After a record warm fall and record breaking December gardeners cannot help but wonder how the unseasonal weather will affect their gardens. We have enjoyed an almost continual stream of mild temperatures.
The weather is thanks to El Nino. An El Nino weather event is caused when temperatures in the Pacific Ocean become so warm that they distort weather patterns around the world. The periodic warming of Pacific waters is not caused by climate change. However, warmer ocean temperatures caused by climate change contribute to El Nino’s magnitude.
The gradual arrival of seasonal winter temperatures should have little effect on our plants, but a hard freeze may kill the buds on the plants that have budded out early. Warm winter can cause buds on spring blooming plants to swell, an indication they are preparing to bloom. At this stage they are unable to harden off and the buds are likely to die and drop off with a hard freeze. It depends on how suddenly cold weather arrives, how low the temperature drops and how long the freeze lasts.
Plants need to harden off and gradually acclimate to cold temperatures. Some plants are more susceptible to abrupt change than others. The worst case result may be fewer spring flowers on certain plants and a reduced fruit crop. Warm temperatures don’t kill healthy plants.
Many plants require a chilling period (aka vernalization) to produce fruit or bloom at their best. A plant’s chilling requirement is the number of hours necessary, generally at 32 to 45 degrees, before it breaks dormancy and flowers. For some plants the chilling requirement is below 60 degrees, for others, below freezing. Hours need to be cumulative, not necessarily consecutive. Most perennials benefit from a chilling period. Some won’t bloom without it. Lack of adequate chilling time can result in stunted, smaller or fewer flowers. You may wonder how plants keep track of the hours. Scientists have not yet figured that out.
Non tropical fruits have a chilling requirement. The hours and temperature vary with variety. Fruits are particularly vulnerable to temperature fluctuations. If trees and bushes bloom too early a freeze will kill the swollen buds and flowers, and that will result in no fruit crop. Keep track of the buds on your fruits like grapes, raspberries, peaches, apricots, cherries and blackberries, buds are visible all winter.
Biennial vegetables (many are root vegetables and greens) are typically grown as annuals. They produce their crop the first year, but do not bloom and set seed unless they are chilled and then exposed to warmer temperatures and longer days. They will then bolt and produce flowers and seed in their second season. You need to know this if you collect their seeds.
The amount of chilling varies with species. FYI: Artichokes, beets, broccoli, Brussels sprouts, cabbage, carrots, cauliflower, chard, celery, collards, endive, kale, kohlrabi, leeks, onion, parsley, parsnips, rutabaga, turnips and salsify are all biennials.
The most notorious flowers for requiring vernalization (chilling) are bulbs. If you grow foxgloves, hollyhocks, Sweet William or Canterbury bells, you likely know they are biennials. They will not flower unless they have undergone winter’s cooling. The same is true for perennials campanula and aquilegia.
Cold winters kill insect eggs, pupae and overwintering insects, which means that we can expect more insect pests after a warm winter. Not only insect pests but diseases controlled by cold can also be more problematic. Weeds, too, will be more plentiful because fewer seeds have been killed by winter cold.
If warm winters become a regular event we risk that the life cycles of beneficial insects will become out of synch with the insects they previously managed. Some pollinator life cycles, too, may not coordinate with bloom time.
We don’t know what the rest of the winter will bring, or whether next winter will be unusually warm or cold. If you don’t already keep garden notes start a journal. Our weather is not likely to stabilize anytime soon. Keeping track of weather extremes and how they affect our plants will help us learn to adapt our gardens for the future.
Reach Debbie Menchek, a Clemson Master Gardener, at firstname.lastname@example.org.