In lowland Hawaii, you don’t have to. No freeze reaches the tubers, so dahlias, cannas and gladiolus can stay in the ground all year, and there is no first-frost date to count from. Digging still earns its place for dividing crowded clumps, rescuing roots from waterlogged winter beds, or replanting gladiolus corms. What triggers the job here is leaf dieback and soggy soil, not a thermometer. Frost belongs to the summits.
Do you have to dig them up in Hawaii?
For almost every garden in the islands, the answer is no: leave dahlias, cannas and gladiolus in the ground and skip the autumn dig. The USDA Plant Hardiness Zone Map (2023 edition, built on 1991-2020 normals) places the territory in zones 9a to 13a. That is a statewide range, not one number, and the USDA system rests on a single variable: the average annual extreme minimum temperature. Honolulu sits at the mild end of what matters here. NOAA’s NCEI Climate Normals for 1991-2020 put the mean daily minimum of its coldest month at 66.8°F. The station records zero days per year at or below 32°F.
That is the whole reason northern gardeners dig. A dahlia tuber or canna rhizome is mostly water, and when soil freezes deeply, ice crystals rupture the cells. The tissue turns to mush by spring. Nothing like that happens where the coldest nights of the year still sit well above freezing. Frost shows up only on the high volcanic summits (Mauna Kea, Mauna Loa and Haleakala), and nobody gardens up there.
What decides things instead
Rainfall, elevation and exposure do the work that frost does on the mainland. A bed on a windward slope that catches rain for days at a time is a very different place from a dry leeward lot. Higher-elevation gardens run cooler than the coast, and that cooler air is gentler on dahlias, the fussiest of the three. But cooler does not mean frozen, and the gap between a seaside lot and an upcountry garden is a matter of degrees, not a season of ice.
A layer of mulch still helps, though its job changes. On the mainland it insulates against cold. Here it buffers heavy downpours, holds moisture during dry spells and keeps weeds down. Keep it a little back from the crown so water doesn’t sit against the stems.
For the broader question of which bulbs need a winter chill or a lift and which do not, Which Spring Bulbs Survive Your Winter sorts it out by zone. Because the question never turns on a freeze date here, there is no calendar to print on the fridge. There are three situations where you do reach for the digging fork anyway, and the next sections cover them.
When to lift them
On the mainland, the signal is a killing frost that blackens the foliage. Gardeners then lift the plants within about a week or two so the tubers can finish maturing. That signal never arrives in Hawaii. The NCEI normals for station USW00022521 flag every freeze percentile at 32°F and 36°F as not computable, because there is nothing to compute. Anyone who hands you a “first fall frost” date for an inhabited part of the islands is borrowing it from somewhere else. You can read more on the subject in First and Last Frost Dates in Hawaii.
So the cue moves to the plant itself. Lift when the foliage has yellowed and died back on its own after flowering, not on a date. Dieback is the plant’s way of saying the tops have finished feeding the underground storage organs. Cut the stems back and the tuber, rhizome or corm is ready to come out. In practice that moment is spread across the year, and it differs by plant and by garden. A dahlia that quits during a hot, muggy stretch will not match a canna that is still putting up flower spikes next door.
Why earlier lifting backfires
Pulling a plant while the leaves are still green is the mistake that costs the most. Green foliage is still sending sugars down into the tuber. Dig it early and you get a thin-skinned, undersized piece that never finished toughening. Stored, it loses moisture unevenly, bruises easily and rots from the cut ends. A tuber lifted after the plant has shut itself down carries a firmer skin and enough reserves to sit out a dormant period.
The frost signal does the same thing on the mainland. It forces the plant to stop and sends the energy downward in one pulse, which is why northern gardeners wait for it. Hawaii has no such pulse, so the plant sets its own timing. Waiting for the leaves to fail is the nearest equivalent, and it protects you from the soft, bruised tubers that early digging produces.
Three situations justify lifting at all:
- A clump has grown crowded and flowering has dropped off, which means it is time to divide.
- The bed stays saturated through a rainy stretch and the tubers are at risk of rot in place.
- You want to move or replant gladiolus corms and sort out the new ones from the spent.
In each case the signal is dieback, or sodden soil, not a calendar page.
Storing them over winter
Hawaii has no winter in the northern sense, and the storage question follows from that. You store because you chose to lift, perhaps to wait out a soggy spell or to hold divisions until a drier bed is ready. The goal is the same as anywhere: keep the tissue alive without letting it rot or shrivel. That means curing, a light cleaning, then a cool, dark spot with just enough moisture. The first job is a short cure in a shaded, airy place so cut surfaces dry and the skins firm up. Brush off loose soil and do not scrub. Washing tubers wets the crevices where rot starts.
Warmth is the problem here. Mainland gardeners rely on a cool basement or unheated garage, and a lot of island homes have neither. A closet on the shaded side of the house, an interior pantry or a cooler is workable. Corms of gladiolus can go in a refrigerator crisper drawer, kept away from ripening fruit, which gives off ethylene gas that can affect stored plant tissue. The Planting Spring Bulbs in Hawaii guide covers how to handle warm-climate bulb storage generally.
The three plants want different treatment, and mixing them in one box is a common way to lose some of them.
| Plant | What you store | How to pack it | What to watch for |
|---|---|---|---|
| Dahlia | Tuberous roots with a piece of the crown | Lightly damp medium such as peat, vermiculite or wood shavings, in a container that breathes | Shriveling if too dry, soft spots and mold if too wet |
| Canna | Thick rhizomes | Barely damp medium; they dislike drying out completely | Rhizomes turning papery or mushy |
| Gladiolus | Corms, with the old shriveled corm removed | Dry, ventilated storage in a paper or mesh bag, no damp packing | Mold, soft corms, thrips damage |
Dahlias are the most sensitive to getting the moisture wrong, since a tuber that dries out will never plump up again, and one that stays wet rots from the neck. Cannas are tougher but still resent being bone dry. Gladiolus corms are different animals altogether. Each season the plant builds a new corm on top of the old one, so what you keep is the fresh corm, and the withered one underneath gets discarded. They store best dry, with air moving around them, and the damp packing that suits a dahlia would turn them moldy.
Humidity changes the rules in Hawaii compared with a dry mainland basement. Air that is already moist means you rarely need to add much dampness to the packing material, and you should lean toward ventilation. Check everything once a month, and that habit is the step that saves the collection. Pick up each piece, press it, and look for soft spots. Remove anything that has gone bad before it spreads. If a tuber has begun to wrinkle, mist the packing lightly. If you see fuzz, let the box dry out and discard the affected piece.
There is no set storage length, because lifting dates differ from one garden to another. Replant when the bed has drained and the plants show signs of life, such as swelling eyes on dahlias or firm, plump rhizomes on cannas.
What goes wrong
Rot is the most common failure, and it announces itself. A rotting tuber turns soft and dark, smells sour and may carry gray or white mold. Cut off a small spot and the healthy tissue underneath looks clean; if the whole piece is mushy, throw it out. It happens when storage is too damp or too warm, or when the piece was bruised during digging. Prevention comes down to a proper cure, light packing and the monthly check.
Shriveling is the opposite problem. The tuber looks wrinkled, feels light and has lost its snap. Dry air, too little packing medium or too long between checks causes it. Caught early, a bit of dampness in the packing revives a dahlia or canna. Left alone, the piece becomes too dry to sprout. A gladiolus corm that has shrunk to a husk is finished.
Lifting too early gives you thin-skinned pieces that rot or dry out in storage, as covered above. The fix is patience: let the foliage die back on its own.
Then there is the mistake of following mainland instructions to the letter. Advice written for gardeners in colder places says to lift every fall, and following it in Hawaii costs plants for no benefit. Where the ground does not freeze, leaving them in is not a gamble. The reverse error is the one to watch: if your garden sits on poorly drained soil, or in a spot that stays soaked, do not leave tubers in the ground through a rainy stretch. Wet roots are the danger here, not frozen ones, and the symptoms are the same soft, sour rot you would see in storage.
On the mainland, gardeners worry about leaving plants in the ground one zone too far north. The Hawaii equivalent is leaving them in a bed that floods. Plan your drainage the way a Minnesota gardener plans their lifting date. For the wider list of what to grow and when, the Hawaii guides are a good starting point.
Common questions
Is there a first-frost date I can use for lifting?
No. NOAA’s 1991-2020 normals show no freeze in the inhabited parts of Hawaii. Frost is limited to the summits of Mauna Kea, Mauna Loa and Haleakala, so a frost-based calendar does not apply to garden beds.
Can I leave dahlias in the ground all year?
In most gardens, yes. Cold is not the limit, since Honolulu’s coldest-month mean daily minimum is 66.8°F. Drainage is the limit, so lift or relocate them if a bed stays waterlogged.
Do cannas need to be dug up?
Not for winter protection. Cannas are the sturdiest of the three, and you only need to dig them to divide a crowded clump. After the foliage dies back, cut the stems down and lift the rhizomes.
How often should I check stored tubers and corms?
Once a month. Press each piece, look for soft spots or mold, and add or remove moisture as needed. A single rotting tuber can spoil its neighbors, which makes the monthly check worth the few minutes.
One last point on timing: if you garden at a higher elevation, the cooler air may slow the plants’ cycle, so the dieback cue can arrive later than it would near the coast. The signal stays the same, only the date shifts.