Frost on Dune Sunflower
Picture by: Pam Brown
December is finally here and that means consistent nights of watching for dropping temperatures. Tropical plants and newly installed shrubs are susceptible to cold injury. Those colorful, blooming plants that have added a tropical look to the landscape all summer may begin to suffer when the temperatures drop below 500 F. Leaves may turn yellow and flowering stops. These plants will need to be moved inside or have temporary greenhouse built around them. But, hardy plants that haven’t established a sufficient root system will also need additional attention when the temperatures drop dramatically.
The ability of plants to endure a freeze depends on the species and the weather leading up to the extra cold night. Gradual decreases in temperatures helps plants acclimate to winter. But, a sudden one day drop of 40-50 degrees results in a rapid freeze that causes ice to form inside the plant cells. Leaf and stem tissue expands so quickly that it splits, resulting in parts of the plant incapable of transporting water and nutrients, as well as, performing photosynthesis. However, it may be late the following spring before the damage is noticed, when that section of the plant has slowly staved to death.
Cracked bark from frost
Additionally, plants can experience desiccation or drying out. This happens when dry winds and solar radiation result in the loss of more water from the leaves than can be transported by a cold root system. The resulting symptom is marginal and/or tip burning of leaves that leads to totally brown leaves.
Freezes (when the temperature drops to 320 F) are characterized as radiational or advective. Radiational freezes occur on calm, clear nights when heat radiates from the surfaces of plants, making them colder than the air due to the rapid loss of heat. If there is moisture in the air, ice will also form on the surfaces. Under these conditions, ice forms between plant cells, rather than within the individual cells. Most hardy plants can tolerate these type of freezes if they are properly hydrated.
Advective freezes occur when cold northern winds move in rapidly, dropping temperatures quickly, and causing widespread foliage desiccation. Cultivation and maintenance practices can impact a plant’s ability to endure extended periods of low temperatures. Shade-tolerant species installed under the canopy of a tree typically display less injury from radiational freezes because the trapped heat from the ground and the overhead foliage creates a microclimate. Well-watered soil around a plant will absorb solar radiation during the day and re-radiate heat over night, raising the temperature around the plants. Shrubs that are not pruned in the late summer or fall have leaves that can withstand frost and wind. But, the removal of foliage late in the growing season triggers a flush of new growth, that is very sensitive to lower temperatures. The same response can result from fertilizer applications after plant’s have slowed down in growth.
Irrigation Frost Protection on Citrus
Picture by: Univ of Maryland Extension
So, what can you do to prepare for upcoming freezing temperatures? Begin by avoiding pruning and fertilizing at the end of the season and making sure that the plants have been watered within 24 hours of a cold night. Next, insulate against water loss and increase heat radiation by adding a three-inch of mulch, as well as, covering the trunks of sensitive trees with a commercial tree wrap. Then, consider what needs to be covered. Frost cloth or other breathable fabrics can trap heat for the night and provide a protective layer from frost settling on the leaves. It needs to be placed by mid-afternoon and removed the next day when temperatures are above 320 F. Plastic is not recommended unless the timing regime can be followed reliably and a structure is used under the material to keep the plastic off the foliage. Anchoring of the cover is critically important in the event of an advective freeze.
Finally, turn off the sprinkler system. Commercial agriculture often uses a running irrigation system to keep the leaf surface temperatures near, but not below, 320 F because sprinkling utilizes latent heat released when water changes from a liquid to a solid state. The thin layer of ice melts and re-freezes on the surface throughout the night, without ice forming within the plant tissues. For the technique to work, sprinkling must begin as freezing temperatures are reached and continue until thawing is complete. Landscape systems are not designed to deliver the amount of water over the length of time required to accomplish this type of frost protection.
When the cold nights have passed, don’t forget to check your plants for water. But, wait until winter has passed before pruning out the frost damaged stems.
From time to time we get questions from clients who are unsatisfied with the flavor of the fruit from their citrus trees. Usually the complaints are because of dry or fibrous fruit. This is usually due to irregular irrigation and/or excessive rains during fruit development. However, we sometimes get asked about fruit that is too sour. There are three common reasons why fruit may taste more sour than expected: 1) The fruit came from the rootstock portion of the tree; 2) The fruit wasn’t fully mature when picked; or 3) the tree is infected with Huanglongbing (HLB) a.k.a. citrus greening or yellow dragon disease.
The majority of citrus trees are grafted onto a rootstock. Grafting is the practice of conjoining a plant with desirable fruiting characteristics onto a plant with specific disease resistance, stress tolerance (such as cold tolerance), and/or growth characteristics (such as rooting depth characteristics or dwarfing characteristics). Citrus trees are usually true to seed, but the majority of trees available at nurseries and garden centers are grafted onto a completely different citrus species. Some of the commonly available rootstocks produce sweet fruit, but most produce sour or poor tasting fruit. Common citrus rootstocks include: Swingle orange; sour orange; and trifoliate orange. For a comprehensive list of citrus rootstocks, please visit the Florida Citrus Rootstock Selection Guide. A rootstock will still produce viable shoots, which can become dominant leaders on a tree. In the picture below, a sour orange rootstock is producing a portion of the fruit on the left hand side of this tangerine tree. The trunk coming from the sour orange rootstock has many more spines than the tangerine producing trunks.
A tangerine tree on a sour orange rootstock that is producing fruit on the left hand side of the tree. Photo Credit: Matt Lollar, University of Florida/IFAS Extension
Florida grown citrus generally matures from the months of October through May depending on species and variety. Satsumas mature in October and taste best after nighttime temperatures drop into the 50s. Most tangerines are mature in late November and December. Oranges and grapefruit are mature December through April depending on variety. The interesting thing about citrus fruit is that they can be stored on the tree after becoming ripe. So when in doubt, harvest only a few fruit at a time to determine the maturity window for your particular tree. A table with Florida citrus ripeness dates can be found at this Florida Citrus Harvest Calendar.
Citrus Greening (HLB) is a plant disease caused by the bacterium Candidatus Liberibacter asiaticus, which is vectored by the Asian citrus psyllid. The disease causes the fruit to be misshapen and discolored. The fruit from infected trees does not ripen properly and rarely sweetens up. A list of publications about citrus greening can be found at the link Citrus Greening (Huanglongbing, HLB).
A graphic of various citrus greening symptoms. Photo Credit: University of Florida/IFAS Extension
Nice fall crop of satsuma fruit. Photo credit: Larry Williams
When asked what kind of citrus to grow here in North Florida, my default response is satsuma. I usually get a funny look, followed by an attempt by the person who’s asking to repeat the name satsuma. The individual may ask, “What is satsuma… is that a citrus?” I guess the person expected to hear orange, grapefruit, lemon or maybe tangerine.
Satsuma is a type of citrus, technically classified as a mandarin and is sometimes referred to as satsuma mandarin. The satsuma mandarin is a good candidate for the North Florida citrus enthusiast for a number of reasons.
- Historically, mature dormant trees have survived minimum temperatures of 14°F to 18°F when budded/grafted to a cold-hardy rootstock such as trifoliate orange or swingle, a trifoliate orange cross. Young trees are not as cold-hardy but, due to their smaller size, are more easily covered with a cloth such as a sheet or lightweight blanket for protection during freezes.
- Satsuma fruit are ready to harvest October through December, ripening before the coldest winter temperatures. This is not true with most sweet citrus types such as oranges, which are harvested during winter months. Harvesting during winter works well in Central and South Florida where winters are mild but does not work well here in extreme North Florida. The potentially colder winter temperatures of North Florida are likely to result in the fruit on sweet oranges freezing on the tree before they are ripe, potentially ruining the fruit.
- Our cooler fall temperatures result in higher sugar content and sweeter fruit.
- Fruit are easily peeled by hand, have few to no seed and are sweet and juicy.
- Trees are self-fruitful, which means that only one tree is needed for fruit production. This is important where space is limited in a home landscape.
- Trees are relatively small at maturity, reaching a mature height of 15 to 20 feet with an equal spread.
- Branches are nearly thornless. This may not be true with shoots originating at or below the graft union. Shoots coming from the rootstock may have long stiff thorns. These shoots should be removed (pruned out) as they originate.
Satsuma fruit are harvested in fall but trees are best planted during springtime when temperatures are mild and as soil is warming. Availability of trees is normally better in spring, as well. For additional cold protection, purchase a satsuma grafted on trifoliate orange rootstock and plant the tree on the south or west side of a building. There are a number of cultivars from which to choose.
For more info on selecting and growing satsuma mandarin, contact the UF/IFAS Extension Office in your County or visit the following website.
Citrus Rust Mite “sharkskin” closeup – Image Credit Matthew Orwat, UF / IFAS Extension
In recent years, not a summer has gone by in which I did not see citrus rust mite (CRM) damage in a garden. I thought this year would be the first. Unfortunately, recently I saw my first rust mite damage of the year.
Unlike the myriad of pests that have been recently introduced into Florida from abroad, the citrus rust mite (Phyllocoptruta oleivora) has been documented as present in Florida since the late 1800s. Along with its companion, pink citrus rust mite (Aculops pelekassi) It can be a major summer pest for satsuma mandarins grown in the Florida Panhandle gardens.
Citrus Rust Mite (CRM) damage manifests itself on fruit in two ways, “sharkskin” and “bronzing“. Sharkskin is caused when mites have fed on developing fruit, and destroyed the top epidermal layer. As the fruit grows, the epidermal layer breaks and as the fruit heals, the brown “sharkskin” look develops. Bronzing occurs when rust mites feed on fruit that’s nearer to mature size. Since the skin is not fractured by growth, the fruits develop a polished bronze look. In both cases, the interior of the fruit may remain undamaged. However, extreme damage can cases cause fruit drop and reduced fruit size. Regardless of the condition of the interior, damaged fruit is not aesthetically pleasing, but fine for slicing or juicing.
“Sharkskin Damage” to fruit caused by past feeding by the Citrus Rust Mite. Image Credit, Matthew Orwat
If a CRM population is present, they will begin increasing on fresh spring new growth in late April, and usually reach peak levels in June and July. By August the damage has often already been done, but is first noticed due to the increased growth of the fruit. Depending upon weather conditions, CRM can have a resurgence in October and November, just as Satsuma and other citrus is getting ready to be harvested, so careful monitoring is necessary. For more information, check out this publication: Guide to Citrus Rust Mite Identification.
Sun spot resulting from where citrus rust mite avoids feeding on most sun exposed portion of the fruit. Image and Caption courtesy of EDIs publication HS-806
If control of CRM is warranted, there are several miticides available for use, but it is not advisable for home gardeners to use these on their citrus plants since they will also kill beneficial insects. Horticultural oil is an alternative to miticide, which is less damaging to beneficial insects. Several brands of horticultural oil are formulated to smother CRM, but care must be taken to not apply horticultural oil when daytime temperatures will reach 94 degrees Fahrenheit. Application of oils at times when temperatures are at this level or higher will result in leaf and fruit damage.
Although Citrus Rust Mite (CRM) has the potential to be aesthetically unsightly on citrus fruit in the Florida Panhandle, strategies of monitoring and treatment in homeowner citrus production have been successful in mitigating their damage.
We’re lucky to live an environment where many citrus varieties can be grown. However, many problems can occur when growing citrus, but none more frustrating than fruit drop or fruit split.
Most citrus varieties are susceptible to fruit drop. This is a major cause of low yield in the Navel orange industry. Quite a few variables can cause the condition. This can be difficult to pin point, however. Ethylene gas production goes into effect when a citrus tree is injured, which can spark fruit drop. Our culprit this year may be the late summer excessive periods of rain, high temperatures and areas of poorly drained soils. Low potassium is thought to also be factor in fruit drop, so be sure to follow a fertilizer schedule every year. Competition between fruitlets and young leaves for carbohydrates, water, and other metabolites can be the reason for fruit drop early in the season, as well.
Figure 1: Fruit Split in Naval Orange.
Credit: Ray Bodrey, UF/IFAS Extension Gulf County
Fruit drop can especially affect mature trees. This could mean up to 25% of fruit loss. Brown rot can accompany this condition, if moisture persists for long periods. Fruit drop is usually associated with the lower, shaded areas of the tree canopy.
Fruit split is more severe in Valencia, Hamlin and Navel oranges. Grapefruit (and other acidic fruits like lemons and limes), Tangerines and Temple fruit are much less susceptible. What is thought to be the primary cause of fruit splitting? High temperatures and heavy rainfall during August can easily give rise to the condition. Excess water taken up by a tree during this time will swell the meat of the fruit causing it to grow quickly. Unfortunately, it is believed that the peel does not grow at the same rate. Damage often occurs as the peel eventually caves under the pressure. Nutritional stresses early in fruit development can also be a factor, as low potassium and copper levels have been correlated. The condition is more likely to emerge when no irrigation practices were in place during dry periods that existed earlier in the year
If fruit split has been a problem this year, ensure a recommended fertilizer program next year. Proper tree siting, nutrition and irrigation scheduling are the best defense against fruit drop and fruit split. Although these measures are not a cure all for the conditions, a healthy citrus tree is less likely to be affected. Contact your local county extension office for more information.
Supporting information for this article is from the UF/IFAS Extension EDIS publication: “Citrus Problems in the Home Landscape” by Mongi Zekri and Robert E. Rouse: http://edis.ifas.ufl.edu/pdffiles/HS/HS14100.pdf & information is also provided by Extension Fruit Crop Specialist, Dr. Pete Andersen with IFAS located at Quincy, North Florida Research and Education Center.
UF/IFAS Extension is an equal opportunity institution.
There are a number of plants that are adapted to the climate in the Florida Panhandle. Some can be grown in your yard to provide fresh, nutritious food for your family. In this episode of Home Grown Food, County Agent Doug Mayo, UF/IFAS Extension in Jackson County, Florida shares the rabbit-eye blueberries and satsuma citrus that he grows on the edge of his yard.