invented to keep out the cold
The proper weaving of phase spider webs is apparently one of the secrets humans recovered from the drow long ago, or so she was told. It is the only form of spidersilk they know the proper handling of. Proper handling makes it much easier to harvest the ingredients, but Mother didn't have the circle depth to craft a Portable Hole, and she wasn't going to sell the information. (Starlight is easier to gather on the surface, and she has heard stories of drow parties emerging from the World Beneath just to collect it. It's not as hard here. Unfortunately, that doesn't help if you can't push in the power it needs to work.)
That might have changed. Even if it didn't, Mother still has a Hole from... her flight. Giant spiders are hard to transport, but not impossible, it turns out. She might have more than one, Tencednil didn't need to know the exact details. One layer is restrictive, but she can manage it.
She spends the rest of the night organizing her belongings, setting aside what she would need and what she could lose, and ensuring everything is easily collected in five minutes if she needs to. One hour means ten minutes to cast a Sending, five to excuse an immediate need to flee from her shift, possibly thirty to get back, and that leaves fifteen minutes to actually do anything necessary. Best to leave herself time.