Every year the same question comes up again: when does the real work with the bee colonies actually start? The Grünlandtemperatursumme (GTS), or grassland temperature sum, is a figure from agriculture that helps farmers plan the first cut or the first round of fertilizing. I use it to schedule the first care measures for my colonies.
The calculation is simple: starting January 1st, you add up all positive daily mean temperatures, with January weighted by a factor of 0.5 and February by 0.75. From March onward, daily mean temperatures count in full. Once the sum reaches 200 degree-days, it’s considered the start of sustained vegetation growth. In agriculture, the rule of thumb is that fertilizer applied before this threshold is reached can’t be taken up by the plants.
And what about the bees? Bees take their cues from how the vegetation develops, so it’s a natural fit to work with the GTS here too. After the winter break, the queen often starts laying again before I see anything blooming in my neighborhood. Here’s why: a worker bee takes 21 days to develop from egg to emerged adult. Once she’s hatched, she spends her first three weeks taking on various jobs inside the hive - cleaner, nurse bee, builder, guard - and only becomes a forager after those three weeks. That’s six weeks in total from egg to nectar and pollen supplier.
At the same time, the old winter bees start to die off. Between late February and April or May, they’re completely replaced by a new, young generation of summer bees. We beekeepers call this “Durchlenzung” (literally “getting through spring”), and it’s the most critical stretch of the year: is there still enough food left in the combs from last season? Especially in early spring, when stores are running low but there isn’t much nectar to be had outside yet, things can get tight. If a colony ramps up its engine too early, there are suddenly too many hungry mouths to feed. Conversely, if a colony oversleeps the start of spring, it lags behind for the following months too.
As a beekeeper, there’s at least one thing I can help the bees with: their stores. If I see these running low - whether during an inspection, or without even opening the hive, using a hive scale - I can support them with fondant. The key figure here is GTS 200: around that point I open the colony as part of the spring inspection, without stressing it too much.
What I like about the GTS is that it’s a bit more objective than I am. A sunny 12-degree day feels like spring but says little about the actual state of things. The GTS, on the other hand, integrates over weeks and smooths out brief warm or cold snaps. It’s essentially a simple, but readily applicable model.
Besides GTS 200, there’s another figure that matters to me: around a GTS of 400, one of the loveliest moments of the beekeeping year comes up - putting on the honey supers.
Worth a look online: the GTS monitor run by the Upper Austrian beekeeping association is a really well-made website and a practical helper for me.