On the mild south coast, dahlias, cannas and gladiolus can often stay in the ground under mulch. In colder districts they have to come up. Where you lift, wait for a frost that blackens the foliage, then dig within a week or two, which near Vancouver usually means mid to late November. Cure the tubers, pack them cool, dark and barely damp, and look at them once a month. Hardy bulbs that stay put are a different job, covered in Spring Bulbs.
Do you have to dig them up in British Columbia?
Leave them in the ground under a thick mulch on the mild coast, and lift them wherever the soil freezes in a normal winter. That is the decision to write on your garden calendar. The coast and the rest of the province sit at opposite ends of it.
The reference city, Vancouver, falls in Canadian hardiness zone 9a, according to Natural Resources Canada’s Canada’s Plant Hardiness Site. That is a Canadian zone, and it does not convert to a USDA zone. The Canadian system blends seven climate variables, while the USDA system uses one, and Natural Resources Canada says there is no simple conversion between them. Compare it with American zones only loosely, if at all.
The number behind the verdict is the coldest month. At the Vancouver station, Environment and Climate Change Canada’s 1981-2010 normals put the mean daily minimum of the coldest month at 0.8 °C (33.5 °F). The average night in midwinter stays just above freezing, so the ground there does not freeze deeply. Tubers sit in cool soil, not frozen soil.
Can dahlias, cannas and gladiolus stay in the ground on the coast?
Often, yes, if the soil drains well. A coastal winter is wet, and standing water around a tuber does more harm than a cold night. Raised beds and sloped ground do best. A loose, coarse mulch laid on in late fall covers the crown and gives some protection against a short cold snap. An average is only an average, and a few nights can drop well below it.
Cannas are the most cautious call. Their rhizomes dislike cold, saturated soil, so a gardener with heavy clay might lift them even on the coast. Gladiolus corms are usually the easiest to leave in a well-drained bed. Dahlias fall between the two.
Which parts of British Columbia need lifting?
Vancouver is one station, and a province is not one climate. Northern and upland districts run weeks behind the coast in spring and arrive at the first fall frost earlier. Inland, hard freezes can reach the soil for days at a time. In those places, tender bulbs have to come up every year, and they come up sooner than on the coast.
The line falls where frozen ground becomes ordinary. If the soil under mulch freezes solid in a typical winter in your neighborhood, lift. If you are unsure, a local nursery or master gardener program will know the neighborhood’s pattern. Gardeners in the Prairie provinces face the same job in harsher conditions, and the routines in lifting tender bulbs in Alberta and in Manitoba show how it works where winter is severe. For hardy bulbs that stay planted, see Which Spring Bulbs Survive Your Winter.
When to lift them
The signal is a frost that blackens the leaves. Before then the plant is still feeding its underground storage, and the foliage is its power supply. Once the frost kills the tops, the plant sends its last reserves down and the tubers harden.
What is the lifting window near Vancouver?
Environment and Climate Change Canada’s 1981-2010 normals for the Vancouver International Airport station give the first fall frost, on average, as November 10. That is a midpoint, and the frost arrives earlier in one year out of two. Do not treat it as a safe date. A cold, clear night in late October could end the season for your plants.
Lift within a week or two after the blackening frost. In an average coastal year that means roughly mid to late November, one to two weeks after the first killing frost. In a year with an early frost, move the window earlier. In a mild fall with no frost until December, wait for the foliage to collapse, which cold, wet weather can also cause. Watch the plants, and use the calendar only as a guide. The First and Last Frost Dates in British Columbia will help you place your own garden against the coastal average.
Colder districts have an earlier first fall frost, so their lifting window opens earlier as well. A garden at higher elevation or farther north should plan to dig while the coastal gardener is still deadheading. The weather station is a good reference for the coast only.
How do you lift them without damaging the tubers?
Cut dahlia and canna stems back to a short stub. Dahlia stems are hollow and can funnel water into the crown, so trimming them helps. Choose a dry day, because wet soil clings and tears. Push a garden fork into the ground well away from the stem and lever the clump up gradually. Spearing a tuber leaves a wound where rot starts. Gladiolus corms come up as soon as the leaves yellow or the frost browns them, and the old shriveled corm at the base can be removed later.
Why not lift earlier?
Tubers dug while the tops are still green have thin skins and soft flesh. They have not finished maturing, so they lose moisture or rot in storage, often in midwinter, when it is too late to replace them. A tuber that has had a week or two after the frost has firmer skin and keeps better. If you lift too early, and the tops are still green, you are trading a few extra weeks of garden tidiness for a box of mush in February.
Storing them over winter
The goal is to keep tubers cool, dark and slightly moist, so that they neither rot nor shrivel. A tuber that is too wet rots, and one that is too dry shrivels. The balance is easy to find and easy to lose.
How do you cure and clean them?
After lifting, let the clumps dry for a few days in a sheltered, airy place out of rain and direct sun. Dry soil falls off by itself, and a light brush or a gentle shake is enough. A hard wash leaves moisture in the crevices where rot starts, so skip it unless you can dry the tubers thoroughly afterward. Dahlias and cannas cure fairly quickly. Gladiolus corms need longer, a couple of weeks, until the husks are papery.
How do dahlias, cannas and gladiolus differ in storage?
The three do not want the same thing. The table sets out the differences.
| Plant | Storage part | Packing | Moisture |
|---|---|---|---|
| Dahlia | Tubers | Boxes with peat moss, coir or dry shavings | Barely damp, enough to prevent wrinkling |
| Canna | Rhizomes | Boxes with coir or shavings, kept in clumps | Slightly moist, since rhizomes dry out faster |
| Gladiolus | Corms | Paper bags, mesh bags or shallow trays | Dry and airy, with no packing medium |
Dahlias tolerate a little more dampness than the others, because their tubers are fleshy and shrivel quickly. Cannas are similar but dislike drying out completely. Gladiolus corms store like onions, and wet packing makes them mold. Keep all three somewhere above freezing and well below room temperature, such as an unheated cellar or frost-proof garage. In colder districts, a spot that stays frost-free all winter matters most.
Check your boxes about once a month all winter. A quick look is enough. Remove any piece that has softened or molded before it spreads to its neighbors. Lightly mist a tuber that has begun to wrinkle, or add a pinch of damp medium to the packing. This is the habit that saves most stored tubers, because problems stay small when you catch them early. Checking also tells you whether the storage spot has become too warm or too damp.
What goes wrong
How do you spot rot and shriveling?
Rot shows up as soft, dark patches, a sour smell and sometimes gray or white mold. It comes from storing tubers wet, packing them too tightly or lifting before they matured. Cut away small soft spots back to clean, firm tissue, let the cut dry, and move the tuber to drier packing with better airflow. Discard tubers that are mushy throughout.
Shriveling is the opposite problem. The tubers wrinkle and go leathery, usually because the room is too warm or the packing too dry. A tuber that is only slightly wrinkled recovers with a little more moisture in the packing, but once it is hollow and light, it will not sprout.
What if you lifted too early or left them in too far north?
Lifting too early gives thin-skinned tubers that rot or dry out by midwinter. Wait for the frost-blackened foliage and dig within a week or two afterward. The opposite error is leaving bulbs in the ground in a district colder than the coast. Mulch cannot protect tubers when frost reaches them, and they turn black and mushy by spring. Gardeners often copy coastal advice without checking whether their soil freezes. If it does, lift. The Planting Spring Bulbs in British Columbia guide covers bulbs that are meant to stay in the ground, and the main British Columbia guides list other seasonal jobs.
Common questions
Can I leave dahlias in the ground on the British Columbia coast?
Often you can, if the bed drains well and you cover the crowns with mulch in late fall. Wet soil is the main risk. If your ground is heavy clay or you have lost tubers to rot before, lifting is the safer choice.
What if frost has not arrived by late November?
Wait for the foliage to collapse. Cold, wet weather can end the season much like a frost, and the signal is the same: leaves that have blackened or browned and stems that have gone limp. Then lift within a week or two.
Should I wash the tubers before storing them?
A light brush after the soil has dried is enough. Washing leaves moisture in the crevices, and that is where rot begins. If you do rinse them, dry them thoroughly in an airy place before packing.
When do I replant them in spring?
Plant after the last spring frost has passed and the soil has begun to warm. The last frost comes later in colder districts, so use your local date. The First and Last Frost Dates in British Columbia give a starting point, and the sprouting of the stored tubers shows when they are ready.