February 14, 2015

1 - Newly started seedlings may need fertilizer to ensure rapid growth while growing in sunny windows in commercial seed-starting potting soil.

Yard and Garden February 14, 2015


I have started tomato and chile plants inside my home. The seedlings have started growing, but after several weeks they are still very small. I planted them in a store-bought potting soil, keep them in a sunny window, and do not let them dry out. What can I do to make the plants grow more rapidly?


Seedlings of tomatoes and chiles tend to grow fairly slowly at first, especially when compared to beans or corn plants which have much larger seeds. The food for the growth of the new seedlings is contained in the seed and smaller seeds have less food. The varieties of tomato or chile plants may also determine their growth rate. I have noticed that habanero chiles grow much more slowly than poblano and jalapeƱo chiles. In addition, these plants may grow more slowly than seedlings of lettuce or kale because which grow rapidly at lower temperatures.

Another factor is that many seed-starting potting "soils" do not contain nutrients. Some are sold with slow release fertilizers in the mix, but others are just peat, vermiculite, and perlite. It is important to begin feeding the seedlings with a diluted fertilizer solution after a couple of weeks. This will enhance their growth significantly. The primary nutrient needed initially is nitrogen. The ratio of nutrients on the fertilizer label lists the nitrogen first. Some gardeners prefer to use organic materials to supply nutrients. These may include compost tea or manure tea or fish emulsion fertilizer. These are less likely to burn the plants than traditional mineral fertilizers, but it is more difficult to regulate their nutrients. If you use manure or compost tea, be sure there are no plant pathogens in the manure or compost. You can use frequent applications of very dilute fertilizer to keep nutrients available to the plants without burning the tender little seedlings.

I grow my seedlings on a heat mat maintained by thermostat above 70 degrees for the first two to three weeks. I then transfer them to a sunny window that receives at least 4 hours of sunlight each day. As they begin growing more rapidly, it becomes important to move them to larger containers if the seed starting containers are small. Larger containers encourage development of a larger and stronger root system that can supply more nutrients to the plants. You may notice a significant increase in growth of plants in larger containers compared to those remaining in smaller containers.

If you have access to a cold frame that can maintain temperatures above 40 degrees at night, you can move plants to large containers before you must plant them outside in your garden. They may even begin flowering and producing fruit before planting into the garden if the containers are large enough. By moving to larger containers you can resist the urge to plant early when late frosts may kill or injure your plants.

Starting your garden from seedlings indoors is challenging, but also fun. Just remember that constant moisture, adequate light, warm temperatures, and nutrient applications will give you the best plants when it comes time to put them in your garden. Happy gardening!

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.


For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms.

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