Math for the Home Gardener

Math for the Home Gardener

Math isn’t always fun! Here is Euclid explaining math in Raphael’s The School of Athens. Credit: Creative Commons.

There are a lot of things gardeners need to know to cultivate a beautiful landscape. Between plant zones, scientific names, soil chemistry, and pest identification, being a gardener takes curiosity and willingness to learn new things. Unfortunately, one topic that needs to be well understood, especially when it comes to applying pesticides and fertilizers, is a subject that many cringe when they hear it – mathematics!

Some of the most common mathematical concepts to have a good grasp on for gardening include area, converting decimals to percentages and vice versa, estimating volume and converting units, and determining how much fertilizer to be applied based on your fertilizer grade.

Here’s a couple tips to help you gardeners out with math.


Most often, we need to know the square footage of our gardens. Rectangles are straightforward, you multiply length (in feet) by width (in feet) to get feet squared. Since most yards are not rectangles, we sometimes need to use other shapes to best calculate the area, such as circles and/or triangles. The area (A) of a circle is , where “π” is 3.14 and “r” is the radius (half of the diameter or the distance from the center to the edge). Remember that the little raised 2 means you multiply the radius by itself twice, not by two.

Most of the time, square feet is the best area unit to use. But just in case you need to know, one acre equals 43,560 square feet. So, if you find your lawn is 10,000 square feet, that means you have 0.23 of an acre (10,000/43,560).

Get your calculator’s out! Credit: iStockphoto.

Percent Conversions

This one may be better understood by most, but this becomes important later when we determine fertilizer needed or doing volume conversions. Basically, any percent can be divided by 100 to convert it to a decimal that can be used easily in other calculations. For example, 15% is the same as 0.15 (15/100). The easy way to remember is that 100% is the same as one, and 50% is the same as 0.5.

Volume Estimation and Conversions

Determining volume is required when figuring out how much mulch is needed for a garden bed or soil required for a raised bed. Converting one unit to another is especially important when determining how much pesticide product is needed to prepare a mixture.

For mulch and soil, the cubic feet (cf) or cubic yards (cy) are needed to figure out how many bags or truckloads will be needed. Most bulk products are sold by the cubic yard. Since we are dealing with volume, we need three measurements, the length, width, and depth. If we want to add three inches of mulch to a 500 square foot garden, we multiply the 500 by 0.25 feet (3”/12” equals 0.25 inches) to get 125 cubic feet. That is about 62 bags of mulch from the hardware store, which are often sold in 2 cubic foot bags. If we divide 125 cubic feet by 27 (three feet in a yard so 3’ x 3’ x 3’ = 27) we now have the answer in cubic yards and find that we need about 4.5 cubic yards of mulch. One cubic yard is roughly a half of a full-size pickup truck bed.

When it comes to converting liquid volume units to help with pesticide mixtures, fortunately we have the internet to help. However, it is a good idea to at least be familiar with converting ounces to gallons and vice versa. Since 128 ounces equals 1 gallon, to go from gallons to ounces you multiply by 128. For example, 0.5 gallons equals 64 ounces (0.5 x 128). To go from ounces to gallons, we just divide by 128. For example, 192 ounces equals 1.5 gallons (192/128).

Applying the proper amount of fertilizer keeps your landscape healthier and protects local waterbodies. Credit: UF/IFAS.

Calculating Fertilizer Needed

Okay, bear with me, as we are about to do some hard math and piece together several of the above concepts. First of all, most fertilizer rates are provided by extension resources and given in terms of pounds of a particular nutrient for a set area, usually 1,000 square feet. For example, when using slow-release fertilizer a homeowner should apply no more than one pound of nitrogen for every 1,000 square feet of lawn. To figure out how much actual fertilizer to apply to meet the recommendation, we will need 1) the recommended rate, 2) the size of the lawn, and 3) the fertilizer grade on the product (the three numbers that represent the percent N, P, and K). Our rate is one pound nitrogen per 1,000 square feet. The size of our lawn is 50 feet by 30 feet, so 1,500 square feet. We are going to use a 16-0-8 fertilizer. Now, here is the hard part! To figure out how much of our 16% nitrogen fertilizer will provide one pound of nitrogen, we need to convert 16% to a decimal (16/100 = 0.16) and then divide the amount of fertilizer we need by that number – 1 pound/0.16 – to find that 6.25 pounds of our fertilizer product are required for each 1,000 square feet. Since we have 1,500 square feet, we need to multiply our answer by 1.5, which gives us a total of 9.4 pounds.

Math is hard and there are usually many methods to get the same solution. When using pesticides and fertilizers in the home landscape, it’s important to make sure we’re using the right amount of materials to minimize the chance of harming ourself, our plants, and the local environment. If you need help, or would like someone to check your work, contact your local extension office.

Safety of Wood Products in the Garden

Safety of Wood Products in the Garden

Despite a sharp increase in costs recently, wood products remain a common landscape and garden material. They can be used for building structures, such as arbors and sheds, or for hardscapes and garden accessories, like raised vegetable garden beds or landscape timbers used for edging. Many homeowners may be confused on the safety of using certain wood products, especially around plants grown for consumption. This article hopes to explain the various options and the known safety concerns.

wooden garden box

Raised beds are a common use of treated lumber in the garden. Credit: Molly Jameson/UF IFAS

Gardeners seem to be mostly concerned with the safety of using preserved wood products around food plants. Pressure-treated lumber is usually suggested whenever the material will be exposed to the elements and especially when in contact with the soil. Non-treated lumber, while free of any preservatives, will simply not last as long in the landscape, especially in Florida where we have a long growing season, are wet, and have lots of organisms – termites, fungus, etc. – that love to break down wood. If the wood is in direct contact with the soil, such as in a raised bed garden, you can expect non-treated lumber to last a year, maybe two, compared to three to five years with pressure-treated lumber. Wood products used for structures not in contact with soil can last significantly longer but, even then, non-treated products will need to be protected with sealers or paints to extend their longevity.

To address the safety of pressure-treated wood products, the wood products industry, and the federal government, in 2004, phased out the use of potentially hazardous chemicals used in the process – namely arsenic and chromium. Wood products preserved with these compounds were either chromated copper arsenate (CCA), ammoniacal copper arsenate (ACA), or acid copper chromate (ACC). Since 2004, much of the lumber available at hardware stores is micronized copper azole (CA) or alkaline copper quaternary ammonium (ACQ). The abbreviations for each are usually found on the little label stapled on to the lumber. So, while copper used to prevent fungal damage is still a component of pressure-treated lumber, the arsenic and chromium used to protect the wood from insect damage has been removed. Railroad ties, sometimes still in use and for sale, are much more toxic than even the older pressure-treated products because of the use of creosote and other oil-based compounds to preserve the wood.

a stack of wooden boards

Preserved wood products contain the preservation method on the tag stapled to the product. Credit: USDA Forest Products Lab

By removing these potentially hazardous compounds from pressure-treated wood products, gardeners should expect no adverse effects from it’s use. A Human and Ecological Risk Assessment of ACQ treated wood that was done in 2007 found exposure levels well below health benchmarks when directly contacting the wood and even with some ingestion of the material, such as contacting the wood and putting hands directly in mouth.

Even though these hazardous materials have been removed, there are still some precautions you should take when using pressure-treated lumber. These precautions include wearing a mask when cutting the lumber and not burning the lumber. This is to prevent inhalation of the chemically treated sawdust or smoke. It’s also not recommended to compost pressure-treated lumber since the chemicals can affect the microbes in your compost pile.

For those still concerned with the use of pressure-treated wood products around food crops, there are other options. In addition to using non-treated wood products or a plastic liner to separate the pressure-treated lumber from your edible garden plants. There are many other materials that can be used instead. Concrete blocks or other stone materials can be a good option. Logs from cut trees, although will rot like non-treated lumbers, can add a natural type of look to the garden.

garden beds

Stone, metal, and concrete blocks are good alternatives to lumber. Credit: Mark Tancig/UF IFAS.

For additional information on pressure-treated lumber, visit this Clemson University website – If you have any questions about the use of wood products in the garden or on how to build a raised bed garden, contact your local UF/IFAS Extension office.

Yay for Yarrow

Yay for Yarrow

Gardeners delight in finding a versatile and resilient landscape plant, especially one that is easily shared. Unfortunately, when a plant checks off those characteristics, it usually finds itself on the invasive species list (see IFAS Assessment). Well, in the case of yarrow (Achillea millefolium), we get a tough plant that is easily propagated, has attractive blooms and foliage, attracts butterflies, and is considered native! As I’ll discuss below, it doesn’t come totally flawless.

Yarrow, in the Asteraceae family, is a great addition to the landscape.

Yarrow is considered a cosmopolitan species. It is found across the entire northern hemisphere and there has been a lot of mixing of native and introduced plants, causing much confusion amongst botanists. It is currently considered a single, though complicated, species. Much of the mixing is due to its history with man, being carried along all sorts of expeditions, even the mythical character Achilles, where the plants genus name comes from. The species name comes from the finely divided leaves – like a thousand leaves. It is in the daisy and sunflower family, called the Asteraceae, or composite family, due to the flower heads being composed of many individual flowers.

The cluster of flowers over the feathery leaved foliage is what makes yarrow stand out. The classic yarrow is white-flowered, but breeders have developed many cultivars in an array of colors, including red (‘Rosea’ or ‘Paprika’), pink (‘Cerise Queen’), purple (‘New Vintage Rose’), and yellow (‘Gold’ or ‘Lemon’). Yarrow is also great for its drought tolerance and has few pests or diseases that bother it. It is even reported to be deer resistant! It can be propagated by seed and is easily divided.

Yarrow ‘Paprika’ is a commonly found yarrow cultivar. Source: Timeh87, Creative Commons license.

The common yarrow shows off beautiful white blooms over the feathery foliage. Credit: Rachel Mathes, UF/IFAS.

With all these great attributes comes one potential problem – it is considered toxic to dogs, cats, and horses. Yarrow contains a toxic alkaloid, called achilleine, that can poison some mammals. So, if you have an adventurous pet that likes to chew on random plants, then you may want to skip adding yarrow to your landscape. Achilleine is the same compound that has led it to be used by humans for centuries as a blood clotting agent. Achilles was said to have carried yarrow to the battlefield in Troy for his soldiers and the plant has been known as herba militaris and soldier’s woundwort. Of course, always consult a doctor for medical advice!

While not a good choice around Fido, yarrow can be a great plant for Florida gardeners. For more information on growing yarrow, see this Ask IFAS publication and this profile from Evergreen State University. You can also contact your local county extension agent for additional information on gardening and more.

Update on Citrus Greening in North Florida

Update on Citrus Greening in North Florida

The Asian citrus psyllid, Diaphorina citri, spreads the bacterium responsible for citrus greening. Photo by Michael Rogers, UF/IFAS.

Asian citrus psyllid, Diaphorina citri. UF/IFAS/Entomology Photo: Michael Rogers.

In late 2016, as many of us were enjoying the harvests from our backyard citrus, a bacterial plant disease that can affect all citrus, citrus greening, was widespread in central and south Florida but had not made it this far north. That year, the vector, the insect that spreads the disease from tree to tree, had been found in Leon County and a few other surrounding Panhandle counties, but the disease had not. By mid-2017, the disease had been confirmed in Franklin County and we hoped that our cooler temperatures could keep the insect and disease at bay. Well, I regret to inform you that the disease has also now been confirmed in Leon County, growing in a residential yard in Tallahassee. Now that it is confirmed in non-coastal (and cooler) north Florida locations, I thought a review of the signs and symptoms – as well as what to do with your tree if you suspect or confirm greening – would be helpful.

The tricky part about diagnosing citrus greening is that it has symptoms that look very similar to soil nutrient deficiency symptoms, especially when first infected. This is a good time to mention that citrus require certain micro-nutrients for optimal growth and a citrus-specific fertilizer product should be used when applying fertilizer. Both the disease and certain nutrient deficiencies cause yellowing of the leaves. With greening, the yellowing is typically blotchy and/or not in any particular pattern. Nutrient deficiencies typically cause unique patterns of yellowing, such as a V-shape or artistic-like symmetrical patterns on each side of the leaf’s midvein. The soil’s acidity, or pH, can also cause some nutrients to not be taken up by the plant even if they are present. Soil testing, available from your local UF/IFAS Extension office, and scheduled fertilizations with a citrus-specific fertilizer can ensure that nutrients are not to blame for the discoloring of leaves. More advanced stages of the disease cause such symptoms as leaf drop, fruit drop, lop-sided fruit, uneven inner fruit cores, and reduced fruit quality.

Citrus greening symptoms of the fruit. Photo by Brooke Moffis.

Citrus greening symptoms of the fruit. Photo by Brooke Moffis.

A more obvious sign of potential problems for your citrus are the presence of the insect vector, the Asian citrus psyllid. This is a tiny little insect that goes from one leaf to the other sucking up plant saps for food and unknowingly spreading the bacterium responsible for citrus greening. You can monitor for them by looking closely at the new flushes of growth. If the psyllids are present, you will likely notice most their small, peach-colored eggs and/or white, waxy secretions. If found, it doesn’t necessarily mean that your tree has greening, but you will want to minimize the chance that they could carry it to your tree. The psyllids can be treated with pesticides, ranging from the less harsh options (horticulture oils, neem oil, kaolin clay) to the more hardcore stuff (malathion, carbaryl, imidacloprid). Of course, always read the label of any pesticide before use and/or consult a qualified landscape professional for assistance.

Blotchy leaf symptom of citrus greening. Photo by Jamie D. Burrow.

Blotchy leaf symptom of citrus greening. Photo by Jamie D. Burrow.

If you suspect your tree is infected, a diagnostic test can be performed by UF/IFAS plant pathologists at the North Florida Research and Education Center (NFREC) in Quincy to confirm. The test does cost $50, which may seem a little steep, but it’s an expensive lab analysis to run and may be worth piece of mind.

If citrus greening is confirmed in your tree, the right thing to do, unfortunately, is to remove and burn the plant material as there is no known cure. While the Florida Department of Agriculture and Consumer Services (FDACS) doesn’t have any removal requirements for infected dooryard citrus, tree removal is best to prevent additional spread of the disease to other trees, especially those grown by commercial producers in our area. This may seem drastic but eventually the health of the tree and quality of the fruit will decline to a point where you will want to remove it anyway. Don’t forget that movement of any citrus plant outside of the state is prohibited for the very reason of preventing spread of citrus diseases.

You may be asking, “Is it okay to replace it with another citrus tree?” The answer is yes you can, but you do risk re-infection and will want to be monitoring the new planting.

You may also be thinking, “What is going to happen to Florida citrus?” That’s a question that many researchers at UF/IFAS are trying to answer. There is some hope that intense irrigation and nutrient management, as well as specific pruning practices, can help infected trees continue to be profitable for commercial growers. Recently, UF/IFAS researchers were awarded several grants to try and figure a way out of this problem. Some of the lines of research focus on exploring the resistance found in different citrus varieties, including an Australian lime that appears to be greening resistant. This could potentially be used as a future rootstock. Another approach is to try and treat the plants with a particular peptide that would prevent the disease from binding in the insect’s gut. Isn’t that amazing?

Until a fix is found, we should be monitoring for this disease in our area and taking steps to reduce its presence through controlling the psyllids and removing infected trees. If you suspect a tree has greening, please contact your local county Extension office to review the symptoms and discuss your options.

Much more information on citrus greening is available at the following Ask IFAS website: and from this 2017 article –

Bzzzz, It’s Mosquito Season

Bzzzz, It’s Mosquito Season

Now that we’re getting plenty of rain and the temperatures are nice and toasty, our nemesis to enjoying the outdoors is back in full force. Yes, I’m talking mosquitoes, the reason for inventing window screens! If you’re gardening outside these days, you’ve probably been annoyed by one of the many mosquito species that occur in our area. Many of these mosquitoes are native species that play an important role in the food chain, feeding many aquatic and terrestrial wildlife, but some of them, like the daytime biting Asian tiger mosquito, are invasive species that were accidentally introduced. While mosquitoes are an important food source to more charismatic critters, they are annoying and can spread disease, and so we can benefit by reducing their presence.

The Asian tiger mosquito, another annoying invasive species. Photo credit: Susan Ellis,

In addition to the age-old advice of draining any standing water, there are other control methods that can be very effective at reducing the mosquito population. One of the most effective and least-toxic options is the use of Bti products. These products come in granular or “donut” forms with the smaller granules being best for various uses around the home and the dunks/donuts for larger areas of standing water like ponds. These Bti products are considered a type of biological control since it is a species of bacteria, Bacillus thuringiensis israelensis to be precise, that causes mosquito, blackfly, and fungus gnat larvae to perish as they wiggle around and grow into their pupal stage. Because it only affects a narrow range of species, it is considered a selective pesticide that does not cause harm to non-target species, such as bees, birds, butterflies, frogs, lizards, and other desirable garden visitors. Bti can be sprinkled into rain barrels, bird baths, bromeliads, gutters, and other places where water may stand more than 5-7 days, the amount of time needed for mosquito eggs to develop into adults.

Bti granules prevent mosquito larvae from becoming biting adults. Photo by: Mark Tancig, UF/IFAS.

If considering other methods of mosquito control, such as the use of foggers, keep in mind that many of the pesticides used to control adult mosquitoes are not selective products and can kill the pollinators you may be trying to invite to your garden. Additionally, planting citronella plants, eating copious amounts of garlic, wearing repellent bracelets, or using ultra-sonic devices or cell phone apps has not been shown to repel mosquitoes so stick to what is known to work.

For a more enjoyable mosquito season, keep the window screens tight, wear long pants and sleeves and use appropriate repellents when outdoors, and do your best to minimize standing areas of water. If you have questions about mosquitoes and their control, visit the UF/IFAS Florida Medical Entomology Lab website ( and browse the many resources available.