April in Southern California is the time for soil amending, sowing seeds and transplanting.
In the earlier part of April, plants to sow from seed or to transplant are: celery, herbs, okra, kale, carrots, beets, peanuts, potatoes, spinach, etc. During this time, only plant varieties of the following that mature early: eggplants, tomatoes, cucumbers, and squash- select varieties of these vegetables that can tolerate cooler temperatures. (Some varieties are more tolerant.)
Protect them from an unexpected frost by covering them overnight with plastic or straw.
Wait until late April or early May to plant varieties of the following that can tolerate and prefer warmer temperatures: beans, corn, cucumbers, melons, peppers, pumpkins and squash.
A good tip for seeds: Make sure they are not too old; make sure they have been stored properly; make sure they are planted at the correct depth; Pay attention to the soil temperature- not too hot or cold; keep track of soil moisture- seeds cannot tolerate soils that are too dry or too wet.
Onion tip: Onion bulbs will split if the moisture levels of the soil are too dramatic early in the season. This can happen also if there is too much nitrogen in the soil during the two months prior to harvest.