Starting Tomato Plants from Seed

Courtesy photo &nbsp&nbsp Once the seeds have germinated, the tomato plants need a strong light source. A shop light or fluorescent lamp will do the trick.

Starting Tomato Plants from Seed

Though nurseries offer a great selection of tomato plants in the spring, anybody wanting to try an obscure variety will need to start ahead of time from seed — now is not too early to begin planning for indoor tomato seed sprouting. Alameda Backyard Growers’ informal tomato growing team always gets busy just after the New Year sowing unusual varieties for Alameda’s Earth Day Festival the following April. Requests for tomato-starting pointers abound. In the spirit of giving, those pointers are presented here.

Use a quality seed-starting soil mix to plant the seeds rather than standard potting soil, which is too heavy for seeds to sprout easily. Have arranged on a tray new or used nursery six-pack containers or small pots. Sterilize any used pots in a bath of one part bleach to nine parts water, rinse and air-dry before filling; this kills any lingering pathogens.

Mix the seed-starting soil with enough water to make it moist but not wet. Fill every pot without packing the soil mix down. Poke quarter-inch deep holes for the seeds and drop them in one at a time, patting gently to cover. If more than one variety is planted, use Popsicle sticks to mark them.

Tomato seeds need consistent heat between 70 and 80 degrees Fahrenheit in order to germinate properly. Have a commercially-available seedling heat mat ready to place the tray on, or set up a do-it-yourself alternative ahead of time. Many can be found online. Do not use a heating pad or electric blanket; neither are made for round-the-clock usage or in an area where water might spill. Partially cover the seed tray with a plastic dome-type lid if available. Keep a spray bottle handy to mist dry soil patches until seedlings sprout. 

Light is not crucial for germination, but once the seeds have sprouted they need a strong light source under which to grow. Take the tray off the heat source and place a shop light or fluorescent lamp close to the seedlings. Leave this light on 14 to 16 hours a day. Window light is simply not strong or consistent enough to do the job.

Once the seedlings are outgrowing their pots, they can be transplanted up to a larger size. Use half seed-starting soil and half regular potting soil. Keep out of bright sunlight for a few days after transplanting. 

Begin fertilizing with a dilute liquid fertilizer per directions. Gradually accustom the transplants to the outdoors in a sheltered spot until planting time, when the soil in the garden is warm enough to stand in barefoot.

Alameda Backyard Growers (ABG) is a network of gardeners in Alameda interested in growing food and donating fresh produce to neighbors. Find out more at