Dahlia Care Tips

DAHLIA BASIC NEEDS

Full sun, well-draining soil (ideally on the slightly acidic side with a pH of 6.5-7.0). If you live in an area with high mid summer temperatures, afternoon shade is ideal. Partial shade is okay, but you will get fewer blooms here. 

PRE-SPROUTING (OPTIONAL)

Here in Zone 5b, I have found the most success with pre-sprouting my tubers indoors about a month before planting out after our last frost free date. I allow the tubers to do this in bags of peat moss or in plant containers covered in potting soil. Different varieties will take longer to eye up or sprout than others. By pre-sprouting, I've personally had much better rates of in-ground loss due to tuber rot. 

PLANTING

Whether you've already pre-sprouted your tubers or eyes are beginning to form, the planting is similar. On or after your last frost-free date (according to your hardiness zone) is when you should plant out your tuber unless you're okay with running out in inclement weather to protect and cover any young sprouts. Generally if still in ground, they will be fine - tubers do not want to freeze!

You can plant dahlias out in the garden or in dedicated in-ground rows or raised beds. If planted in the garden, ensure the new sprouts will get ample sun so they don't get shaded out by the surrounding plants. Many who plant in the garden choose to start those dahlias early so they already have some good leafy growth.

Dig a hole about 6 inches deep and mix in any amendments your soil may need. Here in Colorado, I mix in compost with our clay soils to loosen/provide nutrients as well as a handful of bone meal which can increase soil microbes and improve root growth from the phosphorous and calcium. Ideally the soil is damp but not wet/saturated.

Lay the tuber horizontally with the eye or sprout facing up. Even if you've pre-sprouted and see little leaves, it's okay to bury them the required 4-6 inches. Fill the hole and place your label where you expect the shoots to come up.

Space the dahlia tubers 12-18" apart. Generally, the larger/taller varieties like to be spaced closer to the 18" apart. It's ideal to provide a stake at this time rather when it's needed since driving a stake in later can damage the new tubers/root system. Most varieties will want to be staked, especially if planted in an area not protected from wind.

WATERING

Do not water until there is leafy growth above ground as dahlia tubers are susceptible to rot. Only in dry soil conditions or heat with no rain should you water once or lightly and then wait for growth. This can take up to 4 weeks for late season bloomers. If I spot a planted tuber spot that still hasn't sprouted above ground around this time, I will gently excavate around the tuber to see if I can spot any growth underground or see if the tuber has rotted.

Young dahlias under 12" tall don't need too much water - start with around once a week. Once they begin to grow past the 9-12" mark, water regularly and deeply. Drip irrigation is ideal for this. Overhead watering is okay during the leafy stage, but can lead to powdery mildew later in the season or cause damage to big open blooms. The best watering times are early morning or evening so the water can be absorbed by the roots without evaporating.

In the heat of the summer (around July/August for me), dahlias may require more water. Keep an eye out for wilting or brown/crisp-edged foliage and reduced flowering.

PINCHING

Once plants reach 8-12" and have developed 4 sets of leaves, snip (aka "pinch") the center growth point above the 4th set of leaves. This will encourage lower basal branching which will create a bushier dahlia plant with increased bloom production. 

PEST PROTECTION

Slugs love the new leafy growth of dahlias. Sluggo Plus is an organic treatment you can sprinkle on the soil at the time of planting (and if needed throughout the season) that also works on earwigs which will eat your blooms later in the season.

Japanese beetles have recently made their way to our area! I do not recommend the traps, but the old fashioned method of knocking them off your dahlias into a cup of soapy water (omit soap if you feed them to your chickens like I do!) in the morning when they're still a bit slow. I also use 12" organza bags tied around a bud that's about to unfurl to allow the flower to bloom inside the bag unscathed if I'm needing them for an event or arrangement.