Growing Concerns 485: Mold and mildew, winterizing a pond

Early Fall!

When a garden is slimy and mildewed, move the air and change the watering

Mold is taking over! I have a number of patches in our mulched areas that are light yellow and turn to dark brown. By early September my plants were turning white in the leaves and looked bad. What caused this and how can I help?

- L.W. -


You're looking at two separate problems.

The mold is slime mold, a mulch decomposer that sometimes has enough energy to "bloom" yellow. It's harmless except when large enough to lap over ground-hugging plants during hot weather, killing the covered foliage. You can't get rid of it except by removing all mulch, which is not a good move for the garden overall.

Slime mold is most likely to occur in thick wood or bark mulch, and isn't seen where other soil microorganisms are working well enough to decompose mulch within a year. So don't keep piling on mulch if you use wood that persists from one year to the next, and switch where you can to finer-particle, quicker-decomposing materials like fall leaves.

Your whitened plants are probably infected with powdery mildew, fungal diseases that disfigure but rarely kill. Mildews were not prevalent during the hottest part of the drought but began to crop up once temperatures dropped and we got a bit of rain.

Some plants including zinnia, tall phlox, bee balm, and lilac are simply prone to mildew. They don't "give it" to each other, however. Each plant species is prey to its own fungus species. We simply see the same symptom on different plants.

When mildew is prevalent among several species, we look to poor air circulation and watering problems as the real culprits.

Where air doesn't circulate well, mildew spores have more time to grow and take hold on a leaf. Poor air circulation can be a whole-neighborhood problem -- cleansing breezes are more likely on hilltops and in meadows than in low lying areas and woods. However, you can have dead spots in a yard near solid fences and thick plant growth. Pruning to thin overgrowth can help, as can replacing solid fencing with airier structures.

Some species, such as zinnia, are more prone to mildew if their leaves stay moist for long periods. Other species, like bee balm, are more susceptible when grown too dry. Reduce mildew by rearranging a garden to group plants by their varying water needs, then irrigating more carefully. Replacing what you already have with disease-resistant varieties is also a good move -- throw out 'Starfire' phlox and plant 'Franz Schubert' instead. Research plants in garden encyclopedias and on the Internet before you buy, to learn the disease resistant types in each species.


Short reports

Winterizing a water garden...

... revolves around keeping fall debris out of the water. The less organic material there is in an ice-covered pond, the less decay and more oxygen there will be to keep fish and plants in good health.

Cut back water plants and put that foliage on the compost. Keep fall leaves skimmed off a pond's surface or stretch a leaf-collecting net above the water.

Stop feeding fish. They can't even digest food once temperatures drop to the 40's. Clean your filters. Make a commitment to keep an opening in the ice or divert the flow of water from your pump to a bubbler to keep oxygen levels up.


You washed plants before you brought them in for winter...

...and that's good. A thorough cleansing of the foliage and pot is more effective than an insecticide for fending off unwanted houseguests. Yet neither approach is enough in itself if the plant is prone to pest trouble.

Some insects or eggs always escape either sudsy soak and rinse or toxic spray. These pests rebound within the first weeks, at the same time your plants' natural defenses are down while they're adapting to lower light.

A pest's presence may not be noticeable until it reaches crisis levels in mid-winter. So for this first month back indoors, move plants into the shower every week or ten days to rinse them with a forceful stream of water. This sends pesty survivors and newly hatched insects down the drain.


Green thumbs up

to those who aren't fooled by surface appearances and so continue to water important plants. The soil may have lost its crusty appearance and cool weather persuades us to think the drought passed along with the heat, but below the top inch the ground is still bone-dry in many places. Keep trickling the hose under key trees and shrubs until fall leaf drop finishes.


Green thumbs down

to giving up on any shrub, tree or long-lived perennial after just one disappointing season. Take a lesson from the Detroit Lions' football fans! But don't go too far in that direction. If one year of skimpy growth or lackluster bloom follows another, don't hope for magic. Continuing lackluster performance calls for major change, such as a move to better growing conditions.


Originally published 10/12/02