Pentas hearty enough for summer container gardening

Question: What can I grow in a large pot and provide color to a patio on the east side of my house? I've tried everything, and nothing seems to survive.

Answer: First of all, you mentioned your pot was large. A large pot is required to keep anything alive in the lower-desert heat. The plants need enough soil to protect their root systems. I usually don't plant in anything smaller than 24 inches in diameter and height if I expect it to live. Secondly, you mentioned the eastern exposure. Your pot with summer annuals needs minimal sun exposure or dappled sun to survive and really can't handle the western sun.

I think it is also really important that you get your soil mix right. I like to use a substantial soil mix like a cactus mix with an even ratio of soil, organic matter and some substrate such as pumice for good drainage. This will hold enough moisture so you don't end up over-watering and rotting your summer annuals. Typical "potting soil" will dry out too fast and cause you to water too frequently.

For an easy sturdy planting for summer, try a grouping of Pentas lanceolata or star clusters. This plant is extremely heat tolerant and will take living in pots with weekly watering. Pentas come in white, pastel and red. I especially enjoy white, because they are great at night when you can enjoy the summer evenings. I like to surround my grouping of Pentas with burgundy- and lime-colored sweet potato vines for added interest.

Q: I have a Queen Victoria agave that started to sprout about nine weeks ago. The spike is now about 18 feet tall, and the white buds have begun to open. Someone told me it is going to die, and another person told me that if I cut off the spike, it will survive. Who is correct? Also, it has hundreds of babies all around the bottom. What is the best way to propagate these?

A: I spoke with Starr Urbatsch, our agave collections manager at the Desert Botanical Garden. Here are Starr's thoughts: "The agave bloom is a symbol of years of preparation by the plant to reproduce itself. It takes an incredible amount of energy to do this, so by the time you see the bloom stalk, the plant is already on its way out. Cutting off the stalk won't stop what's already set in motion.

"The good news is, you should have plenty of seed and/or pups (basal offsets) to plant! If you choose to plant the pups, they will root in almost any well-draining soil mix. Protect young pups from the sun. It is preferable to keep plants in pots until they have developed a root system and have grown at least 4-6 inches tall before placing them in the ground. If they have already reached this size and have a good root system when you removed them, they can be planted directly into the ground.

"The Queen Victoria agave can be planted in full sun, but will need some protection from the sun until established. Try planting in filtered sun or protect new plants with shade cloth during their first summer."

Q: I live in a condominium community in Scottsdale that planted several Desert Museum trees two years ago. I believe they are being trimmed too much and watered too frequently. Would you please let me know the correct care for this lovely tree?

A: Desert Museum paloverdes do tend to grow too fast if given excessive water. At this age, the water source needs to be moved away from the trunk out toward the edge of the drip line, assuming you have drip irrigation. You should set your timer between three and six hours, depending on how fast your soil drains, and water only once a week.

Reduce your watering frequency in the fall, tapering to none during December, January and February. If your trees look like others in town, they may have been severely stripped of interior branches and are heavy at the ends. Don't do any major pruning during the summer or fall; concentrate only on reducing the weight at the end of the branches to encourage interior branches to sprout. Excess weight at the ends can also lead to splitting. These trees need to grow more slowly to be able to build up trunk and branch diameter to withstand stress.

Brian Kissinger is director of horticulture at the Desert Botanical Garden. E-mail your garden questions to bkissinger@dbg.org. Read previous columns at home.azcentral.com.