The unusually hot and dry weather we have experienced this winter is causing consternation in some quarters.  What if there should suddenly be a deluge of rain or a cold snap?  Would this radical weather change cause damage to plants that have bloomed or begun spring growth prematurely due to the unseasonably balmy weather we have experienced to date?

Here, it will be useful to mention criteria for flowering.  There are a number of factors that determine when a plant will bloom.   It is important to know what these factors are in order to understand whether your plant’s flowers, or lack of them, are the result of warm winter weather, drought, or some other factor.

Many plant species flower in response to day length.  Thus, you have long-day plants that flower in spring and summer, short-day plants that bloom in fall and winter, and day-neutral plants that may bloom in any season.  Long-day and short-day bloomers will not be much affected by weather conditions but flower mainly in response to the number of daylight hours.  An interesting application of this principle is brought to bear in the case of florists’ chrsyanthemums, which are naturally short-day plants yet may be found in full bloom at your local supermarket throughout the year.   Greenhouses with blackout systems manipulate the number of sunlight hours available so that the chrysanthemums think its fall when it’s actually spring or summer.

When it comes to deciduous fruit trees – apples, peaches, and apricots, for example – a warm winter will lead to irregular and diminished flowering and fruit production.  The reason for this is that deciduous fruit trees (and deciduous ornamentals, too, such as lilacs) depend upon winter cold to synthesize metabolites required for flowering.  Each apple variety, for example, has its own winter cold or chilling requirement, quantified as the number of annual winter hours when the temperature is less than 45 degrees.  When this number of winter chilling hours is reached, once temperatures have begun to warm again, flowering will begin.  You cannot grow ‘Gala,’ ‘Fuji,’ or ‘Winesap’ apples in the San Fernando Valley because the winter is just too warm.  Instead, low chill varieties such as ‘Anna,’ ‘Beverly Hills,’ and ‘Dorsett Golden’ are cultivated here.

Deciduous fruit trees, lilacs, and roses, when they are pruned early in the winter, can be adversely affected by a sudden spell of warm weather.  The reason for this is that pruning is an invigorating process.  Terminal shoot buds produce hormone that is continually sent down the shoot, inhibiting growth of lateral buds below.  When these terminal buds are snipped off, growth of lateral buds is no longer inhibited and side shoots begin to sprout.  During winter, this process is put on hold by cold temperatures.  However, if winter pruning is following by a sudden heat wave, lateral buds will open.  This new growth is tender, however, and may be killed if temperatures turn cold again.  For this reason, it is wisest to prune fruit trees in Los Angeles after February 15^th since it is unlikely that there will be frost, when temperatures dip to 32 degrees, after this date.

Rain and cold in late winter or, for that matter, early spring, can be damaging to fruit tree crops and tender annuals.  One year, my plum tree was covered with blossoms when there was a sudden storm of spring rain.  The rain knocked some blossoms off the tree and caused others to develop a fungus so that I had little fruit that year.  Damp pollen will also discourage visits from bees whose pollinating activity is essential to fruit development.  Where annuals are concerned, impatiens and begonias should probably not be planted for another month since they are sensitive to cold and rain.  At this time of year, it is not unusual to see freshly planted impatiens and begonias turn to mush after a single heavy rain.  Established impatiens and begonias, on the other hand, that were planted at the end of last summer, will make it through February rain and cold, short of a freeze, just fine.

Speaking of cold weather, it is not just the temperature but the duration of it that aversely affects flowers and fruit.  Most types of citrus, for example, can withstand an hour of two of frost.  However, should the temperature stay at 28 degrees or colder for four hours or more, significant damage may result.  If a frost is predicted, it is a good idea to turn on irrigation since, as water on plant surfaces turns to ice, heat is released and plants are spared.

Finally, acclimation of newly planted specimens is another factor that can curtail flowering.  When you bring home from the nursery a perennial that is covered with flowers, it often happens that those flowers quickly disappear with none to take their place.  You may not see any more flowers this year or the next, or the year after that!  This often happens with fruit trees, but it may happen with flowering ornamentals, too.  You purchase an avocado or an orange tree with several fruit on it, plant it in your yard, and don’t see another fruit for several years.  The reason for the delay is that the tree is adjusting to your particular microclimate, which includes such factors as soil type, light exposure, fertilization, and watering regime.  In their growing grounds, before they reach the retail nursery or home improvement center, plants enjoy a steady feed of water and minerals.  A fertigation apparatus may be employed so that a small dose of liquid fertilizer is injected into the irrigation system each time watering takes place. In addition, foliar fertilization with micronutrients could have been practiced, another technique for producing lush, retail ready perennials and fruit trees.  Unless you continue these practices after your plants are brought home, you are likely to see a decrease in flowering and lush growth.

Tip of the Week:  Jeffrey Carlson, who gardens in Redondo Beach, emailed asking about a non-toxic treatment for control of the giant whitefly insect pest.  Endemic to Oaxaca and Guadalajara, Mexico, giant whitefly arrived in California in 1992.   Locally, hibiscus is giant whitefly’s favorite host plant, but it may also take up residence on ‘Richmondensis’ begonia, giant bird of paradise (Strelitzia nicolai), orchid tree (Bauhinia spp.), banana, citrus, and many other species.  Research has shown that, after a plant has been thoroughly pruned out to remove most of the whiteflies, a weekly syringing or blast of water on the underside of leaves is the best control.  Eventually, water blasts two or three weeks apart may be adequate.  This control measure was more effective than any chemical treatment that was applied.  It is also recommended that plants be mulched with earthworm castings and fertilized with a slow release or organic fertilizer, as a steady mineral feed will keep plants at optimum health and non-stressed plants are less susceptible to whitefly predations.