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Heat brings out pests to damage rose bushes

Frank Brines

Master Rosarian

When it feels as though Mother Nature is out to get gardeners living in the Temecula Valley and other regions that predictably experience hot summers, and the wind parches our skin, we have the luxury of going indoors. Meanwhile, our roses have to just stay put.

Roses don't like intense heat any more than most of us do. Their priority is to live. The plant will conserve its resources for roots, canes, leaves and bloom – in that order. When it's hot, roses want lots of water and heat relief rather than food.

Blooms will be smaller with sunburned petals and lessened fragrance. Leaves will turn yellow as chlorophyll is depleted, reducing photosynthesis, then brown just attempting to keep hydrated because their root system can't keep up with transpiration. Do not remove desiccated leaves in hot weather. They provide some shade to protect the cane from sunburn. Remember, if a leaf dies it is easily replaced; if a cane dies, it is gone.

That is why I advocate a conservative style of summer rose care from July to September.

1. Do not prune away spent blossoms – just the petals, leaving the rest to form rose hips.

2. To avoid stressing the plant, reduce or discontinue fertilizing until mid-September; if you do feed, use lower Nitrogen and dilute it.

3. Extreme heat can burn the roots of a stressed plant: Continue irrigation schedule or increase according to temperature.

4. If not mulched, consider adding some to a minimum of 3 inches.

5. Keep the ground clear of debris to help control diseases and pests.

Hot dry summer days and cool nights create perfect conditions for powdery mildew. Examine daily and begin treatment at the first indication of any problem. Start with a strong blast of water early in the day to top and under sides of the leaves to knock it off. This has to be done early in the stage of development before the spore embeds into the leaf. This is likely only temporary and gives you some extra time.

This is also perfect weather for rust, the spores that form on the undersides of leaves and (as its name implies) looks like rust on metal. Since it begins on the lower leaves it can go undetected before you discover it is present. Remove each leaf by cutting it off close to the cane to minimize the spores falling onto other leaves and the ground. Spores on the ground can easily be splashed back upon the leaves if irrigating with other than a drip system.

Western thrips continue to be a terrible problem. These tiny insects love to get inside the blooms and suck the juice out of the petals, beginning on the outside petals, causing them to lose substance and preventing blooms from opening. Damage is easy to see on light-colored roses: small brown spots on petals and/or edges. Open an affected blossom: Thrips look like tiny hopping fleas running around inside. Clip off and promptly dispose of infested and spent blooms, as well as litter on the ground.

The dreaded Chili Thrip is even smaller and more damaging. This species attacks blooms and tender foliage. They have been detected on other plants as well. Immediately cut out distorted and bronzed new foliage, scorched and deformed buds and blooms, and fallen leaves. There are available products for treating, read the labels so you buy the product you need for the problem. I cannot endorse products here.

As if all the above isn't enough, spider mites are a major destructive pest. They are not insects but more closely related to spiders. They are hard-to-see because they live on the underside of leaves and rasp the tissue. Left alone they can quickly defoliate a bush. Heat increases their reproduction. Look for loss of color on tender green leaves in the middle part of the leaf and purplish yellow on more mature leaves and in severe cases, webbing on the leaves.

Because spider mites overwinter in soil and migrate to the undersides of the lower leaves, an infestation may often go unnoticed until significant damage has been done. A quick light brushing of the underside of the leaf with your finger will readily support your suspicions. The surface will feel like it's covered with a fine grit. If discovered early, a strong spray of water from underneath and a water shower from above to rinse off the dislodged mites may be sufficient to correct the problem. To help prevent a complete infestation, remove all leaves within 8" to 10" of the soil surface.

It goes without saying that water needs have increased dramatically. Perhaps requiring daily watering. It is essential that you check soil dampness frequently during hot days. Use a water probe or stick your finger to a depth of four to six inches (that is, if you have fluffy soil or unnaturally long fingers). If your soil is too compacted to do this, use a small garden trowel to scratch down to that depth and check the moisture content. A minimum of 4" of good, composted mulch over the entire bed will help conserve moisture.

Potted roses are even more susceptible to heat and drying because soil in a container will heat up rapidly, virtually cooking the roots. In addition, the soil contracts and pulls away from the container's sides, causing water to run through rapidly, washing away soil, and wasting water. Here are a few remedies to help struggling containerized plants:

• Move potted plants to a cooler area under a patio cover or shade tree.

• Never place containers on concrete or other surfaces that readily absorb heat from the sun – but if you must, use pot feet or other methods to provide an air space between the container and the surface.

• Position light-colored umbrellas and/or shade cloth over the plants.

• Be sure you provide plenty of air circulation around the plants to allow cooling.

• Apply three or four inches of composted mulch (but not against the plant itself).

Roses enjoy a good shower, just as we gardeners do! The difference? Give roses an early morning shower before the sun gets too high and the temperatures are hot for long periods. Jets of water can blast off dirt, dust, and tiny pests (such as mites, mildew spores, aphids, etc.), and hydrate your roses in preparation for a hot day. If you use a water wand aimed upward to spray the underside of the lower leaves you can dislodge spider mites. Because they reproduce so quickly you must do this every few days.

Be sure to visit the Rose Haven Heritage Garden located at 30500 Jedediah Smith Road (the cross street is Cabrillo Avenue) in Temecula, a 3.4-acre rose garden owned and maintained by the Temecula Valley Rose Society, a (501c3) organization, supported with donations from kind people like you. (Look for the donation box when you visit.) Also, visit http://www.TemeculaValleyRoseSociety.org regularly for great information and schedule of events.

 

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