Find the dry strip in your greenhouse
A full greenhouse has 120 crop tiles. Before you commit expensive seedlings, use a morning watering check to find the edges that your sprinklers miss.
Map the soil before buying sprinklers
Stand at the entrance and separate the darker rectangular crop bed from the wooden rim. The bed is twelve columns by ten rows. The rim can carry a sprinkler, even though it cannot carry an ordinary planted seed. This lets you trade an extra sprinkler for planting space. Rain outside never waters this room; refill the can at the trough on the north wall. You can also leave the scarecrow outside.
Cover the bed with six iridium sprinklers
For unmodified iridium sprinklers, number the soil columns 1–12 from the left and rows 1–10 from the top. Put four devices at columns 3 and 8 in rows 3 and 8. Put two more on the right wood border, beside rows 3 and 8. Each waters a five-by-five square except its own tile. The four devices in the bed leave 116 crop spaces; the pair on the border reaches columns 11–12. Rotate the arrangement if your preferred aisle runs along the other side.
Do the morning test before filling the room
Water by hand the day you place the devices. Next morning, check both far corners and the line beside the wood before watering anything yourself. A stripe of dry ground means the relevant sprinkler is too far from that edge. An ordinary or quality sprinkler has a smaller footprint than an iridium one. Keep a clear walking lane to the trough while correcting the layout, especially if you have planted trellises.