Dahlias have been a beloved flower for centuries. Interestingly, people used to eat tubers, similar to how we eat potatoes. Can you imagine doing that today? I think dahlias are way too precious for consumption. I'd much rather admire them in the garden or in a bouquet than on my plate!
The guide covers everything you need to know, from soil preparation to winter storage, growing in containers, and taking cuttings. It's organized by season and includes lots of valuable information.
- When to plant
- Soil Preparation
- Where to plant
- How to plant
- Cutting Flowers
- Pest and Diseases
FALL & WINTER:
- Growing dahlias in containers
- Starting dahlias indoors
- Taking dahlia cuttings
- Growing dahlias from seed
When to plant your dahlias:
Dahlias should be planted in the spring when the soil temperature is 60F degrees or warmer (the same time your gardening friends are planting out tomatoes).
For example, in Washington State, we begin planting at the end of April. Folks in California, Hawaii, and Florida can start planting in March. And if you live further north, your planting season may not begin until May.
1. Add a 4-inch layer of compost to your planting beds, then till or work it in with a pitchfork to mix the compost and native soil together.
Compost breaks up clay soil and improves drainage, while making sandy soil hold more moisture. It also adds nutrients back into your soil that were depleted from last year's garden. If you have heavier soil, you can add sand, peat moss, or steer or cow manure to lighten and loosen the soil texture.
We live in a very wet climate, and our fields become puddles and clay mud. Everything we plant has to go into a raised bed, which we create by raising the soil, not necessarily by building raised beds out of wood. For our new dahlia field, we raised the soil level by laying down 3 inches of sand, then 4 inches of compost. You can read more about that here.
Where to plant your dahlias:
Pick a well-draining, sunny spot. Good drainage is a must for dahlias since the tubers are prone to rotting. And the more sun, the more blooms you'll get! If you plant in an area that doesn’t receive enough sunlight, your plants will be taller and thinner with fewer blooms.
In hot climates, morning sun with some afternoon shade is ideal.
How to plant your dahlias:
1. Make sure that your soil is slightly moist but not soggy.
2. Dig a 4-6 inch deep hole and place the clump or tuber into the hole, laying the tuber horizontally with the eye facing up.
3. Add a stake for support right away so you don't disturb the roots later. Then cover the tuber completely with soil.
4. If you are planting in the landscape, give each plant 3 feet on each side to spread out. If growing in rows, place the tubers 12 inches apart. An average 4-foot-tall dahlia plant will be 18 inches in diameter.
5. If your soil is moist at the time of planting, DO NOT WATER! Unless you are growing in containers or live in a hot climate, do not water until the first sprouts appear. In hot climates, we recommend watering once a week after planting until the sprouts appear.
6. Apply slug and snail bait right away around your planted area, as they love to munch on dahlias' tender leaves.
Many areas of the United States receive enough rain for the dahlia tubers until the sprouts appear. Do not water your dahlias after planting unless your soil is bone dry and no rain is expected. If growing in a hot climate, water once a week until sprouts appear.
Once the sprouts appear and the plants start growing above ground, they will require deep watering 3-4 times per week for 45-60 minutes by sprinkler or soaker hose, and even longer in the heat of the summer. Deep watering means that the water will need to reach down to the planted tuber roots.
For container growing, you will need to water 1-2 times per day until the water runs out of the bottom of the container.
Dahlias require deep watering to reach the roots once there are green sprouts above ground level. Water your plants 3-4 times per week for 60 minutes using a sprinkler or soaker hose, and even longer during the heat of the summer. Deep watering means that the water will need to reach down to the planted tuber roots.
Keep in mind that your soil type and climate will determine the amount you will need to water.
Cutting flowers promotes new growth on the plant, so don't hesitate to bring those dahlias inside! If you want to keep the flowers in the landscape, be sure to deadhead them as soon as they start to wilt.
The best time to cut flowers is in the cool morning hours or in the evening. Cut deep into the plant to promote the growth of new, long stems. You might cut a few buds that haven't opened yet, that's ok. There will be more!
Bring a bucket of fresh, cool water with you and place the stems immediately into the water.
Vase Life Trick
Dahlia flowers are not known for their long vase life. To increase their longevity, place the cut stems in 3 inches of very hot water (cooled down slightly from boiling temperature) and keep them there until the water has cooled. This will help your dahlias last for 4-6 days.
Then, re-cut the stems to the desired length to fit your vase and place them in cool water with flower food.
Change the water in your vase daily and keep any leaves or dropped petals out of the water.
Insects & Diseases
Dahlias are loved by many insects and pests. Fun fact: white dahlia flowers attract especially many insects!
The healthier you keep your plants, the less susceptible they’ll be to insect damage. Keep in mind that whatever you decide to spray will likely come in contact with you, your children, and pets. We would rather have a few bug-eaten flowers but know that our garden is a safe place for our family.
The best way to avoid an infestation is early detection and prevention.
Slugs and snails: Apply Sluggo (an organic option that’s safe around kids and pets) at the time of planting or as soon as the first sprouts emerge.
Earwigs: Use Sluggo Plus to deter these munching bugs. It’s also an organic product that’s safe around your family.
Aphids: Spray your plants with organic insecticidal soap. Arbico Organics has lots of great options.
Organza bags: If you are growing your flowers for cutting, you can place organza bags over individual buds while you wait for the flowers to open up.
Beneficial insects: Lacewings, Ladybugs, Hoverflies, Syrphid flies, and Parasitic wasps help to destroy harmful insects such as aphids, thrips, and mites.
Plant lots of plants and flowers that attract these insects to help combat the pests naturally.
Ladybugs are great for aphid control! Make sure that there is a water source for them (like an irrigation system, a wet paper towel, or a very shallow dish).
Just like other flowers dahlias are prone to bacterial, fungal and viral diseases.
Crown Gall is a bacterial disease identified by bumpy, cauliflower-like growth around the neck of the tuber. Remove and throw away any infected plants and tuber in the trash! Unfortunately, there is no treatment for this disease. Do not throw in the compost to prevent spreading to any other plants.
Viral diseases can be identified by yellow streaking or spots on the leaves and veins and stunned plant growth. Viruses live both in the plant and the tuber. It’s best to dispose of the plants and tubers. Unfortunately, there is no known treatment. Do not throw in the compost to prevent spreading to other plants.
Fungal diseases include powdery mildew, botrytis, and leaf spot. They are spread by air-borne spores. They commonly show up during damp and humid weather conditions. The best way to prevent them is by giving plants good airflow and by keeping the garden clean of any diseased debris. You can use a fungicide spray if the issue has gotten out of hand.
Hand weed your dahlia beds regularly to give your plants all the space and nutrients they need. Do not spray any weed killer as that will kill your plants as well. Weed fabric is also not advisable if you plan to dig up your tubers at the end of the season.
FALL & WINTER
Dahlias are susceptible to rot and freezing. If the freeze reaches a depth of 4-6 inches or more, do not leave your dahlias in the ground. In milder climates, including the Pacific Northwest, you may try leaving the dahlias in the ground, but make sure you have good drainage in the planted area to avoid rotting from winter rains.
Approximately two weeks after your first frost, cut the plant's stalks down to ground level and cover your dahlia beds with plastic and straw, leaves, or any other mulch for insulation. Containers should be moved to a location where they will not freeze, such as a garage.
If you are growing in a hot climate (like Florida, So Cal, Hawaii, etc.), you should dig up your dahlias and store them in a cool place (40-50°F) for 90 days to give them some rest between growing seasons.
Start digging your dahlias 2 weeks after the first frost or approximately 120 days after planting. If you dig too early, you may risk that the tubers will still be in a "green stage" and will not have hardened off enough for winter storage. Their skins need to thicken like potatoes.
Use a pitchfork to carefully lift the tubers out of the ground without breaking their necks. If you don't plan on dividing the tubers until spring, store them unwashed.
Dividing dahlias is the most common way to increase your stock. A single tuber can produce 3 to 10 tubers in just half a year. You can divide dahlia clumps in the fall or in the spring using needle-nose snips, pruners, or a linoleum knife.
- Wash the tubers thoroughly with a strong stream of water, allowing the dirt to fall away from the necks of the tubers.
- Allow the clumps to dry for 1-2 days before dividing. Don't let them sit there for too long! Once they are clean, they will start to dry out and shrivel.
- A viable tuber must have an eye to grow. The eyes will be located on the center stalk. Eyes are sometimes difficult to see, and not all tubers will have an eye. If you have a very large clump, start by splitting the clump in half to make it more manageable. If you're having a hard time finding the eyes, you can continue splitting the clump into quarters. Each section should easily contain at least 1 eye.
- Remove any broken and damaged tubers and trim off the hair roots. Then start at the top of the clump and "cut out" individual tubers by slicing through the crown, making sure to include the eye.
- Make sure that each tuber has a strong neck. If the neck breaks in the process, discard the tuber as it will not grow.
- Different varieties will produce different tuber sizes and shapes. Some varieties will have very obvious eyes while others will be in a different stage of dormancy and more difficult to see. If you're unsure whether you have found an eye, store the tuber over the winter and wait until spring when the tubers start to wake up and sprout. If there is an eye, you will see it starting to swell and color up.
- Make sure to trim any excess from the crown and around the neck as that is likely to rot and shrivel, and with it, the entire tuber.
TIP: I like to dip the cut ends of the tubers into powdered cinnamon. It acts as a fungicide and helps prevent mold growth.
Dahlias are extremely cold-sensitive and need to be stored in a controlled environment. The ideal temperature should be between 40 and 50 degrees Fahrenheit.
Proper humidity is also crucial. If it's too wet, they will rot, and if the tubers are stored too dry, they will shrivel up. The ideal humidity is 93-95%. To achieve this, you can store the tubers in plastic containers with vermiculite, peat moss, or sawdust/shavings. You can drill a few small holes into the bins to allow moisture to escape. If storing in crates or cardboard boxes which allow more air circulation, dampen the peat moss or sawdust lightly.
Use the lasagna method to layer your tubers with the chosen medium and don't allow the tubers to touch. If they touch and one molds or rots, it will likely spread to the other tubers.
Check on your tubers monthly! Remove any molding or rotting tubers to prevent it from spreading to others. If you notice shriveling, moisten down your medium with a little water to prevent further dehydration. If you notice moisture building up, open up the storage containers to vent.
A whole unwashed dahlia clump can be placed into big plastic bins. If you use a cardboard box, line it with a plastic bag to prevent the tubers from drying out. Note that clumps take up quite a big more space than tubers.
GROWING DAHLIAS IN CONTAINERS AND RAISED BEDS
Tall dahlias do best in the ground but some shorter varieties will do well in containers. When choosing a container make sure that there’s enough room for tuber growth. I’ve pulled clumps out that were 2ft in diameter!
The container should be at least 15” x 15” across and deep enough to allow proper growth and drainage. You will be able to plant one dahlia in a container of that size. Raised beds should be at least 12” deep for proper drainage and growth.
Fill the container with ⅔ garden soil/topsoil or compost and ⅓ with potting soil. Potting soil alone is too porous and will dry out too quickly. Last year, we grew all of our dahlias in raised beds filled with compost and they thrived!
The biggest difference between growing in containers vs. in-ground is watering. Containers dry out much more quickly, so you will need to start watering right after planting. Make sure to keep the soil slightly damp, not soaking wet. Overwatering can cause tuber rot. After the dahlias have sprouted, water daily as you would with any other container plant.
STARTING DAHLIAS EARLY INDOORS
Dahlias should not be planted outside until the soil is 60°F warm. If you live in an area with a short growing season or want to have earlier blooms, pot up your dahlias and give them a head start indoors.
Start your dahlias no more than 6 weeks before transplanting them outside. Count 6 weeks back from your last frost date. Starting too early may cause weak and spindly plants.
Fill individual pots or a tray with plain potting soil, peat moss, or a combination of the two. If planting in a tray, lay the tubers horizontally and cover them up. They can be buried 1-4 inches deep. If planting in individual pots, place the tuber with the eye facing up. If you plan on taking cuttings, leave the neck exposed.
Keep the tubers in an area that’s 65-70°F warm. Once the sprouts emerge, give them 8+ more hours of light.
The soil should be kept slightly damp while you wait for the tubers to start sprouting. Water regularly once the plants are showing signs of growth. Make sure you have proper drainage if you planted in a tray or repot into individual containers with drainage holes after the tubers have sprouted.
Harden off for a week by planting the pots outside during the day and bring them back inside at night. Transplant outside after the soil has warmed and all danger of frost has passed. You can pinch the central stem back before planting outside to encourage stronger stems and branching.
That’s it! Plants should be about 12” tall when you plant them outdoors.
TAKING DAHLIA CUTTINGS
Taking cuttings is the cheapest way to multiply your favorite plants without buying more tubers. And it's easier than you think!
Cuttings may be taken in January and February from tubers you grew the previous year.
Plant your tubers into individual pots with drainage holes, leaving at least 1” of the neck pointing out of the soil so that the eyes are easily accessible.
Place the potted tubers in a 65-70F warm spot to wake up. Don’t use a heat mat as that might rot the tubers from below. It should take 2-3 weeks for the sprouts to begin to emerge depending on the variety
If you are a visual learner you can watch How to Video here.
GROWING DAHLIAS FROM SEED
Growing dahlias from seed is a wonderful way to discover your own new variety! Dahlias grown from seed do not grow true to the mother plant, but each seed produces a new dahlia. This is how dahlias are bred. And yes, dahlias that are grown from seed produce tubers too!
Start your seed indoors 4-6 weeks before last frost. Fill pots or a tray with potting soil. Place the seeds in the pots and cover up with ¼” of soil. Keep the soil moist but avoid overwatering. Don’t let the seedlings get root-bound (they start growing little tubers right away). Move to larger pots when necessary. Transplant outside after all danger of frost has passed.
In areas with a long growing season you may plant your seeds directly into the garden. Make sure that the soil is at least 60F and keep the planted area moist.
Days to germination
Seeds should emerge in 3-5 days if temperatures are kept between 65-70°F.
HARVESTING SEED (coming soon!)