Water 2 to 3 times a week. Dahlias have large deep roots so the water they get needs to get down to where the roots are, not just on the foliage like bedding plants.
Fertilize with a low nitrogen fertilizer like 10-20-20 or a bloom food. Granular, liquid, or foliar fertilizer is fine and the numbers are not all that important, just low nitrogen as you will get more blooms and stronger stems. If using a granular fertilizer do not get it on the plant or you will burn the leaves, put it on the ground in a circle around the bush, a 1/4 to 1/2 cup is plenty.
If bottom leaves are turning yellow and brown, you could have spider mites. They are bad when the weather is hot and dry. Good healthy growth helps keep them at bay. Check the underside of the yellow leaves. If there is a webbing, you have them. Strip or clip all yellow and brown leaves off the stock, if they have already consumed the whole plant, cut it down. The leaves and the plant will recover, but by taking off the infected leaves, you have gotten rid of all those mites and their eggs. Put these leaves into your regular garbage so they don't contaminate your compost. Then get a good miticide from the garden store, one that gets 2 spotted spider mites. If you have the red mites, they are actually eating the spotted ones which are white and harder to see, but are the ones sucking the chlorophyll out of the plants, hence the yellow appearance.
If you have holes in your flower petals you have probably earwigs or green cucumber beetles ( they are not green ladybugs!). You can pinch all you find or sue an insecticide, check the labels. Note: earwigs eat the new sprouts in the spring just like slugs, so if you take care of them early, you won't have them in the summer jumping out at you from the dahlia flowers :)
Cut dahlias when you can in the cool of the morning take a bucket or can of water with you to put the blooms into right after cutting. Cut dahlias before the bloom opens all the way as you will be cutting a younger bloom and they will last longer and continue to open. Cut Collarette and other open centered type dahlias when the center it still has a waxy feel to the center. We use a hot water treatment on the blooms when we can, this also helps them to last longer.
Hot water treatment: Bring the dahlia blooms in and put the stems into 2 inches of 160 degree water. A coffee pot of hot water is easy and cool with a little tepid tap water. Take the temp a few times and you will get the feel of it with your finger after a while. Add some flower fresh or tsp of sugar to the hot water for food. Let set in this water for 15 minutes at least. You can then cut them to whatever length you want without treating again.
Keep the dahlias dead headed. That means to keep the old blooms cut or picked off. The plants will keep producing more blooms and won't start to set seed. Cut long stems also, down past several buds, this will help the plant to be more bushy, give you longer stems to work with and actually give you more blooms later in the season. Dahlias will bloom until the first hard frost in mid to late October depending where you are. Sometimes they go to Thanksgiving if protected.
Dahlias work well in vases with just water, vases with oasis and as boutonnieres or corsages just like other flowers. So cut them, cut them, cut them and enjoy!!
Frey's Dahlias 12054 Brick Rd. SE, Turner, OR 97392
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