Flower Boxes Breathe: Chill, Humidity and Ethylene in a 24-Hour Cut-Flower Shipment

Four hundred bunches of roses left a Nairobi packhouse at 4 degrees and arrived in Amsterdam with grey-blue botrytis on the outer petals of perhaps 6 percent of stems. The log was clean, the box had been closed within spec, and the packs were still partly frozen. What the log did not show was 96 percent relative humidity for nineteen hours, because bare gel packs had been packed against the sleeves.

Flowers are shipped cold to slow respiration, but they are damaged by the wrong kind of cold: too wet, too still, or too cold at the wrong moment. This article covers how gel packs interact with flower respiration and humidity, the target band that actually protects petals, and the packing changes that keep a 24-hour box clean.

Why Flowers Need Cold, Dry and Moving Air Together

Cut stems continue to respire and transpire. A rose at 20 degrees produces roughly 20 to 30 millilitres of water vapour per kilogram per day, and a 400-stem box at 8 kg of stems produces 160 to 240 millilitres daily, all of it trapped in a sealed carton. At 2 degrees respiration drops by a factor of five or six, which is the whole point of cooling, but the vapour still accumulates because the box is closed.

Botrytis germination needs free water or humidity above about 93 percent for several hours. A sealed box with a sweating gel pack delivers exactly that. The temperature may be perfect while the flower is sitting in a saturated microclimate, and the damage appears two days later at the auction.

The Target Band and the Numbers Behind It

The standard cut-flower target is 0.5 to 2 degrees for most roses, carnations and chrysanthemums, with 90 to 95 percent relative humidity, and that narrow band is deliberate: below 85 percent the stems desiccate and petals lose turgor, above 95 percent the condensation risk climbs steeply. Gel packs push the box toward saturation because their surface is colder than the air, so water condenses on the film rather than on the flowers. That is actually helpful, provided the condensate is contained.

Containment is the whole design problem. A pack in a sealed 30 micron bag holds its meltwater and presents a dry outer surface to the box. A bare pack drips onto sleeves within a few hours of the first thaw cycle.

Pack Weight for a 400-Stem Box

A typical 400-stem rose box carries 7 to 9 kg of stems in a 55 by 35 by 30 cm carton with a UA near 1.0 W/K. Against a 28 degree ambient and a 2 degree target, that is 26 watts. Over 24 hours it is 624 watt-hours, or 2.2 megajoules, about 6.7 kg of ice-equivalent, plus the field heat of stems loaded at 8 degrees rather than 2, which for 8 kg of stems at 3.9 kilojoules per kilo-kelvin is another 190 kilojoules.

In practice, 3.5 to 4.5 kg of gel packs per box holds a 24-hour lane, split as 1 kg on the base, 2 kg along the two long walls and 1 kg across the top. More than 5 kg is usually wasted money unless the lane exceeds 36 hours, and it increases condensation risk without buying a lower flower temperature.

Three Packing Changes and the Receiving Check That Proves Them

Bag every pack, always. Use a perforated liner rather than a sealed poly bag over the whole box, so vapour can escape at a controlled rate instead of saturating the headspace. And keep packs off direct contact with petals: a 5 mm layer of kraft or a sleeve wall is enough to prevent cold-surface condensation from touching the bloom.

Ethylene is the other invisible factor. Flowers are sensitive at parts per billion, and exhaust from a tarmac vehicle or ripening fruit in the same consolidation area can cause petal drop. Gel packs do nothing about ethylene; a sachet of potassium permanganate or a 1-MCP treatment does, and it belongs in the box alongside the cold, not instead of it.

At destination, open one box per pallet and look at three things in the first minute: free water on the inner liner, condensation on the sleeve film, and the temperature of a stem base rather than the air. A stem base above 6 degrees after 24 hours means the cold was in the wrong place; visible droplets on sleeves mean the packs were not bagged. Both are packing errors, not pack failures.

If your packs are losing water into the box rather than holding it, the formulation is the place to look, and the freeze-thaw behaviour matters more in flowers than in almost any other lane because the box is opened twice. Our cold-chain SAP for gel packs is supplied with retention and syneresis data so the pack stays a pack, not a puddle, on the second day.