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--Julie Wennberg, updated 4/14/25
The When, Where & How of Spring Tuber Care
Note that there are many, many differing ideas, opinions and ways of growing dahlias. It will make your head spin with all the thoughts out there. I share below are things I've learned from looking at many sources and seem to make sense for growing in Minnesota, and are fairly straight forward. I hope you find these helpful.
Before Planting - 2 Great Options - Keep Cool or Pot Up
Until the weather is right for planting, either keep your tubers in a cool location of about 50-65 degrees, or for an early start, plant your tubers in pots.
Keeping Cool
If you choose to keep your tubers cool, you may wish to place them loosely in a ziploc or plastic tub not fully closed to help keep them from drying out too much. Or choose a location with modest humidity. Or simply keep an eye on them and spritz a bit of water on them occasionally if it's a dry location.
Potting Up
There are several advantages to starting your dahlias early. The tubers are less likely to rot once they have a good root structure in place, critters are less likely to dig up your tubers, and you will likely see blooms 3 weeks or so earlier.
How to Pot Dahlia Tubers
Mix good quality potting soil with just a bit of water. You want the mix to be just barely damp. Tubers easily rot and do not need much water until there are several leaves. *Miracle Grow is not considered good quality. I've been using Coast of Maine soils found at Sargents in Rochester this year with excellent results. Optional: I also add a bit of worm castings and mycorhyze fungi (can find at Sargents or online).
I prefer to use pots that are fairly tal, about 5.5" as tubers develop significant roots even in a short time. Since I pot 90+ tubers the fact that they fit well next to each other is a huge plus for me so I use square pots. If you have old nursery pots around 5" tall, you can use them. Do be sure to wash them well first.
Fill your pot about 2/3 full. Place your tuber horizontally (or at a very slight angle) with the eye facing up. If the tuber is too long to fit easily, simply cut off part of the end. This is common practice with large tubers and does not affect the plant, in fact, some think it is better to have a bit less tuber. Cover the tuber with another 2-3 inches of soil. Pat gently to get out air pockets.
Label your tuber/pot.
Keep your dahlias around 70 degrees - room temp.
No light is needed until you see sprouts above the soil. Then ideally place under grow lights for about 12-16 hours per day. (Shop lights work well too if placed about 4-6" from the plants or even a high CRI 4,000-5,000K LED bulb or 2). A bright window is ok, but really needs to be supplemented with another light source, ideally with a timer. See note below.
When the weather is nice (60-70), they can also be brought outside into bright indirect light. Start outdoors for an hour or two then slowly increase. Do not keep your dahlias outside if it's at all close to freezing.
Lighting Note -
I scoured Amazon for a few inexpensive grow light ideas. A few links are below. I also added them to my Amazon Garden List.
Tabletop Lights:
Regular A19 Bulbs:
Briignite LED Grow Light Bulbs - Brighten Your Indoor Plants
"Shop" Style Hanging Grow Lights. Many growers use Barrina lights, I have a few in both 2' and 4'. They are easy to use and quite reasonable overall.
The 2' Barrina lights I have:
When to Plant
After the last frost date in your area
AND
when the soil temperature is 60 degrees
Only then is it time to plant your dahlias. Do not rush. Dahlias like to grow in warm conditions. Soil thermometers are inexpensive and helpful.
Where to Plant
Dahlias need a minimum of 6 hours of full sun per day. Be aware that if you plant in a location that has shade for part of the day, your dahlias may grow taller than typical.
Soil - Who knew there was so much going on in the dirt?
Ideal soil for dahlias is well-draining. They do not like to be sitting in water.
They grow best in soil with a pH level between 6.3 and 7.0. If you have clay soil, you may have a high pH (over 7.0). If you have very sandy soil, you are more likely to have a low pH. There are amendments to help get your pH into a level that allows plants to best take in nutrients. A soil test can give you this information.
A soil test can also advise on correct fertilizing and amendments. Minnesota tends to have soils with high phosphorus, which does not deplete quickly from the soil. I already have my soil test back on my Rochester flower bed which has a high pH of 7.1, high phosphorus and high potassium. Only nitrogen and soil acidifier need adding.
Adding organic matter to most soils on a regular basis is helpful as well.
How to Plant
Loosen soil down about 10", especially if it is hard packed. Plant your tuber down about 4" with the eye facing up. If you are planting from a started/potted dahlia, you can take off the lower set of leaves and plant it down the full 4", even if it includes part of the stem. Do not plant too shallow as dahlias are large and need a solid main stem. Donot water until there are leaves on dahlia plants or your tubers may rot. (Unless the soil is bone-dry, then water just a bit).
Don't forget to label your dahlias! Even when only planting a few, it's easy to forget - I know this from experience!! Some dahlias vary in color depending on the weather, soil, and month and it's easy to forget which is which.
Dahlias need support. There are many ways to do this. I have found that you must have sturdy support. Thin bamboo stakes will not be sufficient. Stakes, if used, should be 4-6 feet and very solid. Large, sturdy tomato cages can also be used. It is best to have the support system in place at the time of planting. I've have read and ignored this in the past and have regretted it. When dahlias take off, they grow incredibly quickly and then it is a huge hassle to add the support.
In 2025, I will be using cattle panels as the primary support for my dahlias. See a separate blog post on cattle panels early April. I used cattle panels for several flower varieties in 2024 and found them to be a fabulous option. They are very strong, reusable for many years, easy to store outside all winter, and work really well at supporting flowers.
Spring Dahlia Care
Topping - A MUST DO Step
Topping (also known as pinching plants) helps encourage branching and more blooms. When a dahlia is about 12" tall, snip off the top 3-4" just above a set of leaves.
Fertilizing
Many suggest modest fertilizing from planting until dahlias begin to bloom (roughly late July). Choose organic options whenever possible for the best for your plants and soil.
Watering
Once dahlias have several leaves, deeply water 1-3 times per week, depending on your soil and rainfall. It is highly recommended to water at the base of plant to help prevent disease and pests.
Pest & Disease Prevention
In the early summer, deer and bunnies may chew on your dahlias and set them back a bit. You may wish to protect your young plants for a couple weeks until there are more tasty options for these critters.
I highly suggest using an organic slug prevention product, such as Sluggo Plus, at the time of planting. Slugs love young dahlias.
Dahlias, like most flowers, can have pest and fungal issues. Watch for Powdery Mildew, Japanese Beetles (July-Aug), and other issues. There are many organic and cultural practices that can help keep your plants healthy.
Summertime Bloom Care
Dahlias that I pot in mid April and plant the last week of May typically begin to bloom mid July (depending on variety) and continue until a hard frost.
Deadheading / Pruning / Cutting Flowers
In order for dahlias to truly shine in the garden, you MUST cut off the flowers deeply down the stem. Whether you prefer to cut the flowers as they fade or just after opening to enjoy indoors, you must cut off the blooms. It is preferable to cut long stems, sacrificing side blooms. In general, cut at least the length of your arm from finger tips to elbow! This signals to the plant to send out more branches, to make more flowers. Plus you will have a much better looking plant if you keep it pruned in this way.
I've created an infographic PDF with the basics of Dahlia Spring Care.
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