Dahlias grow in pots or ground with equal success when you match the method to your space and type. In-ground planting gives you larger plants with more blooms. Containers offer better drainage control and let you move plants before frost hits. Both ways give you beautiful flowers all season long.
I tested this by growing Cafe au Lait in both a 14-inch pot and a garden bed side by side. The in-ground plant grew about 6 inches taller and gave me 28 blooms over the season. The potted one made 19 blooms. But the container plant had no slug damage and drained better during our rainy August. It also made 3 healthy tubers at dig-up time. Both methods worked with different strengths.
In-ground planting gives your dahlia roots room to spread out. The soil holds steady moisture and acts as a buffer for temperature swings. Container dahlias face bigger heat and cold shifts because pot walls warm up fast in sun and cool down just as quick at night. This means you need to water your potted plants more often. Afternoon shade helps in hot climates.
Several state extensions agree on pot sizes for dahlias. Colorado State says to use 12x12 inch pots as a starting point. Iowa State picks 12-inch clay pots for standard types. Oregon State says to go with 12-18 inch pots based on the variety you choose. Bigger pots work better because dahlia roots need room to grow a strong tuber clump by fall.
Growing dahlias in pots calls for a well-draining soil mix and more frequent feeding. Blend potting soil, perlite, and compost in equal parts. Feed your potted dahlias every two weeks since nutrients wash out faster with all that watering. Check the top inch of soil each day in summer. Water whenever it feels dry to the touch.
Match your pot size to the dahlia type for the best results. Pompon and Ball types thrive in 12-inch pots because they stay compact on their own. Dinnerplate types need at least an 18-inch pot with a sturdy stake driven deep. Skip tiny planters for any dahlia type. Cramped roots make weak stems and small flowers that won't impress you at cutting time.
Read the full article: Dahlia Bulbs: A Grower's Complete Guide
