Tender-Bulb Overwintering Scheduler
Get the dig window, storage conditions and spring replant date for tender bulbs in your zone.
Tender bulbs live on borrowed time in cold zones: dahlias, cannas and caladiums grow like natives all summer and die in the first hard ground freeze. The rescue is a schedule, not a scramble: dig in the right window for each bulb, cure it, store it at its own temperature, and replant on a date counted from your last spring frost. This scheduler lines all of that up from your zone.
Each bulb has its own trigger in the list below, from the caladium that must come up before chilly nights to the dahlia that should wait until frost blackens its tops. The frost dates themselves come from the First Frost Date Calculator.
How this is worked out
The dig window hangs off the average first frost by the offset each bulb’s biology demands. Dahlias and cannas store best after frost has blackened the foliage, because the cold signal finishes moving sugars into the tuber, so their windows open at the frost. Caladiums are the opposite: their tubers are damaged by chilly soil well above freezing, so they come up two to four weeks before it. Gladiolus follows its foliage, yellowing six to eight weeks after flowering, which lands around the frost in most zones. Storage temperatures differ the same way: most tubers want a cool 40 to 50 F, while caladiums rot below about 55 F and need a warm corner.
Assumptions:
- In zones 8 and warmer, most of these bulbs overwinter in the ground with mulch; the scheduler still gives dig dates, useful for dividing crowded clumps, but the survival stakes are lower.
- The window is a planning span of about two weeks, not a deadline day. Dry weather inside the window beats the exact date in the rain.
- Replanting counts from the following spring’s last frost; tender tubers rot in cold wet soil even when no frost follows, so the ten day buffer is deliberate.
Worked examples
1. Dahlias, Zone 6b. First frost about Oct 27: dig Oct 27 to Nov 6, after a frost has blackened the tops, cure a week, store at 40 to 50 F, replant about Apr 23 next spring (or pot up indoors in March for earlier flowers).
2. Caladiums, Zone 7a. First frost about Nov 2: the window runs Oct 5 to Oct 19, before the first frost, because waiting for it is exactly the mistake this bulb punishes.
3. Gladiolus, Zone 5b. First frost about Oct 13: dig Sep 22 to Oct 20 by the foliage rule, cure the corms two weeks, and store cold and dry; the small cormels can be kept for growing on.
Each bulb’s trigger
| Bulb | Dig trigger |
|---|---|
| Dahlia | Dig after the first frost blackens the foliage. |
| Gladiolus | Dig when the leaves yellow, about 6-8 weeks after flowering. |
| Canna | Dig after the first light frost nips the leaves. |
| Caladium | Dig before nights drop below 50 F; the tubers chill easily. |
| Elephant ear (Colocasia) | Dig after the first light frost. |
| Tuberous begonia | Dig after the tops yellow in fall. |
| Calla lily | Dig after foliage dies back; rhizomes rot in cold wet soil. |
| Ranunculus (tender) | Dig after the foliage yellows in warm zones only. |
Cure everything the same way: shake off soil, dry in a single layer in moving air for one to two weeks out of the sun, trim tops to an inch or two, and discard anything soft, mouldy or damaged before it can spoil a stored batch.
Common mistakes
- Digging dahlias before the frost signal. Dug green, the tubers store poorly and the eyes are hard to see at division time. A light frost costs the tops, not the tubers; it is the starting gun, not the enemy.
- Storing caladiums with the dahlias. The cold room that dahlias love (40 to 50 F) rots caladiums. Warm storage for caladiums is the single most ignored instruction in this whole list.
- Washing and bagging damp tubers in plastic. Stored damp in a sealed bag is a mould incubator. Dry cure, breathable packing, and a monthly squeeze test through winter catch losses while they are still one tuber, not the box.
- No labels. Unlabelled storage is a lottery planted in spring. Label at digging, when you still know which plant was which.
Putting the result to work
Put the dig window in the calendar at the start of fall, and pick the first dry day inside it. Set up the curing space before you dig, not after: a garage bench or a shaded porch with air moving. Come spring, pot dahlias and cannas up indoors about four weeks before the replant date for a head start, and hold the date itself against the forecast. The full year these dates live in is the Seasonal Garden Calendar.
When do I dig and store tender bulbs?
- Pick the bulb; each has its own dig trigger and storage temperature.
- Pick your zone to anchor the window to your average first frost.
- Dig inside the window, on a dry day, after the bulb’s trigger event.
- Cure one to two weeks in dry moving air, trim, label, and pack in breathable material.
- Store at the bulb’s temperature, check monthly, and replant about ten days after last frost.
When do I dig dahlias?
After the first frost blackens the foliage, within about ten days. The frost is a signal, not a disaster: it finishes the tops and firms the tubers, while the tubers themselves are safe in soil that has not yet frozen hard. Cut the stems to a few inches, lift the clump gently with a fork, and cure before storing at 40 to 50 F.
Can tender bulbs survive winter in the ground?
In Zone 8 and warmer, mostly yes, with a thick mulch. In Zone 7, dahlias and cannas survive mild winters under deep mulch and die in hard ones, which is a gamble on the year. Caladiums are the least cold-tolerant of the common tender bulbs and should be lifted anywhere the soil gets properly cold.
Why did my stored tubers rot or shrivel?
Rot means they went in damp, were stored too warm and sealed, or one damaged tuber spoiled the batch. Shrivel means storage too dry and warm; pack dahlias in barely damp peat or shavings and mist the packing once mid-winter if tubers feel light and wrinkled. A monthly check catches both directions while they are still fixable.
How do I store caladiums differently?
Warm. Caladium tubers are damaged below about 50 F and want 55 to 65 F, the opposite of the cold room that suits dahlias. Keep them in dry peat in the warmest cool place in the house, and pot them up early in spring, because they are slow to wake.
When do I replant in spring?
About ten days after your average last frost, when the soil is warming. Tender tubers planted into cold wet soil rot before they sprout, so the buffer matters more than the calendar. For earlier flowers, pot dahlias and cannas indoors about four weeks ahead and plant out growing.
Dig triggers, curing and storage temperatures follow standard US extension guidance for tender bulbs and tubers. Frost anchors are zone averages consistent with the 2023 USDA Plant Hardiness Zone Map climate basis.
