How to manage roses in hot weather

Frank Brines

ARS Master Rosarian

It's been a wetter than "normal" winter, but there's a better than even chance that the developing El Niño will bring warmer than average temperatures to Southern California. So, as always, gardeners must be watchful and learn how to efficiently manage the amount of water they apply in their gardens.

With summer and the warmer temperatures to come it will help diminish heat damage and stress to the plants. The strategies I will discuss here include delivering water efficiently; keeping water in the soil using mulch and allowing your roses a summer dormancy period.

Deliver water efficiently

Installing the most efficient delivery system is one method to save or conserve water. Learning your garden's soil type will help you make a decision on which systems work best and how much water to deliver at any one time. Growing in pots is another story.

Typical mature, full-size, hybrid teas in Southern California require about 6-9 gallons of water a week in moderate temperatures, i.e. 70s. As temperatures rise into the 80s they require about 9 gallons per week. In the 90s, at least 12 gallons. Roses can stay alive on considerably less, but they may come through the experience debilitated.

Drip systems are the most efficient way to deliver water because they don't produce a spray that can be carried away by the breeze and they deliver water slowly so it soaks deep rather than running off. If you have a drip system, be sure it's in good shape before you go on to the next step and cover it with mulch. Open each irrigation valve one at a time and repair leaks.

I like Netafim products for their integrated pressure-regulating emitters that can even be buried beneath soil or mulch. Find the information at http://netafimusa.com.

To estimate how long to run each system, multiply the number of emitters by their delivery rate, one gallon per hour, then divide by the number of roses. For example if you have 40 emitters each delivering one gallon per hour, you deliver 40 gallons per hour. If you have 10 roses, that's four gallons per rose. To deliver 12 gallons per week, run each system for one hour three times a week.

It should work well in a typical loam soil. You want the water to soak down at least 12 inches for optimal rose health. A loam soil doesn't allow water to just run through it, so irrigating for an hour at a time can be fairly efficient.

On the other hand, if your soil is particularly sandy and the water permeates more quickly, watering for an hour may waste water. In this case run the system twice as often for half as long. You may need to make adjustments based on the performance of individual bushes.

Apply mulch

If you've read my past columns, you know that I advocate a three inch to four inch layer of mulch. Mulch moderates the soil temperatures, retains moisture and allows it to spread throughout the root zone, discourages weeds and over time enriches with nutrients and biomass. There are many materials you can use, but I recommend composted mulch.

You might experiment with a variety of materials, but you'll probably get the best results if you don't mix them in any one garden bed. For example, some gardeners have access to pine needles. They provide a cool airy barrier and break down very slowly to impart a more acidic soil environment which makes mineral nutrients more available to plants.

Another material is any size of wood chips specifically intended as mulch. I recommend the finer cut forms. Possible drawbacks are if they are not specifically manufactured for garden use.

When a neighbor has tree waste chipped up there is the potential for matting due to fungal growth, which can make the mulch impermeable to water. As the wood breaks down it tends to rob the soil of nitrogen, meaning you have to add more. I'm not an advocate for dyed wood products.

Whatever material you choose, be careful to not apply it on or over the bud union; the place where most commercially available rose varieties are grafted onto "rootstock." Leave a clear area around the base of the plant of about 12 inches in diameter.

If you can maintain that 12-inch distance, then as your composted mulch disintegrates it will not raise the soil level around the bud unions and won't cause the lower canes to send out lots of fibrous roots. Keep foliage pruned to at least 8 inches above the mulch layer to reduce infestations from pests, like spider mites.

Allow a summer dormancy

Allowing your roses to go dormant during the hot summer months will reduce the stress on your plants. You won't be missing out much because when you allow roses to power through the summer, most blooms are poor quality with burned petals and leaves.

To encourage this dormancy, stop feeding established roses near the end of June, but be sure to water them deeply. For your June fertilizing program, I suggest using a product with higher phosphate – the middle number of a fertilizer product using the three NPK system. It helps grow roots, so the plant can better cope with the water demands placed by higher summer temperatures.

NPK are the three primary nutrients which all plants need: nitrogen, phosphorus and potassium.

As blossoms fade remove only the petals. Do not deadhead them, that is, allow hips to form. It discourages new growth and flower formation, thus reducing demand for water.

Remove fallen leaves and discard them along with the petals into your green yard waste bin. Do not compost them unless you know for certain that your compost pile reaches a sufficient temperature to kill pathogens.

In summary, it is always a good practice to keep the garden clean in order to reduce fungal diseases and insect pests, particularly in hot dry weather.

Do not remove sunburned leaves because they provide shade for the cane which can be damaged or killed by sunburn.

Follow these six pointers until at least September: Do not feed your roses, make sure your water delivery system is operating efficiently, apply four inches of mulch over the entire bed, remove petals as flowers mature, do not prune or cut back – allow hips to form and leave burned leaves on the plant.

Potted plants will require more diligent watching, resources and attention to what they are experiencing during this period. Learn to listen to your plants and observe their reaction to the elements.

Summer heat brings with it a host of diseases. It is also perfect weather for rust, the spores that form on the undersides of leaves. As its name implies it 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 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 chilli 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 specific products.

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 inches to 10 inches of the soil surface.

Doesn't look like much work, right? Well, since you'll be taking it easy for the summer, go visit Temecula Valley Rose Haven Heritage Garden, 30592 Jedediah Smith Road in Temecula. For more information, visit http://www.TemeculaValleyRoseSociety.org.