I didn't want to hijack the OZ thread, so am starting this one.
The term "heatpack" is not a universal descriptor. They vary all over the map.
The best ones I have found are Uni-Heat 72 + hour packs. If kept at room temperature, they will heat to about 115°F surface temperature in about 4 hours, then degrade to about 100° over their usable life. However, temperature is not the key, heat output is, and one of those will boost a 1 cubic foot box about 10°-15° over ambient, so you have to use the appropriate number of them. Then there is knowing how to use them.
Some put them right up against the plant, but they are intended to heat the enclosed environment, not the objects in it. If it wouldn't mean so much work, ideally one would affix one smaller pack on each wall of a box. Instead, if you place one, two, or a few strategically around the box, and insulate the box - I use a closed-cell foam, but newspaper is great, too - you can have a nice cozy environment for the plants. I have successfully shipped blooming plants to Alaska in February using them.
Even though they appear to be mesh bags of powder, there really are only a relatively few perforations in the enclosure to allow oxygen and humidity in. If you cover those (they are usually marked with a colored stripe), they won't work properly. The same is true if you do an absolutely excellent job of sealing the box.
The basic concept is iron filings and salt in a bag. Moisture and humidity entering the powder causes the iron to rust, which is an exothermic reaction. If you starve the heatpack for either, the process slows. I prefer not to ship in winter with wet potting media, so I'll throw a wet paper towel in the box.
Heat packs are a mixed blessing....I have gotten plants that were severely burnt from their proximity to the heatpack. In my entire ordering history over the past few decades, I have only once gotten plants that were killed by freezing, regardless of heatpacks.
The term "heatpack" is not a universal descriptor. They vary all over the map.
The best ones I have found are Uni-Heat 72 + hour packs. If kept at room temperature, they will heat to about 115°F surface temperature in about 4 hours, then degrade to about 100° over their usable life. However, temperature is not the key, heat output is, and one of those will boost a 1 cubic foot box about 10°-15° over ambient, so you have to use the appropriate number of them. Then there is knowing how to use them.
Some put them right up against the plant, but they are intended to heat the enclosed environment, not the objects in it. If it wouldn't mean so much work, ideally one would affix one smaller pack on each wall of a box. Instead, if you place one, two, or a few strategically around the box, and insulate the box - I use a closed-cell foam, but newspaper is great, too - you can have a nice cozy environment for the plants. I have successfully shipped blooming plants to Alaska in February using them.
Even though they appear to be mesh bags of powder, there really are only a relatively few perforations in the enclosure to allow oxygen and humidity in. If you cover those (they are usually marked with a colored stripe), they won't work properly. The same is true if you do an absolutely excellent job of sealing the box.
The basic concept is iron filings and salt in a bag. Moisture and humidity entering the powder causes the iron to rust, which is an exothermic reaction. If you starve the heatpack for either, the process slows. I prefer not to ship in winter with wet potting media, so I'll throw a wet paper towel in the box.