How to Overwinter Dahlias in 12 Easy Steps
Dahlias are summer and fall blooming flowers, but in colder climates, their tubers will need to be pulled to overwinter where the freezing temps can't kill them off. Overwintering isn't necessarily hard, but it takes some planning. In this article, certified master gardener Liz Jaros outlines the proper steps to overwinter your dahlias this season if you live in a cooler climate.
A delight to the senses from mid summer to late fall, dahlias range in height from 10 inches to 7 feet. Dahlias bloom in all shades of red, pink, white, yellow, and orange. With 42 species and thousands of cultivars, dahlias can be tall and leggy or short and bushy. Their blooms can be as small as ping pong balls or as large as frisbees.
They can be found growing in gardens everywhere from the English countryside to the urban rooftop. They’ll bloom repeatedly for months if they’re well cared for. But at the end of the growing season, when the cold winds return and the days get shorter, your dahlias will let you know that it’s time for some much needed shut-eye.
Browning leaves, dried out blooms, and the absence of new buds are all indications that your dahlias are entering dormancy. In zones 8-10, where they are perennial and can remain in the ground during winter, dahlias will need to be cut back and covered up. In zones 3-7, this means it’s time to dig them up and store them for winter. Unfortunately in cooler climates, they will not survive in freezing temperatures.
Overwintering dahlias can be a little tricky. But the genus is expensive and the flowers are spectacular, so we promise it’ll be worth the effort. What follows is a list of steps you can take to help get them ready for dormancy, ensure they sleep soundly and give them the best odds for blooming again next year.
- 1 Start When Leaves are Brown
- 2 Cut Foliage to Two Inches Above Soil
- 3 In Zones 8-10, Cover With Mulch
- 4 In Zones 3-7, Remove From Ground
- 5 Clean Them Off & Dry Them Out
- 6 Find a Proper Storage Location
- 7 Store The Whole Root Clump
- 8 Divide Before Storage if Needed
- 9 Label Them For Storage
- 10 Pack Loosely
- 11 Monitor Them Regularly
- 12 Replant in Late Spring
- 13 Final Thoughts
Start When Leaves are Brown
Your dahlias will stop blooming sometime in mid-fall, depending on the variety. Since there will be nothing to look at but fading leaves and stems, you may be tempted to cut them down or dig them up at this point/ But for best results, you must wait a little longer.
Dahlias will use the season’s waning days to store energy in their tuberous roots. This process is essential to proper dormancy. It also has a big impact on the quality of next year’s blooms.
In warmer climates (zones 8-11), their leaves will turn brown in late fall when their natural bloom cycle has ended. When grown in colder climates (zones 3-7), dahlias will turn brown shortly after a hard freeze has hit your area. In all areas, completely brown leaves are a signal that it’s time to prepare for winter.
Cut Foliage to Two Inches Above Soil
At this point, both dahlias remaining in the ground and those being dug up for storage will need their foliage cut back to just above ground level. Do not cut leaves off until you are ready to begin the overwintering process, as dahlia stems are hollow and will collect water if they’re exposed. This can lead to root rot and other diseases that may thwart your overwintering plan.
Pro Tip: Before you begin cutting, use plant tags to make note of which variety is which. You’ll thank yourself for doing so later in the process, when you’re dividing, labeling, and storing a whole bunch of tubers that all look pretty much the same.
Using a sharp, sterile, bypass pruner or shears, make a clean cut through stems and leaves at a point roughly 2 inches above ground. Compost discarded foliage if it’s healthy and disease free.
In Zones 8-10, Cover With Mulch
In warmer climates, they can be overwintered in the ground. They do not need to be dug up and brought inside. However, since a dahlia’s tuberous root system is very large and they can be propagated quite easily, you might want to take a peak and see if they need division. If they’ve been in the ground for more than a couple of years, they probably do.
If you determine it’s time to break them up, read ahead for excavation and division instructions, then replant immediately.
However, if you think they’re fine as they are for another season (or if you’ve divided and replanted), cover dahlia beds with a 3 inch layer of hardwood mulch. This will help insulate the ground against rogue cold snaps as well as encourage good drainage.
In Zones 3-7, Remove From Ground
Tuberous roots can expand to two to three times their original width throughout the growing season. Before you begin digging, expect underground clumps to be quite large and give them a wide berth.
Using a pitchfork or a careful shovel, begin digging about 12 inches away from your dahlia’s stems. Proceed in a circular path around each plant and gently pop up root clumps. Tissue will be brittle, so take your time and use extra care.
Clean Them Off & Dry Them Out
Set each root clump on a tarp or hardscape and leave them outside to dry for a couple of days. If frost is expected in your area, dry them in a garage or somewhere that will not freeze.
At this point, use a dry towel or gloved fingers to remove as much of the dirt as possible without causing any nicks or bruises to their roots. If a little soil clings to them, don’t worry about it.
Find a Proper Storage Location
Once they are packed up for winter, your dahlias will need to be held in a dark environment where the temperature stays consistently between 40 and 50 degrees and the humidity is moderate. For many of us, this is the most challenging step. It’s also one of the most important.
Depending on your hardiness zone, this may be the garage, basement, or attic. It may also be a potting shed or a hoop house. As long as the conditions are optimal, any location will do. However, the amount of space you have available will impact a few of the next steps.
Store The Whole Root Clump
If you have the luxury of generous storage space, leave root clusters whole for the winter. They are less likely to rot, dehydrate, or freeze than smaller, divided tubers. And spring growth will be green, which takes some of the guesswork out of the process. They are also a lot easier to label and insulate in large chunks.
If your dahlias were grown in containers, and they can be moved into optimal storage conditions, you’re two steps ahead of the game. Move them into place and let them sleep.
Divide Before Storage if Needed
Those of us with limited overwintering space (and those of us in warmer climates who want to propagate) will want to divide tuberous root clumps in fall. Here are the steps:
Step 1: Locate your dahlias’ ‘eyes.’
They can be found at the point where the stem meets the roots. Similar to the growth you’ll find on a potato left in the pantry too long, a tuber’s eyes will be wart-like in their early stages and worm-like as they grow.
Step 2: Locate your dahlia’s roots.
They will look like skinny sweet potatoes and have root hairs at their lowermost points.
Step 3: Cut root clusters.
Using a sharp, clean knife, cut root clusters into chunks that include an eye, a small portion of the stem, and at least one root section.
Step 4: Discard those chunks with no three components.
Discard any remaining pieces that do not contain all three of these components, as they will not produce new growth.
Label Them For Storage
Ideally, you’ve kept tags or a garden map to help identify which dahlia varieties you’re storing. Before you pack them away, make sure each large clump or divided section is labeled. This will help you decide where to plant them in spring. Height and width specs can vary wildly within the genus.
Using a permanent marker, painter’s tape or a string with a tag, identify each future plant with a cultivar and color, if possible. You can write directly on the roots if they are large enough, and it will not hurt the plant.
Ask ten seasoned gardeners how they store dahlia roots for the winter and you will likely get ten different answers. The truth is, what works for one does not work for all, and you are likely to have a few seasons of trial and error before you settle into a storage routine that works for you.
Some of us have good luck storing tubers in plain old garbage bags with the tops left open. Others will swear by cardboard boxes filled with vermiculite, peat moss, wood chips, or shredded newspaper. Still others will insist on lidless plastic storage bins filled with soilless mix. So there’s not one tried and true method to recommend.
The one thing we all agree on, however, is the need for ventilation. Whichever storage method you decide to try, make sure there is plenty of air flow or your tubers will rot and die.
Monitor Them Regularly
Moisture balance is also a top priority when storing dahlias during dormancy. The optimal conditions are slightly humid, but not excessively so. And that can be a tall order. Visit your dahlias once or twice a month to unwrap them and assess their moisture level. They should be plump, not shriveled and have no green shoots or signs of growth.
Condensation anywhere on their container means humidity is too high and their ventilation should be adjusted. Make sure they are not getting too much light, and poke some holes in their storage bag or bin to increase air flow. You may even want to move them to a new location, if possible.
Shriveling and/or shrinking are signs of dehydration. This signals that they are in need of moisture or possibly too cold. Check the room temperature to make sure it’s between 40 and 50 degrees, and mist them every couple of weeks to keep them from drying out.
Replant in Late Spring
Dahlias can not be planted outside until all danger of frost has passed in your region. Check your growing zone’s average last frost date and add a week or two of insulation before putting them back in the ground.
While you wait, you can wake them up a bit to give them a head start on the season. Container grown dahlias that have overwintered in the garage can be pulled out in the sun for a few hours every day and then pushed back in for the cold nights.
Basement or cellar-stored containers can be relocated to a warmer room for a couple of weeks, and then unwrapped and placed in a sunny window or under a grow light. They can also be potted up inside and transferred later to the backyard. Keep moist with a mister and you should start seeing some green shoots very soon.
When freezing temps are no longer a threat, plant them 4-6 inches deep and at least 1 foot apart in well-drained, properly amended soil. Make sure their eyes and stem tissue are pointed up and roots are angled down. Cover loosely with soil, and water evenly until sprouts emerge.
Although it may seem like a tedious process, and success is by no means guaranteed, overwintering dahlias can be both a satisfying and money saving adventure. Think of your first few attempts as experimental in nature. Tweak your methods in subsequent seasons to arrive at the formula that works best for you.
When your yard is so full of dahlias that it can’t hold anymore, start giving them away and sharing your new know-how with friends and neighbors. You’ll be passing along a valuable skill and introducing others to the joy of growing this wonderful flower.