Frost Damage Is Not Cool

Frost damage is a cold reality.

Frost happens. It may not happen every winter. It may not happen everywhere. For a few of the mildest climates, it may not be a major concern. For some climates though, it might have potential to cause significant problems. The best means to avert frost damage is to avoid plants that are vulnerable to a degree of frost that is normal for a particular climate.

Of course, that is not as simple as it sounds. Even in mild climates, angel’s trumpet might get shabby from chill that is not cold enough for frost. Where weather gets cooler, familiar plants such as bougainvillea, avocado, lemon, fuchsia and pelargonium may experience frost damage. Such plants necessitate certain precautions, and must assume innate risk. 

Some plants that are susceptible to frost damage can live in portable pots that can move to sheltered situations prior to frosty weather. Some might be houseplants that live in the garden for part of the year, but come inside at least for winter. Eaves, particularly if above walls of heated buildings, may be adequate protection for marginally susceptible plants.

Plants that are susceptible to frost damage, but live in the ground or are too big to move, may need temporary protection from frost. Such protection might consist of tarps, burlap, old sheets, plastic trash bags or cardboard, suspended above by stakes and string. Thin materials, such as sheets or trash bags, can freeze through, so should not touch foliage.

Protective tenting materials should not remain over sensitive foliage for too long. Ideally, they should be in place immediately prior to frost, and then gone immediately after. Since frost occurs at night here, protection is useful only overnight. During daytime, it obstructs sunlight, but collects heat to stimulate new growth that is more sensitive to frost damage.

Many plants are too big to protect. Fortunately, bigger plants are less susceptible to frost damage than smaller plants. If possible, outer foliage that succumbs should remain until the local last frost date. Although unsightly, it shelters inner growth. Moreover, premature removal of frost damage stimulates new growth that is even more susceptible to subsequent frost damage.

Frost Is Different From Chill

Frost has already stricken some regions.

‘Chill’ could almost be a pleasant euphemism for ‘frost’. Both words describe cool or cold weather that occurs during winter. The obvious difference is that one is good, and one is bad. Chill is a minimum duration of cool weather that some plants require through winter to maintain their schedules. Frost is weather that is cool enough to damage some plants. 

This technical difference is that chill is at or below forty-five degrees Fahrenheit (≤45°F), and frost is at or below thirty-two degrees Fahrenheit (≤32°F). That is a difference of only thirteen degrees Fahrenheit (13°F), just above the freezing temperature of water. Climate and weather are simply not cooperative enough to comply to such precise technicalities.

Climates that provide sufficient chill for many plants that require it are also likely to inflict frost on occasion. Where chill is sufficient for most plants that need it, frost is likely much too severe for plants that are sensitive to it. Climates that lack frost are unlikely to provide enough chill for plants that need just a bit. High chill apples and oranges should not mix.

Of course, just as various chill dependent plants require various degrees of chill, various frost sensitive plants tolerate various degrees of frost. Some orange cultivars can survive frost as cold as twenty-five degrees Fahrenheit (25°F) within climates that low chill apple cultivars are happy with. Angel’s trumpet though, succumbs as soon as ice crystals form. 

It is helpful to know which plants are sensitive to frost, even in frostless climates. Tropical plants might get rather pallid when the weather is too cool for too long, even if frost is not a direct threat. Potted plants are more susceptible to frost damage than they would be in the ground, but can migrate to sheltered situations. Some can be temporary houseplants. 

Frost naturally limits the selection of plants. Various tropical plants that are appropriate to frostless climates are not appropriate for climates with harsh winter weather. That can be confusing with so many distinct climates within such minimal proximity. Coastal, inland, mountain, and all climates are so very different from each other.

Chill Helps Plants Distinguish Seasons

Spring bulbs know what chill means.

The Santa Clara Valley was formerly famous for stone fruit orchards. The San Fernando Valley was formerly famous for citrus orchards. The Wenatchee Region of Washington is still famous for apple orchards. Many variables influence which agricultural commodities grow or grew in each region. Of these, one of the more obvious is temperature and ‘chill’. 

Citrus could not be productive in the Wenatchee Region because it could not survive the chill of winter there. Although productive in home gardens of the Santa Clara Valley, it is not quite as productive as it is where winter is warmer in the San Fernando Valley. Citrus appreciates warmth but not chill. Many tropical and subtropical plants feel the same way. 

However, many of the numerous cultivars of apple that are so famously productive in the Wenatchee Region would be unproductive in the Santa Clara Valley. Only a scant few of these might produce relatively meagerly in the San Fernando Valley. Unlike citrus, which dislike chill, apple trees need chill for production. Some cultivars need more than others.

Chill regulates the schedules of many plants that are originally endemic to climates with cool winters. It confirms the occurrence of winter, which is a convenient time to finish one annual cycle and begin another. Some plants require only a minimal chill. Those that are from climates with harsh winters require coercion by more significant and sustained chill.  

One of the primary reasons that spring bulbs are available for planting early in autumn is that some benefit from spending winter in a cool and damp garden. Although, most bulbs are chilled prior to marketing. Some previously chilled spring bulbs bloom splendidly for their first season, but then bloom unsatisfactorily if chill is inadequate for them afterward.

Most plants that need more chill than they can get locally are simply not available locally. Some are available online though. A few locally popular plants that are marginal for mild climates get confused by the seasons. This is why some flowers such as torch lily, bloom randomly or out of season. Daphne and some late winter flowers may be blooming now, and could get damaged by frost later.

Plants Know What Time It Is

Deciduous trees will eventually begin to defoliate.

Even without significant cool weather, the garden knows that it is now autumn. Most of the late summer blooming flowers are finishing their last bloom phases. Leaves of some of the deciduous trees, shrubs and vines are changing color, and some are already falling. Perennials that are dormant through winter are starting to deteriorate.

One of the several difficulties of living in a climate with so few difficulties is that autumn and winter weather is so very mild. Just as so many warm season annuals and vegetables want to continue to perform when it is time for them to relinquish their space to cool season annuals and vegetables, many other plants that should go dormant in autumn really want to stay awake as long as they can. Some semi-deciduous perennials even start to regenerate new growth before they shed their old growth.

Where winters are cooler, such plants generally shed the growth that developed in the previous year; in other words, they die back. They then stay dormant through the coolest part of winter, to break dormancy and regenerate late in winter or early in spring.

Beard tongue (Penstemon) can really look bad as the last flower spikes deteriorate, and the foliage gets spotty and grungy. It will be tempting to cut them back early. If possible, it is better to prune off only the deteriorating flower spikes, but wait until later in winter for major pruning. Premature pruning stimulates premature development of new growth that does not mature as well or as fast through winter as it would in spring. Such growth can be discolored, sparse and less vigorous until it gets obscured by later growth.

Marguerite daisy, ginger, canna, some salvias, most begonias, the various pelargoniums and all sorts of other perennials will likewise seem to be rather tired this time of year and through winter, but do not necessarily need to be pruned back just yet. Simply plucking or shearing off deteriorating flowers should be enough for now. Ginger and canna should not need to be pruned back until the foliage deteriorates enough to be almost unsightly. Begonias and pelargoniums, particularly common zonal geraniums, will be better insulated from potential frost damage through winter, and may not produce so much sensitive new growth if not pruned early.

Cooler Weather Is Slower Weather

Cooling weather can damage new growth.

Weather is not quite as warm as it had been. Warm days do not last quite as long as they did earlier in summer. Afterward, the longer nights get a bit cooler. Technically, autumn is only a few days from now. Although seasonal changes are mild, and a bit later here than in other regions, they eventually catch up. Plant activity has already been getting slower.

Seasonal changes keep gardening interesting. Plants that are now growing slower than earlier may need less attention. However, some need more attention, precisely because they are growing slower. Some of the work that was so important through summer should conclude until spring. Some of the work that will be important through winter begins now. 

Although evergreen, photinia and pittosporum hedges do not do much between now and next spring. If shorn too late, new growth develops slowly, and may become shabby as a result of cooler and rainier weather later. Late pruning of citrus stimulates vigorous newer growth that may be sensitive to frost through winter. Lemons are particularly susceptible.

Conversely, dormant pruning can begin as deciduous foliage starts to fall. Although most roses and fruit trees supposedly prefer to wait until winter, they may soon be too dormant to notice if pruning is a bit premature. This is partly why autumn is the season of planting. Mostly dormant plants are more resilient to discomforts than they would be while awake.

New Zealand flax, lily of the Nile, African iris and other rugged perennials are conducive to division now. They will soon be about as dormant as they get, but will want to disperse roots for winter anyway. They resume growth before winter ends, so want to be ready for it. Once rainier and cooler weather resumes, they will need no watering until next spring. 

Fertilizer should be passe soon also. Most plants consume less nutrients through cooler weather. Besides, many nutrients are less soluble, and therefore less available to plants while the weather is cool. Turf, cool season vegetables, cool season annuals, and some small palms are a few exceptions that could benefit from minor applications of fertilizers.

Humidity Is The Other Weather

Some delicate foliage prefers more humidity.

It is difficult to always ignore the weather. Regardless of how pleasant it typically is here, it sometimes gets warm or cool. It occasionally gets hot or cold. Rain is wet and perhaps messy. A breeze is comforting while the weather is warm. Strong wind can be damaging. However, humidity is one major component of local weather that gets little consideration. 

Humidity gets more consideration in climates that are either uncomfortably humid or arid. Some parts of Florida get famously humid and hot simultaneously during summer. Some flora and insects enjoy such weather. Unfortunately for the rest of us, humidity enhances the already unpleasant heat. Locally, hot or warm weather is rarely bothersomely humid.

Similarly, local weather is rarely unpleasantly arid (lacking humidity). This is a chaparral climate, which is ‘semiarid’. Relatively minimal humidity makes heat a bit more tolerable than it would be with more humidity. Yet, humidity is generally sufficient to sustain foliage that would desiccate in a more arid desert situation. Actually, this is an excellent climate.

Although, it is not perfect. Flora and fauna have different standards for exemplary climate and weather. The relatively minimal humidity that makes uncomfortably warm weather a bit more tolerable for people and animals is much less appealing to some plants. Except for those that are native to desert or chaparral climates, most plants prefer more humidity.  

Many popular plants are understory plants, that naturally live in the partial shade of taller vegetation. With shelter from desiccating arid wind and harsh sunlight (to enhance heat), most do not mind heat. Otherwise, foliage might roast. Those with finely textured foliage, such as astilbe, ferns, grasses and some Japanese maples, are particularly susceptible.

Some tropical and subtropical plants, such as split leaf philodendron and fuchsia, prefer to be understory plants here, even if they would prefer more exposure within their natural ecosystems. The shelter provided by more resilient vegetation compensates for deficient humidity. Furthermore, adequate irrigation promotes healthy hydration of delicate foliage. 

Succulents For Better Or Worse

There are all sorts of succulents.

Succulents, both old and new, have been something of a fad for quite a while now. There are certainly many reasons for them to be popular. They add bold form, texture and color to the garden. Most adapt excellently to container gardening. Many types are resistant to pathogens. Succulents are generally easy to maintain and equally as easy to propagate. 

However, one of the primary premises of the increased popularity of succulents is simply untrue. Contrary to popular belief, not all succulents are drought tolerant. Only those that are naturally endemic to desert or chaparral climates can survive with minimal irrigation. They neither expect nor require any more moisture than the rain that falls through winter.

Many succulents are naturally endemic to climates that are not arid. Some are even from tropical rain forests. Such succulents rely on watering to compensate for the local lack of rain through the long and warm summers. Furthermore, many succulents from chaparral and desert climates also want water if their undispersed roots are confined to containers. 

Many popular succulents are cacti. They lack foliage, and are instead armed with spines and thorns. (Spines are modified leaves. Thorns are modified stems.) Their fat succulent stems are green to compensate for their lack of foliage, by conducting all photosynthesis. Generally, most cacti actually are tolerant of drought, although less so within containers.

However, many of the most popular and trendy succulents have succulent leaves as well as succulent stems. Some obscure their stout stems within densely set foliage. Although some are chaparral plants that are somewhat drought tolerant, many require watering for adequate hydration. Even Epiphyllum, which are tropical cacti, require regular watering.

Furthermore, many of the succulents that can survive through dry summers without water take drastic measures to do. Various species of Aeonium and Echeveria let much of their older foliage shrivel to conserve moisture for the younger foliage. Echeveria retain much of their shabby dry foliage as insulation. So many of the succulents that have potential to survive without watering are happier with it.

Cold Winters Have Certain Advantages

Winter frost can improve spring bloom.

Even in May, damage from the frosts of last winter is still evident among some of the more sensitive plants. Lemon trees and bougainvilleas that have not yet been pruned may still display bare stems protruding above fresher new growth. Some bougainvilleas did not survive. Those that are recovering will bloom later because replacement of foliage is their priority for now.

However, peonies, although rare, are blooming better than they have since 1991, right after one of the worst frosts in recorded history. Earlier, some lilacs, forsythias and wisterias likewise bloomed unusually well. While so many plants were succumbing to cold weather, many others were enjoying it.

The reason that peonies typically do not seem as happy locally as they are in severe climates is that they really prefer more cold weather while they are dormant through winter. Without it, they may not go completely dormant, or may not stay dormant long enough to get the rest that they need; and then wake up tired in spring. Cold winter weather promotes more adequate dormancy, which stimulates healthier growth and bloom this time of year. It all makes sense considering that peonies are naturally endemic to colder climates.

A preference for cooler winter weather is also why so many varieties of apples and pears that are grown in other regions do not perform well here. The good news is that as the fruit of local apple and pear trees matures through spring and summer, it may be of better quality that it has been for many years. Trees that typically produce sparsely may produce more abundantly this year. The weather that damaged tropical and subtropical plants was an advantage to others.

Nothing can restore cherries, apricots, peaches and any other early blooming fruit that was dislodged by late rain, but any fruit that survived may be just as unusually good as apples and pears. Not only is the best place for gardening, but minor weather problems can have certain advantages.

Natives Are Right At Home

Some native plants should stay wild.

Long before people came here and imported exotic (non-native) plants from all over the world, native plants had been perfectly happy without any pruning, watering or fertilizing. They had always been perfectly happy with local soils, local climates and even occasional wildfires. Many are still happier in the wild than in seemingly more comfortable refined gardens and landscapes.

It really makes sense though. Most exotic plants need to be watered because they are from climates that naturally get more rain. Some want to be fertilized because they are from regions with different soil types. Some plants prefer cooler winters. Others want more humidity. They crave what they would get in their respective native homelands.

However, plants that are native to California are not necessarily native to here. California is a big place with all sorts of climates and soils. For example, the desert fan palm that is native to warm and dry Palm Springs would not be happy in cool and foggy San Francisco. Big leaf maple that likes the cool winters of the Siskiyous does not like the mild winters near the coast of Los Angeles. The best natives are those that are native to a particular region, or similar region.

Also, there are a few native plants that are not so easy to accommodate in every home garden. Both the giant sequoia, which is the biggest tree in the world, and the coastal redwood, which is the tallest trees in the world, are native to California. Even if the local climate is a good fit, the space available may not be.

One of the most difficult problems for so many natives though, is that they are sensitive to the regular watering that most exotic plants require. The regular watering that lawn needs just to survive is enough to rot the roots of plants that do not expect any water between spring and autumn.

Santa Barbara daisy, penstemon and various salvias are some of the favorite native perennials. Wax myrtle and the various ceanothus and manzanitas are interesting shrubbery. Western redbud and toyon can be big shrubs or small trees. California sycamore and various oaks are big trees for big spaces.

Last Frost Dates Help Scheduling

Frost is unlikely until next autumn.

Frost is not as much of a concern here as it is in other climates. It is very rare in some of the coastal climates of Southern California. The potential for frost damage increases farther inland, farther north, and at higher elevations. Regardless, it is generally tolerable locally. Even if it is necessary to protect a few marginal plants prior to frost, the ‘average last frost date’ gets little consideration.

The average last frost date designates the end of the frost season for a particular region. Although a specific date, it is an average of dates of the last frost of previous years. It includes minor frost that caused no major damage. Damaging frost, although possible, is unlikely afterward. It becomes more unlikely as the season advances. The process reverses after the average first frost date.

Obviously, average last frost dates are as variable as climates. They are irrelevant for climates without frost. Climates with cooler winters generally have average last frost dates later than those of milder climates. For most of us on the West Coast of California, the average last frost date happens before we are aware of it. Nonetheless, it is helpful to know the date for our particular regions.

Warm season vegetable and bedding plants should be safe in the garden after the average last frost date. Directly sown seed should get all the warmth it needs to germinate. Young plants will not likely experience damaging frost. The weather will continue to get warmer. The days will continue to get longer. Cool season vegetable and bedding plants will relinquish their space as necessary.

Plants that sustained damage from earlier frost can now be pruned and groomed. Damaged foliage that remained in place to insulate inner stems is no longer necessary. Pruning and removal of ruined vegetation stimulates new growth while it will be safe from frost. Aggressively pruning and grooming damaged plants that are already regenerating fragile new growth may be complicated.

Most local climates are beyond their respective average last frost dates. Soon, the others will be too.