This is a difficult problem that needs a "holistic" level of expertise. I have been growing
coleus since I was a child and I cannot get them to survive in this house, even though by all outward appearances it doesn't look dark or feel cold. I have lost a huge investment of plants and products, so please bear with me. First, my wife has to have temps in the low 70's which hasn't kept me from keeping them before, but every garden center in this area is infested with fungus gnats (which presumably are kept at bay by the heat, until I bring them home). I have tried every kind of house-safe pesticide to sterilize them before bringing them in, and it doesn't even kill the larva when you spray them point-blank. They eventually find their way to every plant in the house including locked rooms; if you wrap the pots in seran they find a way in and if you let the soil dry out they find the least dry spot to breed. I buy the healthiest-looking Wizard rose
I can find each year and the plant is dead from root rot in 10 days, with only the moisture that comes with the 4-pack and a bag of potting soil.
Second, plants at our glass doors (which fill the living room with morning sunlight) start to fade almost immediately, then sport back to their wild green forms. Even an avocado seed
in water produces a plant that looks like it grew in a dark closet. I used a grow-light for months that required me to rearrange the living room every night, but it only attracted gnats and spread them from plant to plant, and only delayed the fading process as the leaves start reverting to green unless they are dangerously close to the bulb. A sun coleus I had on the porch was eaten by bugs and then burned to a crisp over the summer. (When I tried a more powerful outdoor spray I watched the leaves melt.)
There are no yard privileges here and what we can do with the house is very limited (the rooms that need to stay cool are the farthest from the thermostat), but it's the only one we've got. So there is no quick fix to this problem, I've gotten "have you tried ____?" advice from garden centers that only made me invest again based on one person's guess. I could experiment with a cheap sixpack but only at the end of the workday do I find out if it burned, rotted or was eaten up, and then they have sold out and the growing
season is over. I don't think our schedule and living situation are uncommon. Should I give up on coleus?