Fertilizing Tomato Plants in Grow Bags


I am growing indeterminate tomato seedlings in large grow bags for the first time. I am accustomed to top dressing tomato plants in the ground starting 3-4 weeks after planting. However not sure what to do with the tomato seedlings in the grow bags. Should I top dress them too and if so when and with what kind of fertilizer?
Thank You


Hello gardener,

Fabric grow bags are a good inexpensive option for growing tomato plants as long as they are watered regularly and given adequate space and sunshine.  Using grow bags also makes tomato growing possible in tight spaces like balconies, decks and patios.  They are portable (many models have handles) and can be moved around the garden from shade to sun or to a protected area from storms and winds.

Another advantage of grow bags is that they utilize air pruning for a healthy root system. When the roots reach the drier soil on the outside edges of the bag, the roots stop growing and branch out, creating an extensive web of healthy roots.

Here are some tips to successfully grow indeterminate tomatoes in a grow bag.

Adequate space: Make sure each plant has adequate space to grow and that your grow bag is not overpopulated.   The larger the grow bag the better.   Healthy tomatoes develop extensive, deep root systems, so, if the bag is too small, the plant growth may be stunted.  Indeterminate tomatoes require larger grow bags, needing at least a 10 gallon bag per plant.

Planting:   Snip off your tomato plant’s bottom set/s of leaves with clean scissors or garden shears.  Place tomato plant in your grow bag and fill the the rest of the way, up to 2” below the bottom set of remaining leaves.  Plant the tomatoes as deeply as possible.  By burying the bottom section of the tomato stem, it will begin to grow roots for a stronger root system. Thoroughly water.

Compost: Tomatoes are very heavy feeders so you can add a 1″ layer of compost after planting

Mulch: Adding organic mulch to the top few inches of your grow bags will help retain moisture and to reduce the risk of disease. It will help mitigate evaporation/moisture loss from the exposed soil and the bag.  Try a blend of shredded leaves and organic straw or wood chips.

Support:  Secure your tomato plant with stakes, so it doesn’t topple over, but be careful not to pierce the fabric.  One method is to use 3 bamboo stakes to create a tepee structure…place heavy twine under the plant when you are planting it and then wrap gently around the plant …draw the twine up to the tip of the tepee.  The plant can then grow up the twine and be supported by the structure.

Fertilize:  Tomatoes are very heavy feeders, especially once they start to develop fruit. When growing in grow bags remember that the nutrients are leaching/ flowing out of the bag.  Make sure to fertilize every 2-3 weeks with an organic liquid fertilizer such as organic fish emulsion. https://www.torontomastergardeners.ca/askagardener/tomato-fertilizer/ Liquid fertilizers, like compost tea, are perfect for tomato plants in grow bags.  They can absorb into the plants quickly; both through the roots as the liquid leaches into the soil, and through the leaves and stems.

Water:  Water daily.  Potting soil is light and drains well, which means frequent watering is important. If it’s exceptionally hot out, water twice daily to keep the roots cool.

Prune suckers:  Tomatoes are vigorous growers, and not all growth contributes to the production of fruit. Suckers grow between the main stem and the leaf crotches. You can pinch out this side growth. If caught early, they can be plucked off with your fingers.  Prune these suckers weekly.

Good luck.  May your tomatoes grow beautifully.