all problems that come with age in drip irrigation systems. If an emitter is delivering much more or much less water than others on the line, it can change the system pressure and affect the other emitters. The simple solution: replace it.

Plants in pots require more frequent watering than those in the ground. As the soil dries it pulls away from the sides of the pots allowing water to run through the soil without penetrating the soil. Also, the longer the soil is in a pot, the less porous it becomes in the root zone, so it's a good practice to repot every two years or so.

Sun shining on the pot, whether black plastic or clay, can steam the roots of the plant which also requires more water to maintain a cooler temperature of the soil. Painting a pot with white paint can help some in keeping the soil cooler. Plastic is still preferred over clay if you can't keep the soil hydrated because clay loses moisture through its many pores; on the other hand, if kept hydrated, those same pores can provide an evaporative cooling effect – as always, there are trade offs.

"Double potting" is a practice that can moderate drying. It involves placing one pot inside another – both should have drain holes. It provides a curtain of cooling air between the pots, an insulation of some type would be even more efficient.

If you are following my prescribed practice of allowing your roses to rest during the summer, you still have several weeks to take it easy before a mid-season pruning. Remember, a mid-season pruning is light, removing any point along a cane where many stems of blooms came out; for quicker repeat blooming, prune each cane back to just above the outward facing bud at the base of the first five leaflets' leaf. This year I plan to do this pruning in the second half of September.

Now, if you have a special event for which you would like to have fresh rose blooms, count back six to eight weeks from that planned event to determine when you should do your end-of-summer pruning. You can possibly have two more bloom cycles this calendar year.

If you have a slight infestation of chilli thrips, remove all infected growths and then prune as described above. If you have thrips, spray as soon as possible after pruning. Be careful to not remove so much vegetation that the canes are exposed to the hot sun as sunburn can kill a cane.

This time of year with hot temps also attracts spider mites. If you see signs of yellowing foliage you may have an infestation. Check the underside of the lower leaves for a grainy feeling substance or tap onto a paper to see these tiny critters. The easiest way to treat it is to use a strong spray of water from below to give the plant a shower and rinse the mites to the ground. If you see fine webbing, you may need a stronger method. Spider mites have been a big problem this year. They have been seen on many plants causing some to die.

I've noticed another problem as a result of the weather this year. High temps and high humidity combine to increase instances of black spot, which is indicated by yellow leaves with usually round shaped black spots. I have not seen any sign of black spot in my garden yet.

With the humidity comes dewy nights which then tends to incubate powdery mildew. I have been troubled by this mildew throughout this year in my garden. I have discovered damage from chili thrips; however, that is difficult to recognize until it becomes obvious. At the first signs of any of these it is best to start treating with fungicide or a pesticide, preferably one containing spinosad.

After pruning is complete, and at least one thorough application of water, apply a good fertilizer. Read the directions on the container to discern the type of application and what to do. I use granules, powder or liquid and water it in for the quickest effect. Some of my colleagues recommend the use of fish emulsion and seaweed fertilizers at the rate of 1 tablespoon each per gallon of water applied now. Remember: Never fertilize a dry or stressed plant – always water the day before.

Now is a great time to clear the debris in, around and under your rose garden. Due to the heat you may have a lot of leaf drop and old petals build up. Once the debris is clear, you'd be wise to order and apply a layer of composted mulch.

Here is a formula you can use to determine the quantity you will need. An area 10 feet by 50 feet needs 4-5 cubic yards to cover the garden 3-4 inches deep, which is the depth I recommend. It is the best product you can apply to protect your roses roots from heat and cold.

A bi-monthly magazine which covers rose topics is the "American Rose" published by the American Rose Society. For more information, visit http://www.ARS.org or rose.org.

When you have a moment to spare, feel the need to get away or when the day cools down, take your favorite beverage, a picnic basket and visit the Rose Haven Heritage Garden, 30592 Jedediah Smith Road, in Temecula. The cross street is Cabrillo Avenue. Also, visit http://www.TemeculaValleyRoseSociety.org.