USDA Cold Hardiness Zone Map

USDA Cold Hardiness Zone Map

  As gardeners, speaking for myself at least, we always want to try new, exotic, and/or unusual plants and often that entails gardening on the edge of a plant’s USDA Cold Hardiness Zone rating. We happily garden in zone 8B, the warmer half of zone 8, and so we see winter temperatures here that on average fall to about 15oF. Unlike parts of zone 7 and colder winter regions, our soil does not typically freeze to any real depth. This is probably one of the most limiting factors for tropical species, making these showy plants one more possibility for our gardening palette. The more that you know about the species or cultivars that you are working with, the easier it will be to successfully get them through to spring.

Lemon Leaf Bottlebrush with snow and ice, Del Rio, TX


Lemon Leaf Bottlebrush with snow and ice
 Del Rio, TX

Plant cold hardiness ratings are typically set for well-established garden and landscape plants. Many of the hardiest and most reliable tropical plants can easily be overwintered successfully as living crowns beneath a layer of mulch even though the top portion of the plant may freeze. When writing about these types of tropical plants, I often refer to them as “die-back perennials” because the tops literally freeze and then die-back to the crown each winter, but they are still able to become a beautiful and productive part of the garden the following season. Examples are Angel’s Trumpets or Brugmansias (some but not all Daturas), Turks Caps, Confederate Roses, Lantana, and most common types of Hedychiums – like the White Butterfly Ginger. Lantana, for example, is actually a tropical shrub that freezes back to near ground level in our zone 8 gardens and then typically returns from the crown in spring once soil and air temperatures have warmed sufficiently enough for it to initiate growth. Some of the hardier Lantana cultivars, like Miss Huff and the Chapel Hill© series, have proven to be reliable even into zone 7. Crape Myrtles and Vitex develop into large shrubs or small trees here in zone 8 but in zone 6 these same plants are grown as die-back perennials, like our Lantanas here in 8B. Plants like these that are to be grown on the edge of their hardiness ranges should be planted out as early in the growing season as possible to ensure that they have a strong crown and well-established root system with enough resources and vigor with which to be able to successfully resprout the following spring. A secondary option is to overwinter young plants in containers to give them that much more time to develop a strong healthy crown and supporting root system before adding them to the garden and landscape the following spring.

El Capitolio Tropical Hibiscus

El Capitolio Tropical Hibiscus

  Some well-established tropical and subtropical plants may be able to successfully overwinter and regrow but may not perform as well as we might hope the following growing season as garden or landscape plants. One common example of this is the Tropical or Chinese Hibiscus, Hibiscus rosa-sinensis, including many of its cultivars and hybrids. Some of the more vigorous types of Tropical Hibiscus, like El Capitolio shown at left, may survive a zone 8 or 9 winter but may not grow vigorously enough to flower much, if at all, before fall and winter arrive again the following year. If you only have occasional winters that produce freezing temperatures, then this may be acceptable as most years these plants will likely live up to their potential and then when a hard winter does come these plants should hopefully still survive it. Otherwise, it may be necessary to grow these cold sensitive tropical plants in containers and provide them with a warm sunny window or other freeze and frost-free area for winter.

Container Gardens at Longwood Gardens

Container Gardens at Longwood Gardens
October 10, 2010, Kennett Square, PA
 USDA Cold Hardiness Zone 7A

 Not all tropical species can be successfully overwintered in the ground here in our zone 8B gardens and so must be maintained as container plants. Containerized plants, including many plants that may be considered reliably cold hardy in your zone, may need to be protected from extended periods of freezing temperatures. Even though temperatures may remain above the plant's known cold hardiness rating, soils in containers are more susceptible to freezing, thus potentially freezing and killing the tender root system and crown buds of what may be an otherwise reliably hardy garden or landscape plant. It may seem obvious, but it is important to remember that small containers can freeze solid much more quickly than large containers and so even a few hours of below freezing temperatures can be devastating for some of the more sensitive tropical species. Containers that are in good contact with the ground are generally more resistant to freezing than those that are on raised decks, porches, patios, and tables. In the case of containerized young cold hardy species, if that species or cultivar can handle zone 3 or 4 winters then you can be fairly confident that it can handle extended bitterly cold periods as they have proven to be able to survive in these areas where the soil can freeze to significant depths. As container plants, these very hardy plants can probably handle about anything that an average zone 8 winter can throw at them. If a plant’s cold hardiness range is limited to zones 7 or 8 to 10, it may be because the root system of that plant is unable to tolerate frozen soils. Cold hardiness ratings often vary from resource to resource for a given plant so be sure to read the fine print. In many cases, information may be gleaned from other gardeners or horticulturists who have experience with that plant under similar growing conditions as your own.

 Cold in combination with wet soils can be the death of tropical and young newly established plants and so good to excellent drainage can be critically important. Raised beds and sandy well-drained soils can go a long way toward providing optimal conditions for winter survival when it comes to cold sensitive species. This also provides a gardener or landscaper with a broader range of plants to utilize as many more plants are tolerant of well-drained soils than those that can handle heavy, wet winter sites. We’ll discuss why good soil drainage is so important for many plant types in extensive detail below. Another thing to keep in mind is that these plants, especially the more tropical types, may not even consider initiating spring growth until soil and air temperatures are appropriately warm and the days have begun to lengthen so don’t give up on them too early. For some tropicals and even the cold adapted Hardy or Perennial Hibiscus it can be late April, May or even early to mid-June before they start to show themselves.

  For most of the more reliable die-back perennials and for young newly established cold hardy species a mulch is often necessary to insulate and protect the tender buds of the crown from freezing temperatures and exposure to repeated frosts. The dead winter stems of die-back perennials may also provide some additional protection for dormant buds and so, in our own gardens, we typically wait until spring growth initiates before cutting away the dead upper portions. This can also give you a leg up after a mild winter as the plant may not have frozen back as far as normal, providing more living growing points from which they can produce more vigorous early season growth. A protected site can offer additional protection, like on the leeward or downwind side of a structure, that may help to protect them from the coldest harshest drying winter winds. An overhanging eave or evergreen tree or shrub can provide additional protection from repeated or late season frosts.

 It may seem counterintuitive but plants in moist soils are actually more resistant to cold weather extremes than those that are in dry soils. The moisture acts as something of an insulator and fully turgid (or plump and water filled) plant cells are generally more resistant to freezing temperatures than those that are drought stressed. For both containerized plants that will remain outdoors and in-ground garden and landscape plants, be sure to water thoroughly a day or two prior to an upcoming freeze to ensure that the plant's root system will have time to draw in needed moisture as uptake can be slower when temperatures are colder. Another reason to keep your plants moist, never soggy wet, is that cold drying winds can pull moisture from exposed plant parts creating drought stressed conditions even in an otherwise icy landscape.

Arrowwood Viburnum, Viburnum dentatum, fall color

Fall Color of Arrowwood Viburnum
Viburnum dentatum
 USDA Cold Hardiness Zones 2-9

 Fall, winter, and early spring are without a doubt some of the easiest times to establish reliably hardy perennials and woody species in zone 8. This has a lot to do with the fact that the demand for moisture tends to be lowest at these times of year due to slower top growth as well as the fact that their root systems, in these areas where soils generally do not freeze, can continue to grow all winter making them better able to withstand the coming stresses of spring and summer. For tropical and temperate species that are grown as die-back perennials in your area, establish them as early in the season as possible to ensure the development of a large established root system and crown, from which they will be able to vigorously return in spring. Wait to plant these until all danger of freeze and frost have passed and once soil temperatures have begun to warm significantly. In our area, that usually means waiting until about mid-April or early May and the most reliable of these hardy tropicals can be successfully established until about midsummer. With proper preparation and a deeper understanding of a specific plant’s needs, we can get many tropical species through winters beyond their cold hardiness zone. Enjoy the adventures of gardening on the edge!

Adapted from: Almost Eden’s 2020 Fall Newsletter

Textual version of the link: Eden's 2020 Fall Newsletter.pdf

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Thank You & Good Growing,

John, Bonnie, & Jeff McMillian

& the Crew at Almost Eden

1240 Smith Rd

Merryville, LA 70653