Can i use a seedling mat with these that go up to 85 degrees?

yes these are fine with a seedling mat.

When will the inserts be available again?

If you are just looking for the 32 cell trays we have them stocked but not at Amazon yet. We should have them back in Amazon with a month or so. Regarding the pots that go in these we will have them back in our inventory May 10th or so and will be sending them to Amazon as we can but that timeline is less clear.

Can’t I just buy a single tray?

Unfortunately due to the high fixed costs associated with selling on Amazon, we cannot offer a single 32 cell tray.

What dimensions are the inserts? Length, width and height.

Top dimensions are 2.5” square with height of 3.5”. Bottom is 1.75” square. 32 percent inserts will fit in the trays. Trays are consisted with insert tray and water holding tray. Materials are much stronger plastics than other germination trays.