I have some beautiful red dahlias in two flower boxes on my front
porch. Two questions: I noticed some of the leaves at the bottom
of the plants are turning yellow and dropping although the plants
are still blossoming. What to do? Also, do I need to remove them
from the soil for the winter here?
To answer your second question first, you don’t have to dig up your
dahlias unless you prefer to do so. Here is what local gardening expert
Marianne Binetti says on this subject in her Seattle P-I
column:
“Laid-back gardeners should just leave their dahlia tubers in the ground.
Cut back the top growth after the first hard frost and then cover the
dahlia bed with a waterproof oilcloth tablecloth or tarp. Secure with
rocks or bark mulch. This keeps the tubers dry and in the spring you can
remove the covering and see what comes up. For dahlias worked into a
mixed bed, you can pile sword fern leaves on top and weigh these down
with a rock.
“It is the wet more than the cold that kills dahlia tubers.
“If you really need to dig and store dahlias, place them in open paper
bags, never plastic, and let them dry in a covered area for a few days
before storing.”
As for the yellowed leaves, it is hard to diagnose the problem via
e-mail. There is a chance that you could safely just keep removing and
disposing of the yellowed leaves. However, there are problems like
leafhoppers (an insect which feeds on the leaves and causes them to
become speckled, then turn dry and drop off), or viral diseases which can
cause yellow spotting or mottling of the leaves. I looked at The
Gardener’s Guide to Growing Dahlias by Gareth Rowlands (Timber Press,
1999), and found descriptions of leaves which have turned yellow due to a
number of causes, including chlorosis (the leaves are unable to produce
enough chlorophyll, and turn yellow if there us still carotin present, or
white in only xanthophyll remains). When a mature dahlia turns yellow, it
may be due to a nutrient imbalance (such as lack of magnesium or iron).
Without having the plants diagnosed, it is impossible to recommend a
remedy, so you may want to bring samples to a Master Gardener Clinic.