Dahlia Growing Guide

A NOTE FROM FARMER MEREDITH

Dahlias grow from magical, alien looking tubers don't seem that special to the naked eye. However, they are a hidden gem plant, producing late season beauty from a rather unattractive form. Dahlias must be planted in warm, well-drained soils at springtime. We generally plant them at the same time we plant our heat-loving garden vegetables and potatoes.

 Have a wonderful time growing beauty from our sustainable hands to yours! The Barnswallow Flock and myself can’t wait for the next Dahlia Tuber Sale January 2026!

LOCATION Dahlias need a sunny location to thrive. An area that receives at least 8 hours of direct sunlight is best. Hot climates are an exception to this, as dahlias should be planted where they will receive morning sunlight, but no hot afternoon sun. No matter what climate you have, do not plant in areas that are shaded all day. If you do plant in shaded areas, the plants will be tall and spindly; blooms will be sparse and weak headed.

SOIL PREPARATION Prepare your beds a few weeks before planting. Beds should be well drained and in an open, sunny location. You can use aged steer manure, compost or a 10-10-10 granular fertilizer spread evenly over the bed, then till it in. In the past two years at Barnswallow, we only use peat moss and a special bio-char recipe made by David Yoder in Drakesville, Iowa. If you have a heavier soil, work in sand or peat moss to lighten the texture of the soil for better drainage. Bone meal can also be used and put in each hole prior to tuber planting.

PLANTING Ground should be warm and well drained at planting time. Lay the tuber horizontally at a planting depth of 5” to 6” inches and about 18” to 24” inches apart, then cover with soil. For a quicker start, plant tubers 3” to 4” inches deep and then “hill up” soil after plants are up and growing. Hilling to a depth of about 8” inches will help support the plant. Stakes should be put in at the time of planting to avoid damaging the tubers at a later date. Do not water tubers after planting. The spring soil has enough moisture to promote proper growth and the spring rains will provide enough water. In case of an extremely wet spring, you may delay planting until dryer weather prevails. In case of an extremely dry spring, water in tubers a couple days after planting if rain is not in the foreseeable future. Planting times are generally mid-May for zone 5. If you are planting in deck containers or pots we recommend using garden soil or a mixture of 2 parts garden soil and 1 part potting soil. Dahlias planted in 100% potting soil will dry out too often causing poor bud formation. Over watering to keep soil damp may result in rotting tubers in the pot. After trial testing pot growing over many years, we have not found it to be 100% successful, so be aware that dahlias enjoy growing in natural soil vs. in planters or pots.

WATERING Most areas have enough rains to fill water needs at first. We do not recommend watering until sprouts are at least 4” – 6” inches above the ground. This gives the tubers a chance to form roots, which can absorb water. Watering too early will increase the chance of rotting tubers. After plants are established, a deep watering once or twice per week is necessary during warmer, dryer weather or in warmer climates. We now overhead irrigate our tubers in July-September after harvesting for better results in tuber production. In drought, we water weekly.

FERTILIZING One of the largest mistakes made with dahlias is over feeding them. Do not over fertilize, and stop fertilizing a couple of months before the plants go dormant. Then, the tubers have time to harden-off and remain strong in winter storage. We recommend a balanced fertilizer with equal parts Nitrogen, Potassium and Phosphorous fertilizer such as 5-5-5 or 10-10-10. These can be found as vegetable fertilizers at your local garden centers. They should be applied within 30 days of planting and can be repeated at about 6 week intervals, but we apply it only once in our field. If using a stronger (higher number) fertilizer, use less. Stop feeding plants by the end of August. Potassium and phosphorous break down slowly and will become available to your plants during peak blooming. Always check your fertilizer labels for nitrogen amounts when buying fertilizers for dahlias. Nitrogen percentage is always the first number listed on the package, and should not exceed the other two numbers. Consider using compost tea, earthworm casting tea or bio-char, which have all been successful for us and are great for soil health.

WEED CONTROL Recently products have become available that claim to control weeds without affecting the growth of flowers. It is our experience that these products do have a negative impact on dahlias. Do not use commercial weed killers where you are going to plant dahlias. Old-fashioned, hand weeding is still the best remedy. Some growers are putting their dahlia tubers in landscape fabric with good success. Using naturally harvested grains before seeding out like cereal rye and even grass clippings can also be a successful way of reducing weeds and improving soil replenishment for the next year.

PESTS Snails and slugs eat the tender shoots as they try to break through the ground in spring. Use snail bait to prevent these pests from eating up your tender plants. To control other pests, we recommend spraying Neem Oil and dusting with diatomaceous earth or rye flour. We usually put down slug bait at least once a month and sprinkle around each plant.

POWDERY MILDEW Dahlias are also susceptible to powdery mildew; overhead watering does not cause it. It is caused by weather conditions. Dahlias can be sprayed in July and August for powdery mildew. We don’t have issues with mildew due to our open-air environment. Plant Therapy and other products work well. The key is to spray before the problem appears, as it can be difficult to eradicate once it gets established. If your planting area is enclosed with minimal air movement, powdery mildew is very possible to experience.

TOPPING/PINCHING To promote shorter, bushier plants with better stems for cutting, we advise pinching or cutting out the center shoot when the plant is about 12 inches tall. There should be 2-3 leaf sets below your pinching mark. In the past two years we have pinched when the plant is 7”-10”, which significantly helped the plant in horizontal growth. If you do not stake your dahlias, this practice will help keep your plants shorter while producing more blooms.

CUT FLOWERS Cut blooms in the early AM when cool. After harvesting, place cut stems in a clean container with clean water and flower food. Cut dahlias are susceptible to bacteria and will last longer if their water is changed and stems are trimmed. Be sure to remove the old, spent blooms from your plants. Removing old blooms (dead heading) will keep the plant strong and vigorous late into the season. Also, it encourages new blooms and better bloom color throughout the season. Deep cutting at every harvest is encouraged, as dahlias fight hard for new growth.

DIGGING After the first killing frost where the plants will turn black, cut down the dahlia plant leaving 6 inches of stock above the ground. Dig dahlias about 2-3 weeks after that initial hard frost. This allows time for the tubers to “cure” under the ground before you dig. Cured tubers shrivel less in storage. Tubers dug before a frost may be “green” and shrivel badly or rot in winter storage. If your area has not had a hard frost by mid-November, it is okay to dig them out. The cool nights will slow down the plant growth and tubers will “cure.” While harvesting, gently lift the tubers from the soil with a spade or pitchfork being VERY CAREFUL not to break the tuber necks. Wash dirt from the roots with a garden hose and allow to air dry (not in direct sun) in a protected area only as long as it takes the skin surfaces to dry (usually for about one week). We no longer wash our tubers unless we have to harvest in wet, muddy conditions. We feel the extra exposure to water can impact our dry winter storage methods.

DIVIDING You can divide tubers in the fall or in the spring. If you have never divided tubers before, spring is best as it is easier to see the eyes when the new sprouts form. If eyes are difficult to see, we suggest dividing the clump in half or quarters. Not all tubers will have an eye. Cut surfaces should be allowed to dry thoroughly overnight before storing (in fall), or planting (in spring). Barnswallow now dips each cutting wound in cinnamon. This greatly reduced the possibility of mold and bacteria forming. Tuber size does not affect plant growth. The smallest tuber will produce a full sized plant if it has an eye. Different dahlias produce varying size and shapes of tubers. Barnswallow now divides all its tubers right after harvest so we can gauge inventory for online sales. However, we DO NOT a harvest division unless you have several years of experience.

STORAGE There are many ways to store tubers. We now pack our divided tubers in vermiculite in plastic totes and cardboard boxes with air holes. This is due to huge temperature shifts in Iowa that come with large changes in humidity. Check and rotate them regularly to ensure they are not molding or drying out. Never store roots in sealed plastic bags, as they will rot due to lack of air circulation. A cool, dry area is preferred to store tubers over winter (temp. of 40 – 50 degrees is ideal). We run a humidifier and fan all winter that blows cool, wet air over the dahlias. Beware if a cool area may be susceptible to freezing. If the dahlia tubers are exposed to temperatures below 32 degrees, they will rot and die. Bulbs such as daffodils, tulips or lilies can survive below 32 degrees. Dahlia tubers cannot. Check the tubers several times during the winter months to see how they are doing. Remember, there are no absolute rules to storing dahlias. Experiment to find a variation of the above method that works best for your zone, climate and storage conditions.