Just how cold does the closet get? It sounds like to me that the basement is likely to be your best choice. Temperature is very important for germinating seeds. If you have light available at both locations, then temperature may be your deciding factor.

Hi Russ-Ben55 and welcome to GardenWeb!

Yes, as brandon has mentioned, temperature is so important; not only for germination but for growing the seedlings as well. Obviously, temperatures that are too cold would result in poor plant growth. Germination could fail altogether. Temperatures that are a little too warm, cause the plants to grow a little too fast. In that case, it usually becomes hard to supply enough light to keep them from becoming weak and leggy. They do fine outside in really warm temperatures, but they also have plenty of light outside :-) Usually, the best temperature for most seedlings is between about 65 and 70 degrees F. Some like it cooler than that. Much over 70 and youâÂÂll need plenty of light.

How cold does the closet get and how warm does the laundry room in the basement get?

Art

Temperature is as important as light and moisture for good germination. I would grow them in the place that is closest to room temperature. Most seeds require a temperature of from 60 to 80 degrees so a temperature in the middle at 70 is a good temperature. You can adjust the temperature by putting those that require higher temperatures in the center and those that require cooler temps on the outside. Your seed packages will tell you the optimum temp for germination. It need not be exact but a close approximation and I find most seeds germinate well at room temperature.

If you are growing under lights sunlight through windows is not needed. With the eaves shading the house you only get indirect light from windows even if it does look sunny

If like many of us, you become obsessed with starting your own plants, you may end up needing both spaces. You could use the warmer area for germination and early growth and then grow them on in the cooler room. Also - While lights don't put out a lot of heat, 6-8 fluorescent lights will probably raise the temperature in a walk-in closet 10 degrees or so. Because my grow room is cold (60 ish) I use a cheap mylar "emergency blanket" as a curtain over my lighted shelves to hold in heat and light. The top shelf gets up to about 75.

To start/germinate seeds you will need certain level of soil/starter temperature. Probably things like peppers, eggplants need the highest temperature, like 80F -90F. Tomatoes probably are less demanding. Then cool crops like onions, lettuce, parsley .. can do at cooler than room temperatures to germinate.
However, once germinate then it is a different case. Those warm seasons crops would appreciate warmer temps (about 65 f-70f) but the cool crops prefer cooler temps around 60F or even lower.

Now, you have to start where the requirements of what your are trying to grow, are met: Your living room , ..garage ?

JMO

You don't need light initially for germination. I start my seeds on top of the hot water heater in a pitch-black dark closet in deli containers with lids. The trick is to catch them immediately as they germinate (so check them every day) and transfer them under the fluorescent shop lights. Keep the lids on the deli containers for a couple weeks there after.

I start my seeds (tomato and pepper) inside folded damp paper towels inside plastic bags in trays on top of the lights above my tropical fish aquarium. They stay at a nice 80F when the lights are on, dropping to about 75F after the timer turns the lights off.

I check for sprouting every couple of days beginning about five days after starting them. I may have to add moisture to some that get a little dry.

When they sprout, I use a small screwdriver to transfer them to growing trays under lights in my very cool basement (65F or so.)

When they grow their second pair of leaves, I transplant to individual containers.

What seeds are you starting? That is the major factor at play here.