Dig dahlias, cannas and gladiolus in Pennsylvania after the first frost blackens the foliage, and get them out of the ground within about a week or two. In Philadelphia, the mildest corner of the state, NOAA’s 1991-2020 normals put the first 32°F freeze between November 5 and November 16, so lifting there runs from mid to late November. The northern tier and the Alleghenies freeze sooner, so lift earlier. None of the three can count on surviving a Pennsylvania winter in the ground.
Do you have to dig them up in Pennsylvania?
Pennsylvania runs from zone 5a to 7b on the USDA Plant Hardiness Zone Map 2023, and the verdict is the same across that range. Plan on lifting all three every fall. The USDA system rates a place by one number, the average annual extreme minimum temperature, which describes the single coldest night of a typical winter. For tender bulbs the ordinary winter night matters just as much. At the Philadelphia reference station, the mean daily minimum of the coldest month is 26.0°F (NOAA NCEI Climate Normals 1991-2020). That is the mildest corner of the state, and a typical night in its coldest month still sits six degrees under freezing. Every colder part of Pennsylvania does worse.
Can dahlias, cannas or gladiolus overwinter under mulch in Pennsylvania?
Mulch slows the ground’s cooling, but a hard winter outruns it. Dahlias come from Mexico and Central America, cannas from the tropical Americas, and garden gladiolus descend largely from southern African species. None of them evolved with frozen soil. Once a tuber, rhizome or corm freezes, the tissue turns to mush and you find it in May as a black lump. Wet soil adds a second danger. Heavy clay that stays soggy rots tubers even in a winter that never freezes deeply.
A mulched clump may come back after a mild winter, and that occasional success is why people keep trying. The winter that kills it is the one nobody can predict in October. The cost of being wrong is the whole planting, and the cost of lifting is an afternoon with a garden fork.
Does the answer change in the northern tier and the Alleghenies?
It gets firmer. NOAA’s normals note that the northern tier and the Alleghenies run two to four weeks later than Philadelphia in spring. They also freeze earlier in fall, so the lifting window opens sooner there and the storage season runs longer. If you garden at higher elevation, treat the Philadelphia dates as the latest you could possibly wait, and let your own blackened foliage set the day.
Hardy bulbs follow the opposite rule. Tulips, daffodils and crocus stay put and need the cold, and Which Spring Bulbs Survive Your Winter sorts out which ones will make it where you live. For the broader family of Spring Bulbs, the tender summer types in this article are the exceptions that have to come inside.
When to lift them
The signal is a frost that blackens the foliage. Dahlia and canna leaves go limp, dark and slimy overnight, and the garden looks finished. Resist the shovel for a few days. The tubers underneath are fine, and the short wait lets them mature. Plan to lift within about a week or two of that killing frost. In Philadelphia, where the first 32°F freeze falls between November 5 and November 16 (the 30 to 70 percent probability band in NOAA NCEI’s 1991-2020 normals), that puts lifting in mid to late November. Colder parts of the state get their first freeze earlier, so their lifting comes earlier too, often well ahead of the Philadelphia window.
Calendar dates only get you in the neighborhood. A cold pocket in a valley can freeze before a hillside nearby. The First and Last Frost Dates in Pennsylvania will tell you roughly when to start watching, and the foliage tells you when to act.
How do you know it is time to dig?
For dahlias and cannas, the sign is black, collapsed leaves after a frost. Cut the stalks back to a short stub, which also gives you a handle for the fork. Work the fork in well away from the stem, because spearing a dahlia clump leaves a wound that rots. Lift on a dry day if you can, and ease the clump out rather than yanking it.
Gladiolus run a little differently. The foliage often yellows and browns on its own late in the season, and a light frost finishes the job. Dig the corms once the tops have browned, and do not leave them sitting in cold, wet soil. The stalks come off close to the corm after digging.
Why do tubers lifted too early rot in storage?
Until the cold kills the tops, the leaves keep sending sugars down into the roots. Tubers lifted while the plant is still green are immature. Their skins are thin and their flesh is full of water, so they either lose moisture and shrivel or pick up rot. Dahlia stems are hollow, and cutting them while the plant is still active lets water run straight down into the crown. The week or two after the frost gives the skins time to firm up and, on dahlias, the eyes (the buds that grow next year’s shoots) time to set.
The window has a far edge as well. Wait until the ground freezes hard and you are digging mush instead of tubers. While you have the shovel out in late fall, it is also the right season to put hardy bulbs in, and Planting Spring Bulbs in Pennsylvania covers that side of the chore.
Storing them over winter
Cure first, then pack. Spread the lifted plants in a sheltered, airy spot out of rain, sun and frost, such as a porch corner or a garage, until the surface soil is dry and the skins feel dry to the touch. Dahlia clumps can sit upside down so the hollow stems drain. Brush off dry soil by hand. Heavy washing leaves water in the crevices, and that water is where rot starts.
Cool, dark and slightly moist is the target for dahlias and cannas. Frost-free is the hard limit. In Pennsylvania, where even Philadelphia’s coldest-month nights average 26.0°F, an unheated garage can freeze solid, so an interior basement corner is safer. Too warm a spot makes tubers sprout early and soften.
How do dahlias, cannas and gladiolus differ in storage?
| Plant | What you store | Cure until | Packing and moisture |
|---|---|---|---|
| Dahlia | Whole clump of tuberous roots | Skin feels dry; stems have drained | Box of peat moss, vermiculite or wood shavings, barely damp |
| Canna | Rhizome clump with a stub of stalk | Surface soil is dry | Peat or shavings, barely damp, kept from drying out completely |
| Gladiolus | Individual corms, old shriveled corm removed | Husks are papery, in a warm, airy spot | Mesh or paper bags with airflow, dry |
The difference comes from the plant’s own structure. Dahlia tubers and canna rhizomes are fleshy and lose water through thin skin, so a little moisture in the packing keeps them plump. A gladiolus corm wears a papery husk and rots in damp packing, so it wants dry air moving around it. Mixing the three in one damp box is a common way to lose the gladiolus.
How often should you check stored dahlia tubers?
Plan to check them once a month all winter. Open the boxes, lift a few tubers and look at them. Soft spots and fuzzy mold mean rot, and that tuber goes before it spreads. Wrinkled skin means the packing is too dry or the room too warm, so add a little damp medium or move the box. Early sprouting means it is too warm. Two minutes a month is what separates a basement full of healthy tubers from a box of compost.
Storage lasts until the danger of spring frost has passed. NOAA NCEI’s normals show the northern tier and the Alleghenies run two to four weeks later than Philadelphia in spring, so gardeners there keep their bulbs in storage longer and need to watch them longer.
What goes wrong
Nearly every storage failure traces back to moisture, temperature or timing. The damage shows up slowly, which is why the monthly check matters.
Why are my stored tubers rotting?
Rot looks like soft, mushy tissue, a sour smell, and gray or white fuzz on the surface. Causes include packing that was too wet, no airflow, tubers that skipped curing, or tubers lifted too early. Throw out anything that is soft all the way through. If only one end is soft, cut back to firm tissue, let the cut dry, and repack it apart from the others. Prevention is a thorough cure and packing that is barely damp.
Why are my stored tubers shriveled?
Shriveled tubers look wrinkled and feel light, like a raisin. The room is too warm, too dry, or too drafty. Damp the packing slightly and move the box somewhere cooler. A tuber that has gone hard and hollow will not recover, but one that is only wrinkled usually does. The exception is the gladiolus: the old, spent corm shrivels as the new one forms, and that is normal.
What happens if you lift too early or leave them in the ground?
Lifting while the foliage is still green gives immature tubers that rot or shrivel by midwinter. Leaving them in is the other mistake. A neighbor in a warmer part of the state, or advice written for a milder climate, can make it sound safe. Pennsylvania spans USDA zones 5a to 7b, and a clump that survives in one spot may be dead farther north. When the shoots fail to appear in late spring, the tubers are usually black mush underneath.
Common questions
Can I leave cannas in the ground in Pennsylvania if I mulch heavily?
You can try, and a mulched clump may return after a mild winter. Philadelphia’s coldest-month nights average 26.0°F (NOAA NCEI 1991-2020), and colder parts of the state do worse, so one hard winter can kill it. Lifting costs far less than replacing the plants.
Do gladiolus need to be dug every year in Pennsylvania?
Yes. Garden gladiolus corms are tender, and Pennsylvania ground freezes. Dig them when the foliage has browned, cure them until the husks are papery, then store them dry with airflow.
Should I wash dahlia tubers before storing them?
Brush off dry soil and leave it at that. If you rinse a heavily clay-caked clump, let it dry completely before packing, because trapped water in the crevices is a common cause of rot.
Where can I find the right lifting time for my county?
Your county’s Penn State Extension office can tell you your local first-frost timing. The Pennsylvania guides collect seasonal timing for the rest of the garden, and your own blackened foliage is the final signal.