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Overwintering Bulbs How to overwinter bulbs, tubers, tuber roots, rhizomes, & corms to protect them from the ravages of winter.
Two of the first things to consider when deciding whether or not to dig up your bulbs for the winter are: 1- What Plant Hardiness Zone are you in. 2- Are the bulbs Tender Bulbs or Hardy Bulbs. Hardy Bulbs One way to classify bulbs is whether they are tender or hardy. If bulbs are hardy they can tolerate periods of cold temperatures. In fact, hardy bulbs often require periods of cold temperatures to produce reliable flowers each season. Tender Bulbs On the other hand, if bulbs are considered tender they do not tolerate cold temperatures and need to be taken out of the ground before freezing temperatures damages or kills them. Plant Hardiness Zones You should not only look at whether a particular bulb is considered hardy or tender, but what Plant Hardiness Zone do you reside in. Note this… Some tender bulbs that can overwinter generally in a particular zone may still be subjected to a killing frost if there is an unseasonably cold winter year. As a result (unless you are in very warm climates) you should lift all tender bulbs each fall before the soil freezes. If you are in zones 8 or higher you can actually leave them in the ground. In zones 7 you can mulch around the bulbs to help them overwinter. Special Note for Warmer Zones: Most spring blooming bulbs must have a cold period of about 3 months to initiate bloom. This can be done by storing in a refrigerator (do not freeze them). When storing bulbs you would generally adhere to the following: 1- Choose a container that is not airtight. Airtight containers can hold moisture and cause fungus and or rot. Additionally, check your bulbs regularly for mold and fungus. 2- Keep your bulbs dry and above freezing temperatures. 3- Label your bulbs so you can accurately replant them in the spring. When should tender bulbs be dug up for winter? It is usually advised to dig up your bulbs before the first killing frost. How should tender bulbs be dug up? Lift your bulbs with a spading fork and remove all excess soil and foliage. How to store your bulbs: Place your bulbs in a large plastic bag that will allow some air circulation (you could also use paper bags or a box as a possible container). You can store them in vermiculite, peat, sand, or a combination of those. Then store them in a cool frost-free place in your home that is around 40-50 degrees (do not freeze them or put in a place where it will drop below freezing). Note: During the storage process of bulbs, inspect them regularly and dispose of any bulbs showing signs of rotting. Bulbs, Corms, Rhizomes, Tubers & Tuberous Roots: It is worth noting that the term “bulb” is used to generically describe different types of bulb & root type systems. The following definitions demonstrate the differences: Bulbs: These are an underground storage system that consist of fleshy scale leaves surrounding a bud. Roots arise out of a basal plate and the bulbs have a covering of dry leaves called a tunic. Examples of true bulbs are both Lilies and Tulips.  Tubers: These have an enlarged underground stem with latent buds from which the plant then emerges. An example of Tubers is Dahlias. Tuberous Roots: They have an enlarged underground root with buds located near the collar or base the plants stem. An example of Tuberous Roots is Elephant Ears.  Rhizomes: These have an enlarged underground horizontal stem that is generally thinner and longer than tubers. They are generally located right at or just below the soil level. An example of a Rhizome is the Tall Bearded Iris.  Corms: These are very close and similar to true bulbs. They have a swollen base that acts as the food reserve and the buds are usually found on the surface of the corm. Examples of Corms are Gladiolus and Crocus.  More Information on Plant Hardiness Zones: There are several different plant hardiness zone descriptions (also known as growing zones or climate zones). The version many gardeners are familiar with is the USDA Hardiness Zone Maps. This map is broken into 11 temperature zones as follows: USDA Plant Hardiness Zone Map 
Zone 1: (below -50 F) or (below -46 C) Zone 2: (-50 to -40 F) or (-46 to -40 C) Zone 3: (-40 to -30 F) or (-40 to -34 C) Zone 4: (-30 to -20 F) or (-34 to -29 C) Zone 5: (-20 to -10 F) or (-29 to -23 C) Zone 6: (-10 to 0 F) or (-23 to -18 C) Zone 7: (0 to 10 F) or (-18 to -12 C) Zone 8: (10 to 20 F) or (-12 to -7 C) Zone 9: (20 to 30 F) or (-7 to -1 C) Zone 10: (30 to 40 F) or -1 to 4 C) Zone 11: (above 40 F) or (above 4 C) Outside the USA: If you live outside the United States you can figure out your hardiness zone by simply finding out how low the temperatures get in your area. This way you can still go by this method if making a purchase where the growth calculations are related to a particular plant using USDA measurements. Sunset Zones (Problems in the West versus East) In the United States there is another set of measurements that were created for the Western States called The Sunset Zone Maps. These are based on a 24 climate zone system created by Sunset Magazine, and are more precise for those regions than the USDA zones maps. They take into account:
The West has different circumstances that affect plant growth. Elevation and precipitation from weather coming in off the Pacific Ocean becomes gradually direr as it moves over mountain range after mountain range. Cities in the Eastern US can be in the same zones and have very similar growing conditions with weather. On the other hand, cities in the West can be in the same USDA zones but have a very different set of conditions for growing
In the Western United States you’ll find garden centers referring to the Sunset Zones more often than the USDA Zones. The reason USDA Zones are still important nationwide in the USA is that it’s the standard large growers use in general when selling seeds and flowers.
Micro-Environments:
Within every climate zone there are small weather environments unique to the area. Even in your own back yard there can be multiple “micro-environments” depending on:
In addition to natural “micro-climates” you can artificially create these by planting shade trees, putting in large rocks, adding a garden pond, and many other gardening actions to alter your area.
The really neat thing about this is you can have plants and flowers from different hardiness zones growing in your garden at the same time. Examples of climate growing zones from around the world are:
America’s:
Australia:
Europe:
Plant hardiness zone maps are great in general for determining which plants you can grow in the region you live. There are some disadvantages to this method of mapping. The USDA Hardiness Zoning only takes the average minimum temperatures a region will encounter to come up with their mapping.
Why is this an issue? There are several factors beyond pure temperatures that go into whether a plant or flower will grow and thrive in a particular region. Other important things in addition to temperature affecting plant growth are:
2 Popular Bulbs, Tubers 1- Elephant Ears:Elephant Ears are tuberous roots that typically prefer rich, well draining soil and partial to full shade. Elephant Ears stop growing if temperatures go below 10 degrees F. To store Elephant Ear tubers over the winter dig them up after the first fall frost. Allow the tubers to dry out for about a week and then store them in a dry potting medium. 2- Dahlia Bulbs Storage:Dahlias do not technically have bulbs. They are tender (non-hardy) tubers that are kind of like bulbs. Since they are in the tender category Dahlias will not survive winter frost and should be protected from freezing to ensure they will regenerate the following spring. The Dahlia bulb is composed of many distinct tubers with each having a separate lump. In order to sprout next season each tuber must have at least one eye (the whitish shoot) that will grow vertically. This shoot will break up through the soil and turn into stems, leaves and blossoms. To keep Dahlias, you must dig up the tubers before the first hard frost freezes the ground and potentially kills the bulb. Keep part of the dried stem but you can remove dead blooms and smaller roots. Store tubers away from sunlight with some moisture. A temperature of about 35-45 degrees is recommended but you are safe with 50 degrees on most bulbs. You can wrap the tubers in vermiculite or sawdust to help preserve moisture. When replanting in the spring do so with the eye facing upward. Note: Some varieties of Dahlia are hybrids and can be forced to bloom indoors during the winter months. |