Most seeds need planting 4 to 6 weeks before the last predicted frost
Some seeds, like broccoli and tomatoes, need planting indoors 7 to 10 weeks before the last frost
Seed packets have a wealth of planting and care information
As a general rule, the best time to start seeds for spring planting is 4 to 6 weeks before the last frost in your area. However, there are many variables, including the region you live in, what you're planning to grow, and whether you're sowing indoors or direct sowing outside. The packets your seeds come in usually have suggested planting dates or at least the length of time you should start the seeds before the last frost. Follow those suggestions and these seeding tips and watch your seeds take off.
Sowing Seeds in a Heated Indoor Environment
For the earliest crops, using a lightly heated indoor space, like a heated greenhouse, where you can tightly control the temperature, is a good choice. While more costly than other options, a heated greenhouse ensures your plants get a bit of a head start. It allows you to plant between 1 and 3 weeks earlier than unheated indoor plantings.
Plants that require a longer growing season, such as pumpkins, peppers, and brassicas particularly benefit from a heated, earlier start as they'll be bigger, more robust, and further along in their road to maturity by the time the last frost has passed and the plants are ready to be transplanted to their final location.
One crucial point when starting seeds in a heated environment: you must carefully control temperature reduction and hardening off. If you skip this step, your plants will die. Hardening off involves slowly acclimating the plants to outdoor conditions. You do this by gradually reducing the greenhouse's temperature by a few degrees over a week or two.
Then, once the plants can tolerate an unheated greenhouse, you slowly start exposing them to outdoor conditions by moving them outdoors for a few hours at a time, slowly increasing the number of daylight hours they spend outdoors. You can also start simulating outdoor conditions early by using a fan to mimic the natural movement of air, which helps to strengthen stems and roots.
After a couple of weeks, the plants are ready to move to their final outdoor location.
Sowing Seeds in an Unheated Indoor Environment
In an unheated but protected environment, you still get to plant around a week or two earlier than if you were direct sowing, but it depends on what you're growing. Some plants, such as peppers and tomatoes, can only be started indoors in most areas because they need a longer growing season, and they're particularly tender, unable to withstand even the slightest frost.
You'll also need to follow the hardening off process once your seedlings are ready to transplant, but the process is faster because the starting temperature is lower.
Sowing Seeds Outdoors
You can easily sow certain crops outdoors, just make sure your seed is still viable, as not all grass seeds last very long. Roto crops like carrots, beets, potatoes, and radishes should only be sown directly into their final location, as they don't tolerate transplantation.
Let seed packet's planting conditions and recommended dates for direct sowing guide you; you can potentially plant up to two weeks earlier than recommended if you use cloches or fleece blankets to cover the area you plant in.
Mulching can also help warm the soil's temperature and kickstart seed growth earlier.
Sowing Times by Location
One of the biggest determining factors in when to plant seeds is where you live. Sure, hardiness zones give you a good general indicator of when to plant seeds, but within each zone there are many differences that can impact successful sowing times by a couple of weeks.
Using a planting calendar, such as this one from the Old Farmer's Almanac, gives you detailed planting recommendations for many different crops based on your ZIP code.
Pay Attention to the Seed Packet
The seed packet should contain everything you need to know to get your seeds off to the best start. You'll see information about planting indoors vs. outdoors, the depth at which you should plant, spacing, the time it takes to germinate, the time to maturity, and either planting dates by hardiness zone, or planting times by time before last frosts.
Either keep your seed packet so you can refer back to it, or use a gardening journal and write down the cultivar, the recommended planting times, and the date you actually sowed them. Leave room to add in dates of successional sowing. Sowing multiple batches of the same plant, each a week or two apart, is a great way to extend the harvest period and prevent a glut of produce all at once.
Using a planting journal lets you take detailed notes about planting, soil type, things you tried, what worked and what didn't so you can refer back to it and do better the next year.