Take care of roses in the hot summer months

“Gardeners must become water-wise and learn as many ways as possible to use water efficiently. Some water districts may still have restrictions on how and when water can be used for irrigating landscape. Water agencies advise customers to be conservative this year as supply is still limited and even purchasing water from other regions is difficult for the same reasons.”

The above paragraph is from a year ago. Although the current precipitation for the area is 20% to 25% above normal, I encourage gardeners to be as conservative as ever. Agencies haven’t constructed water recovery basins to refill aquifers and some aquifers have collapsed and can't be recovered. Much of the recent rainfall just ran off into drainage. The added rain and humidity have encouraged more fungal diseases this year, fungi like powdery mildew, rust and various forms of botrytis.

There are a few things gardeners can do now to provide some relief for roses once the weather gets really hot. First, as hot weather approaches, cut back on fertilizing established roses; it will encourage them to slow down for the hot summer. Second, water roses deeply as temperatures rise – that means longer, slower irrigation. Next, hose off roses in the early morning to increase humidity and control spider mites, which are found mostly on the underside of the leaves. A strong forceful spray from below is needed to dislodge them. Then give a downward shower to wash them away. Spider mites usually appear when the seasonal temperatures have been hot. Also, keeping vegetation trimmed to approximately 8 inches from the soil level will help as well. Finally, remove only the petals of the blooms and let the rose hips develop; this brings on a mild dormancy.

Typical mature, full-size hybrid teas in Southern California soil require about six to 9 gallons of water a week when temperatures are high. As temperatures rise into the 80 F, those roses require about 9 gallons of water per week. In the 90s, the rose requires about 12 gallons or more per week. These figures are rough and based on the amount of water needed to maintain the highest level of show quality blooms: A rose can stay alive on considerably less. But during summer heat, I don't even try to have show quality blooms.

Use these three strategies for efficiently managing the amount of water you apply in the rose garden:

● Delivering water efficiently

● Keeping water in the soil using mulch

● Allowing roses a summer dormancy period

Delivering Water Efficiently: Drip systems provide the most efficient way to deliver water to roses because such systems don't produce a spray that can be carried away by the slightest breeze, and because they deliver water slowly, allowing it to soak deep into the root zone rather than running off. If a garden has a drip system, be sure it’s in good shape before moving on to the next step and cover it with mulch. Open each irrigation valve one at a time and inspect how it is performing. Repair any leaks, including emitters that are spraying from their attachment point on the tubing – remove the emitter, insert a “goof plug” and install a new emitter an inch or two away from the original one. My gardening friends are recommending the more efficient drip systems similar to a product called Netafim. This product has pre-placed pressure regulating emitters at different distances apart for use. Find the information at http://www.netafimusa.com.

One more thing, estimate the volume of water the system is delivering and manage its use. For example, if every rose has two emitters, which it should have in the typical drip line system that, deliver eight liters or about 2 gallons per hour, then to deliver 4 gallons to the plant, it’ll need to run the system for an hour. This measurement should work well in a typical loam soil. The water should 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 the soil is particularly sandy – which allows water to permeate more quickly – an hour of irrigation may waste some of that water, and it might be better to run the system twice a week for half as long. Experiment. After all, gardening is a scientific pursuit.

Mulch: In my past columns, I have been advocating the application of a deep layer of mulch for years. Mulch provides many benefits. It moderates soil temperatures, retains moisture and allows it to spread more uniformly throughout the root zone, discourages weeds and maintains a soft soil surface while enriching the soil nutrients and biomass. A four-inch layer of mulch is recommended. I highly suggest composted mulch. There are many materials to use, so experiment with a variety of them. Gardeners will probably get the best results if they don’t mix types in any one garden bed.

One material that some gardeners have in abundance is pine needles. They provide an airy cooling 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 chip specifically intended as mulch; I recommend the finer cut forms. Possible drawbacks include if they are not specifically manufactured for garden use is the potential for matting due to fungal growth, which can make the wood chip layer impermeable to water and also the need to apply added nitrogen to break down the wood fibers. I’m not an advocate for the dyed wood products.

I prefer composted mulch that is light and fluffy, so it doesn’t pack down, and contains a higher proportion of hummus, so it slowly integrates with and enriches the soil. One drawback of composted mulch is that after several years, gardeners may find that their garden soil level has risen. If the soil level ends up burying the bud unions, it can be helpful to “lift” the rose – essentially, digging to release a large root ball, levering it up, filling in several inches of good garden soil beneath it and resetting the root ball in the hole or maintain a clear well around the base of the bush.

Whatever mulching material is chosen, be careful to not apply it up to or over the bud union. Leave an area around the base of the plant of about 12 inches in diameter. If that distance is maintained, then as the composted mulch disintegrates it will not raise the soil level around the bud unions. An edging material can be formed into rings that will assist in preventing mulch covering bud unions.

Summer Dormancy: Allowing roses to go dormant during the hot summer months will reduce water use as well as the stress on the plants. Gardeners won’t be missing out much because if they allowed the roses to power through the summer, most blooms would be of poor quality and have burned petals and leaves. So as the roses complete the current bloom cycle, remove only the petals as the flowers fade – do not deadhead them – that is, allow hips to form. It will discourage new growth and flower formation, thus reducing demand for water. Remove any fallen leaves and discard them along with the petals into the green yard waste bin – do not compost them. 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 burned leaves because they provide shade for the cane which can be damaged or killed by sunburn. Discontinue a feeding program so as to not encourage growth at this time because it will only stress the bush more.

In summary, until at least September:

● Make sure the water delivery system is operating efficiently

● Apply 4 inches of mulch over the entire bed

● Remove petals as flowers mature

● Allow hips to form

● Do not prune or cut back

● Leave brown leaves on the plant

● Do not feed

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

Doesn’t look like much work, right? Well, since gardeners will be taking it easy for the summer, go visit Rose Haven Heritage Garden, 30592 Jedediah Smith Road. The cross street is Cabrillo Avenue in Temecula. Also, visit http://www.TemeculaValleyRoseSociety.org.