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September 2009 — 'Ask the Expert' |
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Q. What causes Powdery Mildew?A. Powdery Mildew is a generic term for the unsightly disease that grows on the surface of many plants. There are over 1600 different species of fungus that will cause Powdery Mildew symptoms and they are usually host specific, eg. infected Phlox will not likely spread its disease to adjacent Forsythia. That is not so say that both plants won’t be showing symptoms (if the conditions are favorable it is likely that they will), just that the causal organisms will be different.
Warm days and cool nights with significant dew fall in mid to late summer are prime conditions for the growth of PM. Ensuring good air circulation and dry foliage overnight (i.e. morning irrigation not evening) are paramount cultural techniques for avoidance. Excellent sanitation is also a requirement. The fungal spores over-winter in the scales of dormant buds as well as any discarded foliage.
Growing of resistant cultivars will also decrease frustrations.
Q. Many of my container gardening customers who live in condos or apartments and are asking: Can one successfully over winter Hydrangea in a container? If so, down to what zone and for how many years can one count on successful reappearance and re-blooming? Are there varieties better for this application than others, if so, which ones. What's the minimum size that a container used for this purpose should be? A. Yes it can be done, but with qualifications. There are over 70 different species of Hydrangea with various hardiness requirements. In the North (where the over wintering question is most relevant) most success would likely be obtained from H. arborescens and H. paniculata and their cultivars. Both of these species flower on new wood so there is little danger of the flower buds being killed or damaged with a late spring frost. I would suggest a minimum of a 25 gallon container and if the pot must be over wintered outside on a balcony with little protection, at the very least sheltering against a wall, out of the wind, is advised. If the container can be moved into shelter of an unheated garage, better performance can be expected. Containers should not be moved until all of the plants have entered dormancy, and then the winter storage area should ideally remain between 24F and 40F ( -5C and +4C). All that being said, I would think that zone 5 is probably the limit of successful container overwintering of Hydrangea. But I’d love to learn if I am wrong – anyone out there having success in colder zones?
Q. With the popularity of container gardening for condo and apartment residents, can you tell me if it’s possible to successfully over winter Hosta in a container? If so, down to what zone, and for how many years can one count on the successful reappearance and reblooming? Are there varieties better for this application than others, if so, which ones. What's the minimum size that a container used for this purpose should be? A. Hosta are a much easier proposition for overwintering in containers. First of all they require a much smaller container, making them that much easier to move around either into the protection of a sheltered area, or into an unheated garage. Once the plants are dormant, remove the dead foliage and the pots can even be stacked for the winter – just be sure to unstack before growth starts again in the spring. I know of people in zone 2 that are successful overwintering containerized Hosta.
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