Nine extension services publish a storage temperature for dahlia tubers and the numbers run from 35 to 55 degrees. Maryland’s floor is 45. NC State’s ceiling is 45. Follow one and you break the other. Neither page says what it is storing, or how.
Digging: frost, or a day count
Two cues are in print and they are not the same cue. Iowa State, Minnesota, NC State and Illinois all wait for frost to blacken the tops, then dig a few days later. Penn State and Washington State say frost is not required at all, and give a maturity number instead: at least 120 days in the ground for Penn State, a minimum of 135 for Washington State.
Both day counts come from Master Gardener columns and neither gives a basis. The frost camp is using frost as a proxy for the same maturity the day counts try to measure directly, which nobody states.
One instruction is agreed, with a mechanism attached. Cut the stems right before you dig, not days earlier, because a hollow cut dahlia stem standing in the rain funnels water straight down into the crown. New Hampshire gives that reason. Iowa State sets the stub at 2 to 4 inches for dahlia and 4 to 6 for canna. Missouri cuts gladiolus tops to within half an inch of the corm.
Curing, where the spread is twentyfold and the reason is findable
Oregon State and Purdue say air-dry dahlia tubers for several hours. Iowa State says about a day. Illinois says one to three. Penn State says four or five days out of the sun, and goes further than anyone else by reporting that the commonly recommended 24 to 48 hours came out too short, with briefly dried tubers more prone to rot. That is the only published empirical correction in this whole literature.
The sources are not timing the same job. Penn State washes the clump and divides it before curing, so what it is drying is a set of fresh cut surfaces that have to callus over. The hours-only sources neither wash nor divide first, so they are drying soil off intact skin. Those are different operations with the same name, and not one of the five publications says so.
Gladiolus shows the same problem in reverse. Purdue cures corms at 80 to 85 degrees for two to three weeks, then two or three more days at 80, and it is the only source anywhere that says why curing works: the warmth drives a corky scar layer over the cut. Missouri, Iowa State, Illinois and Wisconsin all keep the two-to-three-week duration and drop the temperature. Two weeks at 60 degrees is not the treatment Purdue described. NC State skips both. Three or four days, unexplained.
The storage temperature argument, and what resolves it
Here is the whole range. Maryland 50 to 55 degrees. NC State 35 to 45. Missouri 40 to 45. Penn State, Washington State and Iowa State 40 to 50. Illinois moved its own floor up from 35 to 40 between 2015 and 2025 with no note. New Hampshire says only below 50 and above freezing, and Utah State says only not 32. Purdue gives no number.
Maryland is the outlier and Maryland is also the one describing a different method. It stores clumps with the soil left on, bedded in moistened peat, no more than two or three layers deep because the bulbs generate heat that causes decay. Warm moist clump storage is not the same physiology as cool dry tuber storage, and 50 to 55 degrees is defensible for the first and wrong for the second. Maryland does not say that, and nobody else cites Maryland.
NC State is worth a second look for a different reason. Its figures are 35, 41, 50 and 70 to 75, which are the exact Fahrenheit equivalents of 2, 5, 10 and 21 to 24 degrees Celsius. Those are converted numbers, not measured ones, and the page gives no source. Finding a spot that actually holds 40 to 45 degrees is the harder half of the problem anyway.
Humidity and packing: one number exists
Across roughly 24 substantive publications from 16 institutions, exactly one relative humidity figure is published for tender bulb storage. Wisconsin says 50 percent, and says it for tender bulbs generally rather than for dahlias. Everyone else writes cool and dry, or low humidity, or lightly moist. Penn State acknowledges that humidity matters and that different media suit different levels, then prints no figure. Any number beyond Wisconsin’s 50 percent is somebody’s inference.
Packing material splits the same way. Purdue and Oregon State prescribe wrapping clumps in newspaper specifically to stop them shriveling in dry air. Penn State says avoid newspaper, cardboard and paper bags because they wick up too much moisture, and packs in cedar shavings in lidded plastic bins with holes drilled in the sides and the lid left loose. Both are right about the failure they are defending against. One guards a dry room against shrivel, the other a damp basement against rot, and neither names the condition that makes its advice hold. Check which room you have first.
Divide now or in spring
Purdue says divide in fall because the eyes are easier to see then. Penn State also says divide in fall, and concedes in the same article that eyes are more visible in spring and that fall division loses some of them. Its stated advantage is only that the tubers cut more easily while fresh.
The spring camp is not arguing about visibility. Illinois, Wisconsin, Iowa State, NC State, Maryland and Utah State all say wait, because every cut is a wound and wounds are the route disease takes into a tuber in storage. Those are two different things being protected, division accuracy against storage loss, and neither camp has measured the other’s. If you are storing in a basement you cannot keep cold and dry, the spring argument is the one that applies to you.
Geraniums: six sources, one lineage
Six institutions publish on carrying geraniums over, and the numbers match almost too well. Bare root at 45 to 50 degrees, cuttings three to four inches, six to eight weeks to root, pot up in March, cut back by half to two thirds. That uniformity is not six confirmations. Purdue and Iowa State, both in 1999, agree nearly word for word on a laborious and distinctive step: take the dormant plants down monthly and soak the roots for one to two hours. No page published since repeats it. Wisconsin mentioned it in 2024 as something it intends to test, soaked against dry. Twenty five years, no published result.
The one independent voice here is the plant pathology side. It says do not do this at all. Penn State’s bacterial blight page: discard all geraniums at the end of the season, and do not save outdoor-grown plants as stock. A Wisconsin Extension bulletin, reposted to a county site in 2023, says do not carry stock over from one season to another. Wisconsin Horticulture endorsed three carryover methods in 2024. Same extension system, opposite instructions, and the reason the pathologists give is that an infected stock plant often shows nothing: below 60 degrees the blight creeps, and at 70 to 81 degrees symptoms take a week to ten days to appear. Not one of the five pages recommending carryover mentions blight. Purdue comes closest, telling you to discard anything with leaf spots or stem darkening before storage, which is advice for the symptoms the pathologists say you will not see.
Sources
- University of Minnesota Extension, Calla and canna lilies
- University of Minnesota Extension, Overwintering geraniums
- Iowa State University Extension, How to Overwinter Tender Perennials
- Iowa State University, How to Overwinter Geraniums (1999)
- Iowa State University, How do I overwinter gladiolus bulbs?
- Penn State Extension, Don’t Ditch Those Dahlias
- Penn State Extension, Bacterial Blight of Geranium
- WSU Extension (Yakima), Fall Dahlia Care (2026)
- UNH Extension, When should I dig up dahlia tubers?
- University of Missouri Extension, Summer Flowering Bulbs: Gladiolus, G6620 (rev. 2022)
- University of Missouri Extension, Dahlias dazzle and delight as gardens dim
- University of Illinois Extension, Tips for overwintering tender bulbs (2025)
- University of Illinois Extension, Plant of the Week: Dahlias (2015)
- University of Wisconsin-Madison Extension Horticulture, Storing Tender “Bulbs” for Winter
- University of Wisconsin-Madison Extension Horticulture, Overwintering Tender Geraniums (2024)
- UW-Extension A2559, Geranium Disorder: Bacterial Stem Rot and Leaf Spot
- Purdue University Cooperative Extension, Winter Storage of Geranium, Canna, Gladiolus, Caladium, and Begonia, HO-85
- NC State Extension, Summer and Fall Flowering Bulbs for the Landscape
- University of Maryland Extension, Bulbs
- Utah State University Extension, Dahlias
- Oregon State University Extension, Dahlia Culture, FS 95
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