Richard Bogren, Owings, Allen D.
News Release Distributed 01/10/14
By Allen Owings
LSU AgCenter horticulturist
HAMMOND, La. – Cold weather in early January has the potential to cause varying problems in our landscapes from north to south Louisiana. We have recently seen damage in our garden trials at the LSU AgCenter Hammond Research Station.
With some isolated single-digit temperatures in north Louisiana to record-breaking cold along the Interstate 10/12 corridor, the damage to some landscape plants will be obvious in the short term. Other plants, however – mainly woody ornamental shrubs – may not show damage until spring.
Winter damage primarily occurs on tropical plants, which cannot tolerate cold weather. Unusually severe cold can sometimes damage normally hardy plants, such as azaleas. Mulching with leaves, pine straw or similar materials is a great way to reduce freeze damage to roots and lower stems. Covering tender plants with fabric or plastic sheets is also a common preventive measure.
To minimize the need to protect tender plants over winter, plant tropicals sparingly and focus primarily on plants that are reliably winter-hardy. Proper pruning, fertilization and irrigation are also important in reducing cold damage. Make sure plants susceptible to cold damage are not drought-stressed during fall.
We mostly think about citrus trees when cold weather approaches. Satsumas and kumquats are most cold-hardy of the citrus commonly grown in Louisiana. Orange, grapefruit, lemon and lime trees are less cold-hardy. Many home gardeners wrap trunks, cover plants with blankets, build temporary greenhouse-like structures or coat trees in ice to protect citrus when temperatures lower than 25 degrees threaten.
Many of our common landscape trees and shrubs are cold hardy down to 10 to 15 degrees. This does not mean that damage will not occur with temperatures in the upper teens or lower 20s. The good thing about this recent freeze is that it occurred in January, and we had already had some cold weather in November and December. Typically, we see less damage with a January freeze than with freezes in late February and March.
Azaleas are our most commonly planted landscape shrub and are generally cold-hardy. Almost every year, though, you can find some isolated cold damage on certain varieties of this plant. Normally bark split occurs when azaleas are damaged by the cold. These symptoms are often sectional as well, with some parts of the plant staying green and other parts dying. Damage shows up in April and May after spring growth commences.
Cold causes internal damage that affects the plant's circulatory system. In other words, it interferes with the plant's ability to move water through the branches and into the leaves.
Expect to see considerable damage on woody tropical-type plants. Ixora, cassia, copper plant, plumbago, tibouchina and hibiscus will be damaged. It is best to wait until new growth commences in the spring, then prune those that are still alive to the point where new growth starts.
You can also check underneath the bark of these plants by scratching the bark with a fingernail. Green tissue indicates wood that is alive. Brown, black or tan indicates dead plant tissue.
One positive outcome of the cold weather is chilling hour accumulation, which is above average statewide for this winter. Many fruit trees and some landscape plants require a certain number of chilling hours during the dormant season. Hostas and dormant, semi-evergreen daylilies benefit from having 500-600 hours of chilling during the winter instead of the 300-400 hours south Louisiana normally receives.
Chilling hours accumulate when temperatures fall below 45 degrees. The most important months for chilling hours are November through February in Louisiana. Temperatures above 70 degrees during this time offset accumulated chilling hours. Through January 7, we have had 450 chilling hours at the Hammond Research Station this winter.
Patience is the key as we wait to see how our landscapes recover over the next couple months. Spring growth will be a good indication. Wait to prune, if possible. In the future, plan to water plants, move plants, cover plants and mulch plants prior to a severe cold period.
You can see more about work being done in landscape horticulture by visiting the LSU AgCenter Hammond Research Station website. Also, like us on Facebook. You can find an abundance of landscape information for both home gardeners and industry professionals at both sites.