Dahlia

Year-round

Dahlia pinnata is a tuberous, tender perennial and the national flower of Mexico. It is not native to Oregon, but it is a popular garden plant in the Portland area if you treat the tubers like the frost sensitive bulbs they are.

• Give it full sun, at least six to eight hours of direct light a day, or the stems get leggy and flowering drops off.
• It wants rich, well drained soil with steady moisture through the growing season. Standing water rots the tubers, which matters in Portland's wet shoulder seasons.
• This is a heavy feeder. Plan on regular fertilizing from planting through late summer to keep the blooms coming.
• Watch for slugs and snails early in the season since they chew on young shoots at night, a common problem in the Willamette Valley's damp spring soil.
• Dahlia pinnata comes from Mexico, not the Pacific Northwest, so it will not feed the specialized native bees or host butterflies the way an Oregon native plant would. It is also listed by the ASPCA as toxic to dogs, cats, and horses, with mild gastrointestinal signs and mild dermatitis reported, so keep pets from chewing on the tubers or foliage.

Fall

Fall brings the first frost that ends the bloom season and signals when to dig and store dahlia tubers for winter.

• Dahlias keep blooming right up until frost blackens the foliage. Timing varies a lot across the Portland metro area depending on neighborhood and elevation, so watch your own yard rather than a single citywide date.
• Once frost kills the foliage, cut the stalks down to a short stub to tidy the bed and mark that it is time to dig.
• Dig the tubers, rinse off the soil, and let them air dry for about a day in a sheltered spot before packing them away. Damp tubers rot in storage.
• Divide the tuber clump into individual pieces, each with an eye, while you are digging. It is easier to see the eyes right after digging than after the tubers dry out.
• Cutting back the blackened stalks promptly after frost also keeps the bed looking intentional instead of collapsed and mushy.

Cut the frost blackened stalks back promptly rather than leaving collapsed foliage in the front bed over the following weeks.

Winter

Winter means keeping the dug dahlia tubers cool, dry, and frost free until it is time to plant them out again in spring.

• Store dug tubers in a cool, frost free spot such as a garage, layered in dry material like sawdust, vermiculite, or newspaper so they are not touching each other.
• Check on stored tubers occasionally through winter. Shriveled ones can take a light misting, while any that go soft or moldy should be tossed out so they do not spread rot to the rest of the batch.
• Dahlias generally do not make it through a real winter outdoors, which is why cooperative extension guidance across cooler and colder climates is to dig and store them rather than leave them in the ground.
• If you do choose to leave tubers in the ground under a thick layer of mulch, know it is the riskier option here and some losses are normal in a harder winter.
• Hold new tubers or divided pieces back from replanting until the frost danger has passed the following spring.

A neatly mulched, empty bed after tubers are dug reads as intentional and tidy rather than neglected over the winter months.

Spring

Spring in the Portland area is tuber planting season. Wait for the soil to warm and the frost risk to pass before you put dahlia tubers in the ground.

• Plant tubers outdoors in mid to late spring once the soil is consistently around 60 degrees, which in the Portland area generally lands between late April and the end of May.
• Lay each tuber on its side about 4 to 6 inches deep with the eye, the small bud on the old stem end, pointing up.
• Hold off watering right after planting. Wait until a sprout breaks the surface, since a tuber sitting in cold wet spring soil before it sprouts is prone to rot.
• Start slug and snail control around two weeks after planting or as soon as the first sprouts appear. They will chew new growth down to nothing overnight if you wait too long.
• Begin feeding once shoots are up and keep it going through the season, since dahlias are heavy feeders.

Summer

Summer is bloom season for dahlias around Portland. Regular water, staking, and deadheading keep the plants upright and flowering through the warm months.

• Water deeply about once a week during dry stretches rather than light daily sprinkles. Dahlias are shallow rooted and dry out fast once summer rain lets up.
• Mulch around the base to hold moisture and cut down on watering frequency.
• Stake taller varieties at planting time with a stake driven a foot or so into the ground, then tie the plant to it as it grows. Doing this early avoids stabbing a tuber later in the season.
• Feed monthly through the summer with a balanced fertilizer, then taper off as late summer approaches so the plant can start slowing down for fall.
• Pinch off spent blooms regularly to keep new flowers coming and keep the bed tidy. Also watch for aphids and keep tools clean, since sap sucking insects can spread dahlia mosaic virus, which has no cure once a plant is infected.

Regular pinching of spent blooms keeps the front bed looking tidy and full of color instead of cluttered with brown, faded flowers.