Keeping roses healthy during the drought

Not only are California gardeners faced with new pests, we are confronted with the most severe drought in decades, and with the drastic rationing that water districts must impose. We must vigilantly manage our water use if we hope to maintain our cherished roses – in fact, we may well be forced to go into survival mode.

In a previous column I noted that: "Typical mature, full-size hybrid teas in Southern California soil require six to nine gallons of water a week when the high temperatures are in the 70s. As temperatures rise into the 80s, the rose will require about nine gallons of water per week. In the 90s, the rose will require about 12 gallons per week and even more. These figures are rough and based on the amount of water needed to maintain the highest level of show quality; the rose will stay alive on considerably less."

Just this week I received a mailing from my water district (Western) advising customers to "Water roses no more than once each week, with three to four gallons of water, allowing it to soak in slowly." Three to four gallons is considerably less than the recommendations for maintaining the highest level of show quality.

If we are that limited in the quantity of water we can use, we must greatly increase the efficiency of how we use that water. The strategies I will discuss here are:

• Deliver water efficiently
• Keep water in the soil using mulch
• Allow roses a summer dormancy period

Deliver water efficiently

Drip systems provide the most efficient way to deliver water to roses because they don't produce a water spray that can be carried away by strong winds, and because they deliver water slowly, allowing it to soak deep into the root zone rather than running off. If you have a drip system, be sure it's in good shape before you 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—you may have to remove the emitter, insert a "goof plug," and install a new emitter an inch or two away from the original one.

One more thing: You'll want to estimate the volume of water the system is delivering so you can better manage your use. For example, if every rose has two emitters that deliver eight liters (about two gallons) per hour, then to deliver four gallons to the plant you'll need to run the system for an hour. This 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—which allows water to permeate more quickly—an hour of irrigation may waste some of that water, and you might be better off running the system twice a week for half as long. Experiment! After all, gardening is a scientific pursuit.

Mulch to keep water in soil

If you have read my past columns, you know that I have been advocating the application of a deep layer of mulch for years. Mulch provides many benefits. It moderates the soil temperatures, retains moisture and allows it to spread more uniformly throughout the root zone, discourages weeds, and maintains a soft soil surface. A four inch layer of mulch is recommended. There are many materials you can use, and you might want to experiment with a variety of them, but you will probably get the best results if you don't mix them 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. One possible drawback if 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.

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 you may find that your garden soil level has risen. If this ends up burying the bud unions, you may find more suckers forming from the root stock. When this happens, 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 then resetting the root ball in the hole.

Whatever mulching material you choose, be careful to not to apply it up to or over the bud union. Leave an area around the base of the plant of about 12-inches diameter. (If you can maintain that distance, then as your composted mulch disintegrates it will not raise the soil level around the bud unions.)

Summer dormancy

Allowing your roses to go dormant during the hot summer months will reduce water use as well as the stress on your plants. You won't be missing out much because if you allowed your roses to power through the summer, most blooms would be of poor quality and have burned petals and leaves.

So as your roses complete this bloom cycle, remove only the petals as the flowers fade – do not deadhead them – that is, allow hips to form. This will discourage new growth and flower formation, thus reducing demand for water. Remove any fallen leaves and discard them along with the petals into your yard green 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 your feeding program—we do not want to encourage growth at this time because it will only stress the bush more.

In summary, until at least September:

• Make sure your water delivery system is operating efficiently
• Apply four 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