It takes patience growing cacti from seed. I know because I've been trying to grow the elusive San Pedro Cactus from some seeds I got from Strictly Medicinal Seed Company. They travel all around the world to collect seeds and carry a wealth of items in their catalogue.
This was my first sproutling as it appeared on November 2, 2020. Not long after, 2 more cacti sprouted. They remained little more than green specks for a long time.
The three cacti grew peacefully for months near the bathroom window. Then, my clumsy sister came back from the high seas to stay with us for a while. She has a habit of dropping or knocking things over. One morning I go to water my cacti and everything is wrong. It looked like the tiniest tornado ever ran through my cactus starting container (mind you, they are planted in a takeout sauce container). I knew instantly they had fallen. They had all been uprooted and had to essentially be replanted. Only one survived.
The original root system dried up and withered away but I propped it up again and continued watering. A new root system formed because I started seeing growth. Although it has kept the same general shape this entire time, it now has visible ridges and texture along with some almost hairlike spines.
I still keep the lone San Pedro cactus in the sauce container. It's old enough to no longer need the extra moisture from having the lid on but also too tall (about 1cm) for it anyway. The lid also helped to keep the fungus gnats at bay, who would have spread fungus and disease to my seedlings.
I admit it was a little risky trying to grow cactus over the winter indoors. Light and water are often a problem apparently but I just kept it light on both. Now it can tolerate a bit more direct sunlight I think. I've been moving the little plant back and forth to windows where there is bright light but am still wary of leaving in full sun for more than a day. Perhaps if I continue conditioning it I can move it to full sun for longer and longer and extend this growth spurt it's having before it goes dormant for the winter.
We will see how tall it becomes by its birthday. I can't wait until it gets its teal blue color. Wish me luck!