How we calculate

This page sets out every rule behind the tools on PlantCare101, separating what comes from a published source from what is our own estimate. If a result looks wrong for your plant, this is where you can see why the tool said it, and where to tell us it should change.

The data: one file per plant

Each of the 80 plants is a structured file with its light range, drying rule, soil, humidity, temperature, feeding, repotting, toxicity, problems, propagation, size, varieties and sources. Every care block lists the sources it comes from, each with the date it was read, and a schema rejects any file with a missing field, a title outside 50 to 60 characters or a source reference that does not exist. A figure that no accepted source gives is left empty rather than estimated. The accepted sources are the NC State Extension Plant Toolbox, Clemson HGIC, the University of Illinois Extension, UF/IFAS, the University of Minnesota Extension, the Missouri Botanical Garden, the Royal Horticultural Society and, for toxicity, the ASPCA.

Watering calculator

Neither Clemson HGIC nor the University of Illinois Extension gives a watering calendar; both say to water by feel because the interval depends on the plant, the pot, the mix, the light, the temperature and the humidity. We take that literally. The calculator starts from the drying rule the plant's sources give, assigns it a reference range for a standard situation, and multiplies that range by one factor for each condition those sources name. The reference ranges and the size of each multiplier are our estimates, chosen to be plausible and slightly cautious on the wet side; they are not measurements, and the result is always shown as a range to start checking, never as a date.

The multipliers are combined by product, the result rounded to whole days, and the upper bound kept at least one day above the lower. Water volume assumes a standard tapered pot about as tall as it is wide (volume ≈ 0.65 × π/4 × diameter³): with a drainage hole, pour about 20 to 30% of the pot volume, which is roughly what it takes for water to run out of the hole as Clemson HGIC recommends; without one, 10 to 15%. The weight test assumes a drained potting mix holds about 40% of its volume as water, 25% for gritty or bark mixes and 50% for peat-heavy ones. Plain tap water weighs about one gram per milliliter.

What the model ignores: root mass (a root-bound plant dries much faster), pot shape, heating vents and drafts, grow lights, and the weather of the week. That is why every result ends with the finger test and the weight test, which Clemson HGIC lists alongside tapping clay pots and judging soil color.

Light estimate

The University of Illinois Extension defines four light levels in foot-candles, which we use everywhere on the site: low 75, medium 150, high 300 and direct 1,500. Lux values are foot-candles × 10.764. The same source places each level at a distance from each window direction, which gives the anchor points of the light finder:

• South: 0 ft 1,500 fc, 2 ft 600 fc, 5 ft 300 fc, 10 ft 150 fc, 18 ft 75 fc, 25 ft 45 fc
• East or west: 0 ft 300 fc, 3 ft 150 fc, 8 ft 75 fc, 16 ft 40 fc
• North: 0 ft 150 fc, 3 ft 75 fc, 10 ft 35 fc

Between anchors the finder interpolates on a logarithmic scale, because light falls off steeply near the glass and slowly deeper in a room. The last point of each window and anything beyond it are our extrapolation and are flagged in the result. A sheer curtain or outdoor shade multiplies the estimate by 0.5, both by 0.25, an order of magnitude for factors Clemson HGIC names without quantifying. A plant “thrives” when the estimate reaches at least three quarters of its preferred level, “survives” when it is inside its range but lower, and is “too dark” below 80% of its minimum.

Pot size, fertilizer, diagnosis

Pot size applies the Clemson HGIC rule of a pot 1 or 2 inches wider, narrowed to 1 inch for plants their guide describes as preferring a snug pot and for succulents and epiphytes, and to 2 inches from a 10-inch pot up. A plant that is not root-bound keeps its pot with fresh mix, as UF/IFAS suggests. The extra mix is the volume difference between the two standard pots.

Fertilizer scales the label dose linearly to your volume of water and the strength you choose (full, half, quarter, or one-tenth for every watering, a route Clemson HGIC describes). Unit conversions use 1 teaspoon = 4.929 mL, 1 tablespoon = 14.787 mL, 1 US cup = 236.6 mL, 1 quart = 946.4 mL and 1 gallon = 3,785.4 mL. Nitrogen in parts per million is computed only when the dose is in grams: grams per liter × nitrogen percentage × 10. The N-P-K reading follows Clemson's definition of the three numbers as percentages by weight.

Diagnosis counts, for the chosen symptom, how many plant guides attribute it to each cause, gives the chosen plant's own entries three times the weight, and doubles or divides by about three the causes that match or contradict the state of the potting mix. These weights are our choice; the shares shown are shares of matches, not probabilities.

Pet safety data

The pet-safe checker uses the ASPCA Animal Poison Control Center lists for cats and dogs, 987 entries copied by a script that can be rerun at any time and that refuses to save a list that looks truncated. Each entry keeps the ASPCA's name, other names, botanical name, family and status for each species. Nothing is inferred: a plant absent from the lists is reported as not found. The plant guides quote the toxic principle and clinical signs from the ASPCA entry they cite.

Checks before every update

The engines are covered by automated tests that pin the reference cases above, the Illinois anchor values, the Clemson pot rule and the dilution arithmetic. The site build fails if a plant or guide file is incomplete, if a title or description falls outside its length, or if two pages share one. Sources are re-opened to catch dead links, and the whole site is reviewed at least once every twelve months, as set out in the editorial policy.