Popping out this week all over Central Florida are giant clusters of bright yellow sunflower-like blooms that are beloved by butterflies, bees, and all sorts of small reptiles that use the giant flower's petals as a platform to catch some midday sun.
However these "sunflowers" are not true sunflowers. They are members of the Aster family that grow to 20-feet and are considered to be desirable weeds by some Florida gardeners.
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Locally these 10-20 foot towering weeds are referred to as "Sunflower Trees," "Mexican Sunflowers," or "Tree Marigolds." Perhaps the latter is a more appropriate name for these giants. In botanical Latin they are Tithonia diversifolia a Mexican and Central American native in the Aster family (Asteraceae).
The aster family is one of the largest angiosperm families, with more than 1,620 genera and 23,600 known species of herbaceous plants, shrubs, and in this case weedy trees, distributed throughout the world. Asters are characterized by their composite flower heads and one-seeded achene fruits.
Today Tithonia diversifolia has a pantropical distribution and has been spread across the tropical regions of the world. It reproduces vegetatively and from seeds. In Florida most new plants are the result of vegetative reproduction but I have had some success with seeds as well.
Sunflower Trees are able to naturally raise soil fertility by producing foliage that is rich in nitrogen, phosphorous, and potassium. They are being studied as an affordable alternative to expensive synthetic fertilizers. In addition to increasing soil nitrogen levels the plant has been shown to increase phosphorus and potassium as well. Some economic studies have shown that when used as a natural fertilizer Tithonia diversifolia can increase yields (and profits) by 50% or more.
Read more at: TGT: The Virtues of a Fertilizing Plant
In Florida sunflower trees to not reproduce fast enough to become a severe problem. They also do not spread in forests or shaded areas. They prefer full sun along fence rows (below) in sandy soil. The trees will bloom if not trimmed. When trimmed blooms are sparse. These examples, below, were allowed to grow untrimmed for a year and will bloom from November to March. At temperatures below 40° F Sunflower Trees will start to show signs of frost damage. Those low temperatures are rare across Central Florida.
Even after heavy frost events the plants usually recover in a matter of weeks, long before native species have come out of their winter hibernation.
The weedy trees and their flowers provide a welcome burst of flowers for wintering butterflies, albeit by providing an unnatural source of nectar. On sunny winter days Sunflower trees are abuzz with insects and reptiles.
Wikipedia incorrectly reports that sunflower trees grow to 10 feet. I have many plants taller than 20 feet. The plants become woody with age and height and drop roots from along the woody stalks. Any of those stalks stuck into the ground will produce a new tree.
Sunflower trees have been domesticated across Asia and Africa. In Asia and Latin America this plant is also referred to as kembang bulan (Indonesian and Javanese), jalacate (Spanish), buatong (Thai) and dã quỳ (Vietnamese).
We love sunflower trees for their winter blooms and the abundant insects they attract. If we don't have frost to kill back the giant weeds then we trim most plants back to about 4-feet tall in late winter to create a bushier, hedge-like ornamental in spring.
Some online sources say sunflower trees not very drought tolerant. We have found them to be among the most drought tolerant, surviving through months of heat and dry, especially in recent extremely hot and dry Florida springs (February to May).
Tithonia as Fertilizer
Tithonia diversifolia can be used as organic fertilizer biomass. I have a compost heap full of trimmings from my Sunflower trees. The biomass refers to materials that are derived from the plant, such as it voilage, being worked into the soil as a dry fertilizer. It works as a very efficient natural fertilizer for tomato plants in Central Florida.
In summer copious green stems and leaves can be removed from plants every couple of weeks, if the plants are kept trimmed to bush level. Biomass from Tithonia diversifolia breaks down rapidly and releases nutrients quickly into the soil.
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I planted these Sunflower trees as a hedge along a 600-foot long fence. Towards the end of the fence I allow them to grow to tree-like heights. My plants started with one piece of woody stalk that a friend broke off of a tree in DeLand, Florida, many years ago. I started planting a few of the trimmings in sand and soon had a 600-foot hedge row that continues to thrive. My soil atop an ancient dune system is about 90% sandy quartz, proving that these giants will grown in just about the worst soil you could possibly imagine.
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