Horticulture Guy

Q. I have a huge, beautiful fruitless plum tree. I’ve never pruned it but it’s time. When is it a good time to prune it? Thank you! Cher - Burbank, California

A. The time to prune it is after it finishes blooming in the spring.

Q. How tall will Prunus Caroliniana ‘bright and tight’ grow?  Will these shrubs grow as vigorously in a wine barrel container? Jamie Jameson - Monterey, CA

A. The Carolina Laurel Cherry is an evergreen tree/shrub that is native to the east coast from North Carolina over to Texas. This variety has a more compact habit than the species as the cultivar name suggests.  In the size container you suggest they probably will top out at 10 feet tall.  If they were grown in the ground they may reach 20 feet (The species can get as large as 40 feet tall).  I am sure you can grow them in California in pots but here in the Puget Sound region you would need to protect the plants during the winter.  These trees are rated to USDA Hardiness Zone 8, which would overwinter here in the ground.  When grown in containers I usually suggest buffering 2 zones.  So here in our Zone 8 areas I would suggest growing Zone 6 plants in containers when they are not protected during the winter.  Protection could be as simple as moving them to an unheated garage or in a protected area near the house.

Q. I have a rhododendron. Dark green upon planting. Western exposure. Leaves and new growth turn pale green and droop. Tries to set buds and flower but buds do not mature and flowers are extremely small and die back quickly upon opening. Paul Faro - San Leandro CA

A.  Sounds to me like you have a fungal root rot disease (Phytophthora spp.).  Although pale leaves could be caused by improper pH/nutrient problems, the drooping and die back sound more like this disease, which is encouraged by poorly drained soils and planting deeper than it was in the container.  Root rot disease is tricky to diagnose since the symptoms can be confused with cultural problems like low pH/nutrient problems (pH should be between 5.5 and 5.6 to allow minerals like iron and magnesium to be absorbed) which would result in pale green leaves or drought which would cause drooping and difficulty in absorbing nutrients.  The way you presented the symptoms makes me thing it is more likely root rot disease.  This is not easy to treat with fungicides and you may be faced with replacing the shrub.  Rhododendrons are not the only species vulnerable to this disease so you will need to ask for a resistant variety of Rhododendron or a species of shrub that is not susceptible. You may be able to bring a sample to your local County Extension for accurate diagnosis and also to find out what a suitable replacement shrub may be for your area. 

Q. Hi - I have had a Christmas cactus for about 2 years now.  Its leaves are now getting about 12 inches long, and very heavy.  Am I watering it too much?  I water it twice a week.  Also, I recently added coffee grounds to help it.  Not sure I should have done this.  HELP! Candice Zoot – Denver, CO
                                                                                                                    
A. The size of your plant is not an indication that you are overwatering your plant.  How much you water depends on the preference of the plant you are growing and the conditions in which it is growing.  Christmas cacti are epiphytic so instead of living on the ground in a desert, lives in the crotch of a tree in a jungle just like orchids.  But like its other cacti relatives they have evolved to live in challenging water environments.  The desert simply provides little rainfall.  Although jungle evironments are much wetter the crotch of a tree doesn’t give the root system much to work with as far as water holding capacity.  So the plants have adapted periodic dry conditions.  I suggest using your finger and feeling the top two inches of the soil and when this has become moderately dry water again.  This may work out to be twice a week as you have been doing all along.  Overwatering of Christmas cactus often results in the root and/or stem rotting.  Coffee grounds are a fine addition of organic matter container but they also can reduce the pH of the soil (which is not necessarily bad but it is good to know).  Too much around the stems of the plant may retain water around the stems so keep the layer thin.  Christmas cacti also prefer a lighter soil mix that has good drainage. 

Gardening questions from CA, NV, UT, AZ, CO & NM

Q. I am a landscape architect working on a new building in Los Angeles. The owner wants to put bamboo such as Bambusa vulgaris in the lobby, which has 25′ glass windows on 2 sides and 10 foot-candles of light from the ceiling. What type of additional lighting would you recommend for the bamboo and how many additional foot-candles? Thanks, Claudia Kath - Laguna Beach, Ca

A. I would recommend 1500 foot-candles. This is the equivalent of the light a plant would get sitting in a south facing bay window.  I would suggest Metal Halide lamps as your additional lighting source since their spectrum is the closest to natural sunlight.  You will have to work out the calculations for how many watts you will need since the distance the lamps are from the plants will determine it.

Q. My peonies have good foliage, however the last two years they grow tall stems have heavy flowers that fall to the ground and don’t last long. Lora Fleener - Vernal, Utah

A. The flowers are not lasting very long because of their contact with the soil, which introduces soil microorganisms that begin to precipitate decomposition of the flower. Flopping (the unofficial term) is fairly common in many peony cultivars. I think the flowers have been bred so large and showy that the plants can’t support them without help. The solution is to support the peonies. There are various commercially available support loops, rings, and grids that can be used to support the peonies. Good ones are vinyl coated green so they are slow to rust and unobtrusive. Short stakes and twine can also be an effective alternative.

Q. I have two Wisterias, outside of the apt., I just moved into. They are growing from each side of a overhanging trestle When is the best time to prune them and how? Thank you Donna Beets - Upper Lake, CA

A. Prune them immediately after they finish blooming. This is to ensure you don’t remove next year’s flower buds.