From the Farm: Early Seeding to Jumpstart Your Garden
In either scenario there are certain risks, and precautions should be taken to ensure that your early seeding pays off. If you decide to go ahead and plant your spring transplants in the ground then you will likely need to protect the young seedlings from frost. This is best accomplished by floating row covers if you are growing on a moderate to large scale, and by small cold frames if you are dealing with a smaller garden.
Irrigation should also be monitored closely, as plants that lack moisture in the cell membranes are more susceptible to frost damage. Once plants like collards and cabbage have become established in the field they are much more tolerant of light frosts.
If you decide to go the other route and leave your plants in the greenhouse then you run the risk of the plants becoming root bound inside the seed trays. This isn’t a total death blow to the plant, but they will be far less productive once they are planted in the ground—if this is allowed to happen. You could always move the seedlings to bigger seed trays or pots, but this is a time consuming process and you are disturbing the roots multiple times which is not a good practice to follow.
Upon weighing the pros and cons of both scenarios, I feel that you are usually better off doing early seeding and setting out seedlings as opposed to waiting for the ideal conditions. This is going to cost more due to the investment in row cover, row cover supports or cold frames, but it will pay off in the long run.
Please keep in mind that if you just want to go the traditional route, you can time your seeding so that your plants will be ready around the historical last frost in your zone or area. Problem solved. However, for those who want to harvest the first greens or carrots in the neighborhood, a little risk can be expected and a little creativity required.