Here’s how to grow tomatoes in a bag organically and in an eco-friendly, DIY manner. When growing tomatoes in a bag, you can grow tomatoes without a garden, even somewhat late in the season if you begin with well-established seedlings. They can set right on the lawn outside your kitchen door, or any convenient place that gets at least seven hours of sunlight.
Get recycled bags. If you live in an area that gets long, hot summers, recycle two white plastic shopping bags for each tomato plant you plan to grow. If you live where summers aren't long and hot, such as in the Pacific Northwest, get a black plastic trash bag for each tomato. Look closely at the black one to make sure it's chemical-free and made of recycled plastic. Don't get one that has built in chemical "germ fighters." Choose one at least the size of a grocery bag, a little larger is fine. Both of these choices will give you strong plastic with handles. Tomatoes in the ground often do well with black plastic to warm their roots. But when it comes to how to grow tomatoes in a bag, the black bag itself will be the heat generator unless your summers are scorching, and in that case, you'll use the white bags to reflect heat instead of absorbing it.
Fill and prepare the bag correctly. Any confined container, including grow bags, can dry out and run out of nutrients. Purchase an organic potting soil that says it contains live beneficial micro-organisms, dolomite and worm castings. If using recycled grocery bags, first double them up before filling. Fill bags 2/3s full. Then add one cup of coir (also called coco pith) now available at garden stores. This is the stuff that absorbs and holds large quantities of water and slowly releases it, and as far as watering, that's how to grow tomatoes in a bag without them drying out so easily. Then add one cup of granulated kelp, also available at garden stores. When you grow tomatoes in a bag, this helps them resist drought if it should occur and get trace minerals they'd otherwise get in the wild by spreading their roots far and wide. Finally, add one cup of alfalfa meal, an extraordinary natural growth stimulant. Mix well and punch a few small drainage holes in the bottom. Use something thin like a toothpick, and make the holes at least three inches apart.
Choose the right tomatoes. A serious component of how to grow tomatoes in a bag is the tomato choice. Tomatoes come in three "types," and the type you want is "determinate." These are self supporting and bushy. The other types need more support and it's hard to stick stakes or tomato cages deep enough into a grow bag to make them work. If not certain, ask the seed company or nursery if the tomato seeds or starts are determinate for sure.
Plant the tomato in the bag. First, moisten the soil in the bag and let it absorb enough water to a nice spongy consistency. If planting seeds directly, plant three per bag near the center, eventually thinning out all but the strongest. If planting a seedling, remove all but the top "umbrella" of leaves and plant the roots and clipped stem down into the soil. Surround it with a ring of stones (if planting seeds wait until they've sprouted and have their first true leaves that come after the "baby" leaves you first see when they spring from the soil). This is also important for how to grow tomatoes in a bag. The stones give a little extra support for holding the roots in place, and they also absorb heat and release it at night.
Keep soil moist. Don't let it dry out, and don't make it soggy.