Elisabeth C. Miller Library logo Miller Library Home UW Botanic Gardens Home UW Botanic Gardens Home book graphic

3501 NE 41st Street, Seattle, WA 98195 | (206) 543 0415 | Open: | Library Schedule

Gardening Answers Knowledgebase

Search Results for: Dahlia | Search the catalog for: Dahlia

Plant Answer Line Question

Keywords: Dahlia

I need to find a photo of the eyes on a dahlia tuber.


There is good information on growing dahlias, including an image of the location of the eye on a tuber, at the website of the American Dahlia Society. To find the eye, locate the point on the shoulder, or crown, of the tuber from which the plant grows. The blog of Lynch Creek Dahlias has good description and illustrations:
"Keep in mind that every dahlia tuber, to be viable, must have at least one eye, which you'll see as tiny pointed protrusions on or near the neck of the tuber (the neck is the tuber's connection to the central part of the root mass)."

Date 2018-04-21
Link to this record only (permalink)

Plant Answer Line Question

Keywords: Whiteflies, Insect pests--Identification, Insect pests--Control, Root weevils, Rhododendrons--Diseases and pests, Dahlia

I have a line of Ward's ruby azaleas. The three weakest ones have a lot of tiny notches in the leaves. I seem to remember the notches from the root weevil as being larger than these. Are the tiny notches from something else?

I also noticed that some of my dahlias have splotched leaves and that when I disturb the leaves, white-looking insects fly off the leaves. These flies apparently have spread to tomatoes as well. Are these whitefly? Will they disappear after the winter or is there some control I should use to prevent them from taking over?


First you need to get an accurate diagnosis of your problems. If you are in King County, you can bring samples to a Master Gardener Clinic. For information about Clinic hours see their website (Plant Clinic Schedule).

Oregon State University offers this information about root weevils and Rhododendron (which includes Azaleas). It describes using beneficial nematodes as a control.

According to Washington State University Cooperative Extension's publication, How to Identify Rhododendron and Azalea Problems (1984), root weevil damage to foliage is not usually a serious problem. You can check for weevils with a flashlight at night to confirm that they are the source of the notches you are seeing. There are some Neem oil-based products that may be helpful, but they must be used at the correct times of year. See WSU's HortSense page.

As for the dahlias and tomatoes, it is important to determine exactly what the insects are before proceeding with treatment. If they are whiteflies, you can put yellow sticky traps around the plants to trap them. University of California, Davis's Integrated Pest Management site has other recommended control methods, including reflective mulch. You may not want to use insecticidal soap:
"Insecticides have only a limited effect on whiteflies. Most kill only those whiteflies that come in direct contact with them. For particularly troublesome situations, try insecticidal soap or an insecticidal oil such as neem oil or narrow-range oil. Because these products only kill whitefly nymphs that are directly sprayed, plants must be thoroughly covered with the spray solution. Be sure to cover undersides of all infested leaves; usually these are the lowest leaves and the most difficult to reach. Use soaps when plants are not drought-stressed and when temperatures are under 80 degrees F to prevent possible damage to plants. Avoid using other pesticides to control whiteflies; not only do most of them kill natural enemies, whiteflies quickly build up resistance to them, and most are not very effective in garden situations."

Date 2017-08-15
Link to this record only (permalink)

Plant Answer Line Question

Keywords: Plant diseases--Control, Fungicides, Powdery mildew diseases, Integrated pest management, Dahlia

What can I do about powdery mildew on my dahlias? Should I throw the bulbs away, or does it only contaminate the plant above the ground? I have heard both too much water and not enough water cause this problem. Is either true?


The main thing you will need to do is destroy all the foliage affected by the mildew. The mildew can survive the winter on infected foliage, and then spread to new foliage.

Powdery mildew thrives where plants are crowded and there isn't enough air circulation, so give your plants space, a sunny site, and try watering in the morning, and watering from beneath the plants (not over the leaves) so they are able to dry off during the course of the day. As you indicated, too little water can also be a problem.

Here are two websites with additional information:
Univ. of California IPM Online Guide
Washington State University Extension Garden Tips

I did not come across any information specifically saying that powdery mildew will affect bulbs or tubers. I spoke to an experienced dahlia and begonia grower here who said that it should be all right to store and replant your tubers, as long as you thoroughly get rid of all the diseased foliage aboveground.

Some sources (such as The Organic Gardener's Handbook of Natural Insect and Disease Control, edited by Barbara Ellis, Rodale Press, 1996) suggest that a baking soda spray (1 tsp. per 1 quart of warm water, with a bit of dish soap) is protective or preventive, but Washington State University Extension professor Linda Chalker Scott disputes the efficacy of this method. She says that other methods work better:
"Other treatments have been more successful in powdery mildew control, including horticultural oils, potassium bicarbonate, potassium phosphate, sulfur, milk, and even water sprays. Probably the most field success has been found in combining SBC [sodium bicarbonate] with horticultural oils, including mineral and vegetable oils (see the Fall 2008 MasterGardener magazine). The mixtures are so effective that they've been successful even on serious powdery mildew epidemics."

Date 2017-08-08
Link to this record only (permalink)

Plant Answer Line Question

Keywords: Limonium, Helichrysum, Skimmia, Elaeagnus, Echinops, Alstroemeria, Callicarpa, Calluna, Aster, Lavandula, Achillea, Quercus, Viburnum, Dahlia, Cotoneaster, Acer

My son and his sweetheart are planning a wedding in Seattle (my hometown) this coming September and would love to use seasonal flowers and greenery. I have not lived in the area for many years and am at a loss. Can you give us some suggestions please?


Here are some of the plants which are available in September: Achillea (Yarrow)
Alstroemeria (Peruvian lily)
Callicarpa bodinieri (beautyberry)
Cotoneaster (for foliage)
Elaeagnus (foliage)
Hebe (flowers and foliage)
Helichrysum (straw flower)
Acer (Maple: foliage)
Quercus (Oak: foliage)
Limonium (Statice)
Viburnum tinus

Here is a link to the Washington Park Arboretum web page of seasonal highlights.

A great book on flowers by season is A Year Full of Flowers: Fresh Ideas to Bring Flowers into Your Life Every Day by Jim McCann and Julie McCann Mulligan.

Date 2017-12-08
Link to this record only (permalink)

Plant Answer Line Question

Keywords: Dahlia

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 9/21/2007 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.

Date 2017-04-15
Link to this record only (permalink)

Garden Tip

Keywords: Hemerocallis, Dahlia

For long lasting summer color look to dahlias and daylilies. Both of these perennials come in all colors except blue. Daylilies (Hemerocallis) are especially hardy and carefree. Both need sun and regular water to bloom their best. Dahlias must have excellent drainage or else must be dug and stored for the winter.

Date: 2007-03-05
Link to this record (permalink)

Garden Tip

Keywords: Plant and garden societies, Dahlia

Dahlia resources:

  • The Gardener's Guide to Growing Dahlias by Gareth Rowlands (Timber, 1999) tells all about growing dahlias with lots of color photos.
  • The American Dahlia Society
  • The Puget Sound Dahlia Association holds monthly meetings, maintains a display garden at Volunteer Park in Seattle, and participates in the American Dahlia Society's annual shows. Members receive a newsletter. To join mail $15.00 to Dale Hylton, Treasurer, Puget Sound Dahlia Association, 10820 Oakwood Avenue South, Seattle, WA 98178.

Date: 2007-04-03
Link to this record (permalink)

Garden Tip

Keywords: Plant and garden societies, Dahlia

Should you dig your dahlias or not? Enthusiasts dig them up every autumn when the clocks get turned back. They dust the tubers with fungicide, divide them up and finally store them in a cool place in slightly damp vermiculite. For those of us not so dedicated to our dahlias we can leave them in the ground provided that the soil is well drained and doesn't freeze. A little patch of plastic can help keep the spot dry over winter. For more dahlia growing tips go to the Puget Sound Dahlia Association website

Date: 2007-04-03
Link to this record (permalink)

Didn't find an answer to your question? Ask us directly!

Browse keywords

Search Again:

August 01 2017 12:36:01