Lift dahlias, cannas and gladiolus in New Hampshire after the first frost that blackens their leaves, then wait a week or two before the fork goes in. At Manchester, NOAA puts the first fall freeze between October 13 and October 26, which puts lifting in late October and early November there. Gardens in the colder north and at higher elevations run earlier. Dig too soon and the tubers rot in the box. Wait through a hard freeze and the cold reaches them underground.
Do you have to dig them up in New Hampshire?
Yes. All three come out of the ground every year, in every part of the state. The USDA Plant Hardiness Zone Map 2023 rates New Hampshire from zone 3b to 6a, so even the warmest corner sits at 6a. That is not mild enough to leave them under mulch. In a place where the ground never freezes deeply, dahlias, cannas and gladiolus can stay put. Here, dig and store all three every fall.
The USDA system is built on a single variable, the average annual extreme minimum temperature, so it tells you about the one coldest night of a typical year. A second figure fills in the picture. NOAA’s 1991-2020 climate normals for Manchester put the mean daily minimum of the coldest month at 17.1°F. That is not a record low or a freak cold snap. It is the ordinary overnight low through the heart of winter, and it sits far below freezing.
These plants evolved in warm places. Dahlias come from the highlands of Mexico and Central America, cannas from the tropical and subtropical Americas, and gladiolus mostly from Africa. Their tubers, rhizomes and corms are fleshy and full of water, so when the soil around them freezes, the cells burst. The damage stays hidden all winter. In spring you dig down and find a handful of gray mush, or you wait for shoots that never come.
Where the line falls inside the state
The line between “leave them” and “lift them” does not run through New Hampshire at all. The whole state is on the lift side. The southern tier and the coast sit toward the warm end of the 3b to 6a range, and the White Mountains and North Country sit toward the cold end. Manchester is a fair reference for the middle, and other parts of the state differ from it in both directions.
Every few years someone reports a canna that came back from a sheltered bed beside a south-facing foundation. That is a gamble that happened to pay off, and it is not a method. Mulch slows the freezing of soil without stopping it, and slowing is not enough in a state where the coldest month averages 17.1°F overnight.
Hardy spring bulbs run the other way. Tulips and daffodils need the cold, and Which Spring Bulbs Survive Your Winter sorts out which ones belong in the ground and which don’t.
When to lift them
The signal to watch
Wait for a frost that turns the foliage black. Dahlia and canna leaves go limp, dark and slimy overnight, and that is the plant’s own announcement that the growing season is over. The frost kills the stems and leaves, but the storage organs underground are insulated by soil and stay fine for a short while.
NOAA’s U.S. Climate Normals for 1991-2020 give the Manchester airport station a first fall freeze (32°F) between October 13 and October 26, using the band where a freeze is neither unlikely nor certain. The recommendation is to lift a week or two after the first frost that blackens the foliage, which at the Manchester reference means roughly late October into early November.
That reference comes from an airport. Suburban and rural gardens typically see the first fall frost earlier than the airport does, often by one to two weeks, and more at elevation. A garden in the hills or farther north might have its blackening frost in early or mid-October and be lifting well before Manchester gardeners. Colder ground means fall jobs move earlier, never later. Look up your own county’s date in First and Last Frost Dates in New Hampshire and count forward from there.
Gladiolus follow a slightly different cue. Their corms don’t need a killing frost to be ready. Gardeners lift them when the leaves have yellowed and started to brown, which often happens close to the first frost anyway. Don’t leave them sitting in cold, saturated soil past that point.
Why earlier is worse
It is tempting to dig in September, when the garden is tired and the weather is pleasant. The plants are still green and working at that point, though. After the tops die back, the plant pulls its remaining energy down into the storage organs, the skins firm up, and on dahlias the growth buds (the “eyes”) near the crown become visible. That finishing period is what makes a tuber storable.
Lift a tuber still feeding a green plant and you get something thin-skinned and waterlogged. In storage it either dries out into a wrinkled husk or goes soft at the first sign of damp. Dahlia stems are hollow, too, and water standing in a freshly cut stalk can run down into the crown and start rot there.
Waiting has its own limit. A light frost followed by mild weather leaves plenty of time. A hard freeze forecast for ground that is already cold and wet is a reason to lift right away, even if you intended to wait the full week or two.
When you do dig, cut the stalks back, work a garden fork in well outside the clump so you don’t spear the tubers, and lever the whole mass up. Shake off the loose soil and move the clumps indoors to a spot out of the rain.
Storing them over winter
Storage comes down to three steps: cure, clean lightly, then keep them cool, dark and just damp enough. Cure by letting the clumps dry for a few days in an airy, rain-free place such as a porch, garage or basement, until the skin feels firm. Clean lightly by knocking off or brushing away soil with your hands. Skip the hose and the scrub brush, because wet tubers heading into storage are the number-one cause of rot.
The storage spot needs to stay above freezing. The 17.1°F mean overnight low of the coldest month in Manchester (NOAA, 1991-2020) means an unheated shed or detached garage can freeze solid in January. A basement corner, a closet against an interior wall or an insulated crawl space is a safer choice.
Dahlias and cannas versus gladiolus
The three plants don’t want exactly the same thing, and the gap is moisture.
| Plant | What you’re storing | Packing | Moisture |
|---|---|---|---|
| Dahlias | Tuberous roots | Boxed in barely damp packing such as wood shavings, peat or vermiculite | Slightly moist, never wet |
| Cannas | Fleshy rhizomes | Boxed in barely damp packing, similar to dahlias | Slightly moist, so they don’t shrivel |
| Gladiolus | Corms | Loose in paper or mesh bags, or a shallow tray | Dry, with good air movement |
Dahlia tubers and canna rhizomes are fleshy and lose water quickly in dry air, so they like a little humidity from their packing material. Gladiolus corms are more like onions. Cure them longer and drier, snap off the shriveled old corm from the bottom, and store the plump new one in a bag that lets air through. Packing a gladiolus corm in damp material is an invitation to mold.
Whatever you store, check the boxes once a month. That one habit saves more plants than any packing trick. Pull out anything soft, moldy or smelly before it infects its neighbors. If the tubers feel light and wrinkled, mist the packing lightly or move the box somewhere a little more humid.
While the dahlia boxes are going down to the basement, the beds are open for hardy bulbs. Planting Spring Bulbs in New Hampshire covers that side of the fall to-do list.
What goes wrong
Rot is the commonest failure, and it looks like a soft, dark patch, sometimes with gray or white mold and a sour smell. It comes from tubers stored wet, packed too tightly, cured too briefly, or lifted green. Cut away small bad spots down to clean flesh and let the cut dry, and discard anything that has gone soft through the middle. Prevention is the cure step and that monthly check.
Shriveling looks like the opposite problem: wrinkled, leathery and noticeably light. The storage space is too dry or too warm, or the packing has dried out. A tuber that is wrinkled but still has firm eyes can often be rescued with slightly damper packing and a cooler spot. One that has gone hard and hollow cannot.
Lifting too early produces both problems at once. Thin skins, weak eyes and plenty of water in the tissue make a tuber that’s rotting or shriveling by midwinter. The fix is patience, meaning a week or two after the foliage-blackening frost, not after the first cool night in September.
Then there is the zone mistake. A catalog describing a plant as hardy for a warmer region, or a neighbor’s one lucky winter, tempts people to leave plants in the ground. With a 3b to 6a range and a coldest-month overnight low of 17.1°F at Manchester, the odds are poor anywhere in the state. Lift every fall rather than gamble on mulch, and treat any plant that overwinters outdoors as a bonus.
Common questions
Can I leave dahlias in the ground in southern New Hampshire under mulch?
You can try, but the odds are poor. Even the warmest part of the state is zone 6a on the USDA Plant Hardiness Zone Map 2023, and Manchester’s mean overnight low in the coldest month is 17.1°F. A sheltered bed may carry a dahlia through a mild winter, but for a plant you would hate to lose, lift it.
What if the first frost comes late?
Wait for it. A late frost just pushes the lifting date later, and the plants keep gathering energy in the meantime. Watch the forecast for a hard freeze, though, because ground that freezes with the tubers still in it is the real danger, and you should lift ahead of that regardless of the calendar.
Should I wash the tubers before storing them?
Not much. Shake or brush off loose soil and leave the rest, since a thin layer of dry soil does no harm. Soaking or heavy washing leaves moisture in the crevices around the crown, and that is where rot starts. If you do rinse, dry the clumps thoroughly before they go into any packing material.
Do tulips and daffodils need the same treatment?
No, they’re the reverse. Hardy bulbs go into the ground in fall and need winter cold, and our Spring Bulbs section covers them, while more state-specific timing sits in the New Hampshire guides. And remember that the Manchester dates come from an airport station. Gardens in the hills often freeze one to two weeks earlier, so your own first frost may well arrive before the reference date.