How we price plants

UK eBay sales: completed sales on ebay.co.uk, in pounds, collected through a sold-listings data service. The figure used is what the buyer paid, postage included. Where a listing states a quantity (for example "x3" or "pack of 3"), the price is divided by it.

UK shop prices: asking prices for plants in stock at UK online nurseries, read from their own sites. A shop price is used only if it was seen in the last 14 days.

US eBay sales are kept apart. Where a plant has them, they are listed in dollars in their own section and never feed a UK price, title or ranking.

Each eBay item counts once, however many times it is seen.

One sale, one plant. A listing that matches more than one plant goes to the most specific match, so a named colour form wins over its plain species. If two plants match equally well, the sale counts for neither.

Colour forms (Albo, Aurea, Mint, Variegated and so on) are separate plants with their own prices. A form never borrows the plain plant's sales or shop prices.

Before any price is worked out, a listing must name the plant (each plant has its own required and accepted words). The filter then drops listings with words from a list of non-plant items, mixed bundles, listings that name a different tracked plant, very low-priced listings, sellers with very poor feedback, and each plant's own exclusions (seeds, for example).

Verified: 3 or more UK eBay sales in the last 90 days for the size the price is given for.

Limited data: Fewer than 3 such sales in the last 90 days, or only older sales from the last 365 days. Always shown with the date of the last sale.

Shop prices: No UK sale in the last 365 days, but UK shops have the plant in stock. These are asking prices, seen in the last 14 days, and are never called sales.

Estimate: No UK sale in the last 365 days and no shop price, but an editorial estimate exists. It is always shown as a range, never as one figure, is never ranked and never triggers a price alert.

No price yet: No UK sale in the last 365 days, no shop price and no estimate.

The headline is what UK buyers paid for one size: a 7cm pot, together with listings that state no size, when those have sold; otherwise the size that sold most. The label above the price names that size, and the sale counts behind Verified and Limited data are for that size only. The headline is never scaled from another size.

A Verified price is a median weighted towards recent sales: a sale's weight halves every 45 days. A Limited data price is the plain median of its few sales.

With 5 or more sales, the page also shows the range most sales fell in (the middle half).

A price for another size appears when that size has 3 or more UK sales in the last 90 days.

Where a size has fewer, and the plant has a Verified price for a 7cm pot or unstated size, the page shows an estimated range for cuttings or corms, 12cm pots and mature plants: that price times how the size sells against it across the site. The multiple is the middle half of the price ratios of at least 10 plants with 3 or more UK sales at both sizes, measured again on every weekly rebuild. It is always shown as a range and called an estimate, and it never sets the headline or a price alert. It is left out when the plant's own sales at that size fall outside it, when it would sit above the plant's own price for a bigger size or below one for a smaller size, and when the plant's price is held for review.

The size chart on a plant page always lists the same four sizes: cutting or corm, 7cm pot, 12cm pot, mature plant. Cuttings, corms, pups, seedlings, plug plants and single tissue culture plantlets count as one size, cutting or corm: they sell at similar prices to each other, a little under a 7cm pot, so their sales make one price, and a headline for that size counts them all. A listing of 3 or more tissue culture plantlets is a wholesale lot; it stays on the sales list but sets no price. A size with no price and no estimate of its own can still show a wider estimate: the plant's 7cm pot price times the usual multiple for that size, widened because less is known. Where that does not agree with the plant's own data, the size is estimated between the plant's own prices at the nearest smaller and bigger sizes instead. These estimates need a Verified or Limited data 7cm pot price and never set the headline, a price alert, a ranking or a collection value.

Sizes are read smallest to largest. Where a cutting or corm typically sold for more than 2 times a Verified price for a 7cm pot or unstated size, or a 12cm pot or mature plant for less than half of it, and that price rests on more sales, the sales beyond that line set no price for their size. Where a size read from one or two sales still sits above a bigger size, or below a smaller one, that has at least as many sales, those sales are not used as its price; an estimate that sits out of order against real sales is not shown. An estimate is never shown over a sale at its own size that it leaves out, so such a size often reads No price yet. The sales stay on the list and the chart, marked. An estimated range never overlaps the price of a smaller or bigger size: it is cut back to start above every smaller size and end below every bigger one, two estimates that overlap are split between them, and an estimate left without a range is not shown.

A Verified price that would move by more than 60% on fewer than 5 new sales keeps its previous figure until more sales confirm the change.

An estimate for a colour form never tops out at more than 20 times the real UK price of its plain species, where that species has one.

Each plant's UK eBay sales are checked on a schedule. If the last check that found UK sales for a plant is more than 90 days old, its Limited data price says "UK sales data last updated" with that date, rather than claiming nothing sold.

The price evidence is rebuilt weekly from the recorded sales and shop prices, checked automatically, and published once reviewed. Every plant page shows the dates behind its price.

The price checker judges an asking price against the same size price the plant page shows. With a middle half of sales for that size, a price under it is Below typical and inside it Fair. Over 2 times the typical price is Well above typical, whatever the middle half; anything else above it is Above typical. With too few sales for a middle half, a price within 25% of the typical price is Fair, and the verdict says it rests on few sales.

A plant's Limited data price is judged the same way, and the verdict says it is on limited data: that price can rest on one sale, or on sales older than 90 days. A plant with only UK shop prices is compared with those asking prices. Estimates, and sizes other than the headline with fewer than 3 sales, give no verdict.

No up or down trend badges. Prices of propagated plants tend to fall as supply grows, and a single high sale is usually one large plant rather than a trend.

No rarity grades. How easy a plant is to find changes faster than a label can keep up; prices and sale counts are shown instead.

No valuations. A typical UK price is what buyers have paid, not what any one plant is worth.