Flower export: gel packs that must not touch the blooms

“The boxes were cold, the flowers were cold, and half of them were brown,” is how a Nairobi exporter described a shipment that had passed every temperature check at the airport. The gel packs had done precisely what the specification asked. They had also been stacked against the flower heads for thirty hours, and a bloom held at minus 2 °C for thirty hours is not a cold-chain success. It is a dead flower with good paperwork.

Cut flowers are the one payload where the pack can be too cold, too close, and too effective. Nothing in a standard gel pack specification warns you about that.

Two degrees is the whole window

Most cut flowers want to travel at 1 to 4 °C. Below 0 °C the cells freeze, the petals go translucent, and on thaw they collapse into a brown mush that no amount of conditioning will recover. Above 8 °C, respiration and ethylene production climb and vase life drops by days. The window is narrow, and it is narrower than the window a frozen gel pack naturally creates at its surface.

A pack at minus 18 in a sealed box will pull the air around it to below zero within an hour. If the box is small and the pack is large, the flower heads nearest the pack will freeze. This is not a theoretical risk; it is the standard failure mode on mixed boxes where the pack is sized for the volume and placed without regard for what is next to it.

The geometry that solves it

Packs go in the base tray, under the sleeve, never at the head end. The stem ends tolerate cold far better than the blooms, and the bunch is already bundled so the stems are packed tight and conduct heat reasonably well. A pack under the stems chills the whole bunch through the stems, which is the direction the cold should travel.

Keep a minimum of 40 mm between the pack face and any flower head, achieved with a corrugated collar rather than with void fill, because loose fill shifts in transit and a shifted pack is a frozen bloom. Where a box must carry packs at both ends, use a lower-capacity pack rather than a smaller number of cold ones: two 300 g packs at 0 °C do less damage than one 800 g pack at minus 18.

Wet packs, dry boxes

Flowers ship with their stems in water or in a hydrating sleeve, so the box already contains moisture. A gel pack that condenses on its outer film adds to it, and free water in a flower box is a Botrytis nursery. Wrapping each pack in a thin absorbent sleeve, or specifying a pack with a condensation-controlling outer wrap, keeps the water where it belongs.

This is the second most common cause of the “cold but brown” complaint. The flowers did not freeze; they sat in a humid box at 3 °C for a day and a half and the botrytis got them at the petal edges, which reads to the buyer as cold damage.

Testing a flower box honestly

A gel pack test with a temperature logger in the air tells you nothing useful on this lane. Put the logger inside a flower head, in the centre of a bunch, and another one at the stem end. Then run the box through the real journey, not a chamber: the packhouse, the truck, the tarmac, the aircraft hold. The head-level curve is the one that matters, and it will show a dip below zero if the geometry is wrong, even when the air temperature looks perfect.

Do this once per box design and once per pack size. Then write the geometry into the packhouse instruction as a photograph, not a paragraph, because a photograph of a pack in the base tray is unambiguous at 05:00 with a queue of boxes waiting.

If you are specifying packs for a flower lane, the low-temperature grades in our SAP powder selection guide can be tuned to hold near 2 °C instead of driving to minus 18, which removes most of the freeze risk at the head end before geometry has to solve it.

Take one box, four loggers and one real flight, and look at the head-level curve before the next consignment is packed. It is the only test that will tell you whether your packs are protecting the flowers or quietly ruining them.