SOIL Dahlias have the same needs as a vegetable garden Well-drained soil Lots of Organic Material Cover Crop in Fall, Compost before planting in Spring, Mulch in early Summer

Fertilize Do a soil test. Organic Recipe: ¼ lb. of 10-20-20 per stake, and/or, ½ cup of Organic Mix per stake 2 parts Bone Meal, 2 parts Blood Meal, 1.5 parts K-Mag or Sul-Po-Mag 1 part Greensand, 2 parts Cottonseed Meal, 1 part Kelp Meal Broadcast fertilizer and spade in with the compost 2 weeks before planting. Light fertilization in July Handful of 10-20-20, broadcast around plant, or, application of Bloom booster

DAHLIA VARIETIES Pick colors, shapes and dahlia forms you prefer. Check bloom size and plant height. Get tubers from a Dahlia Society tuber sale. Throw out weak or unhealthy plants, or blooms you don’t like – replace it with another cultivar. Keep track of the cultivar name. Named varieties have more value when divided and sold at a plant sale, or when blooms are displayed at a competitive fair or show.

EFFORT Planting Dahlias like as much sun as you can give them – at least half-day sun. Plant tubers outside when the lilacs bloom in your area, around May 1st. If a stake is needed, put it in the ground before you plant the tuber. Plant the tuber about 4” deep, about 5” from the stake. About 2 feet apart. Don’t water, tubers can rot before the new roots start to take up available moisture. Watch for Slugs!

Growing You can pinch the top of the plant when it has 4-5 sets of leaves. You can limit the number of stalks to one or two on each plant. You can tie the plant to the stake at about 18” and 36”, then tie laterals as needed. You can disbud the side buds to get larger blooms with longer usable stems. Water when the ground is dry. Dahlias will wilt if they don’t get enough water. DEADHEAD! This will lengthen the blooming life of the plant until mid-Fall.

Pests and Diseases Slugs Aphids, especially the black ones Thrips Earwigs Dahlia viruses – short, stunted plants that may have curly leaves or leaves with yellow splotches should be destroyed. Aphids and thrips can spread dahlia virus from one plant to another. Deer – NOT! Dahlias are usually one of the last things a deer will eat in your garden.

End of the Season Don’t wait for the killing frost in this area. Do cut down your plants when the ground gets so cold that there aren’t any new blooms. The plants usually are pretty beat up by rain and wind then anyway. First part of November? Cut plants to the ground, cover with mulch and look for them to come back next April/May, or, Dig and store in a non-freezing location.

American Dahlia Society –  standard classification info and other official stuff, pictures 
Colorado Dahlia Society – culture, pictures, lists of growers and varieties available  
WSU Whatcom County Extension