What to Plant and When!
What seeds to plant and when??
My general advice is to wait a little longer.
It is mid January- cold, dark and wet. Until there is a bit more natural daylight, germination and plant growth will be slow. Unless you have a super light and heat set up in a greenhouse or grow room then you will have an uphill battle caring for seedlings right now. They will essentially be houseplants.
We have an unheated greenhouse and I do sometimes start seeds in late January for cabbage, greens and lettuce. They will germinate on their own but a heat mat will speed things up.
Definitely wait on direct sowing crops unless you are thinking of spreading echinacea seeds that like a good cold wet spell before germination.
In zone 9, I would start my first rounds of beets, carrots and turnips as early as mid February unless there is a bad weather spell forcast. These seeds can get washed out by too much rain! I usually start planting these when the soil can be worked, not too sloppy, and continue planting them in rounds through May or even June. For those of you in colder zones you may have snow so obviously your direct seeding will need to wait until that has melted.
When I am thinking about when to start seeds indoors my main question is, "When would I ideally transplant these outdoors?" Then I work backwards from there.
Cool season crops like brassicas, lettuce, peas, cilantro, dill and peas can all tolerate some frost and will thrive in cool moist weather. In zone 9 these can sometimes be transplanted outdoors as soon as mid February and almost always in March, though row cover will accelerate growth and protect from surprise hail or snow.
As far as the warm season crops, like tomatoes and peppers, I know a lot of folks are chomping at the bit to get planting. Like I mentioned above, if you have that super system of heat and lights and are ready to up-pot your plants many times, then go for it now. Some people really like putting larger plants in the ground and there is no doubt that you will get fruits a few weeks earlier.
But for folks in zone 9 you generally won't be planting tomatoes and peppers outdoors before mid April to early May. In that case I wouldn't seed tomatoes until mid February to mid-March.
Even if the last frost is in March or April the weather in northern CA can still produce cold and hail that will damage plants. My mom lives in Redding and usually aims for early May to plant outside. I live at 2500 ft in the foothills of Lassen Peak and we aim for the last week of May.
In my experience, pretty much everyone in zone 8 and lower should plan to start transplanting frost tender annuals around June 1. For those folks you don't need to start your peppers and tomatoes until early April! Some years you might get lucky and get plants in earlier but those surprise frosts can really damage plants so I don't risk early planting anymore.
What about the warm season cucurbit crops like squash and cucumbers? For these fast growing crops you only need to start them 2-4 weeks before you plant to transplant. Started too early these can easily become stunted and overgrown. They do not like their roots disturbed much and I often see that there is not really much benefit gained by planting a 6 week old squash vs a 3 week seedling.
Below is my advice on how many weeks before transplant to start seeds.
Cabbage and broccoli: 4-6 weeks, you might want to transplant them into 4 inch pots from small cells to have really robust plants for transplant.
Lettuce: 4 weeks, I generally seed these into 1-2 inch cells
Brassica greens like bok choy, mustard and kale: 3-4 weeks, these grow fast and also do well direct seeded in the garden Feb-April. They will bolt to seed in hot weather so sowing them as soon as the soil can be worked is a good rule of thumb
Cool Season herbs like cilantro and dill: 4-5 weeks, these will also bolt to seed in hot weather so as soon as possible.
Peppers/Eggplants: 6-8 weeks, a heat mat will greatly speed germination and these need to be in full natural light or grow lights as soon as germination occurs or they will get too leggy.
Tomatoes: 4-6 weeks, I recommend using a heat mat to germinate tomatoes as well. These grow quickly and I usually opt to plant smaller 4-12 inch plants that I've only transplanted to a 4 inch pot. Often I plant directly from 2 inch cells.
Squash: 2-4 weeks, Like I said above, don't start these too early. Direct seeded is best if you can. They do best when their roots are not disturbed. Sow into 2-4 inch cells.
Cucumbers/Melons: 3-4 weeks, Same advice as for squash! Sow into 2-4 inch cells.
Corn: 2-3 weeks, I often start corn seeds in trays of 1 inch cells and transplant them when they are no more than 4 inches tall. This way I get great spacing and don't waste any seed. But you must be careful to get them in the ground in a timely manner. If they get too tall in the trays they will be stunted.
Beans: 2-3 weeks, Similar advice as for corn. Get them in as soon as the first true leaves are developed!
I hope this helps you all with planning when to start what. All of this advice is just based on my own experience. After a while planting becomes really intuitive as you get to know the different veggie species as well as your local micro-climate.