How to Grow San Marzano Tomatoes, an Heirloom Vegetable

Updated on December 12, 2017
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Caren White is a Master Gardener and instructor at Home Gardeners School. She has been associated with Rutgers Gardens for over a decade.

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Tomatoes are most often associated with Italy but they are originally from the Andes mountains in South America. As Europeans invaded and conquered the New World, they sent back examples of the new plants they were encountering in the form of plants and seeds.

No one knows for sure how the San Marzano tomato made it to Italy, but the story most often told is that seeds were sent as a gift from the Viceroyalty of Peru to the Kingdom of Naples in 1770. The gifted seeds were planted in the San Marzano region of Neapolitan kingdom. The resulting plants took their name from that region.

How are San Marzano tomatoes different from other tomatoes?

San Marzano tomatoes are plum tomatoes meaning that instead of being round, they are oblong, similar in shape to a plum. San Marzano tomatoes are distinguished from other plum tomatoes by being thinner and having a pointed end. The flesh of the fruit is also thicker and has fewer seeds than other plum tomatoes. The taste is stronger and sweeter but less acidic.

Aside from their looks and their taste, San Marzano tomatoes grow like other tomatoes. They are indeterminate, i.e. will continue to grow and set fruit until killed by the frost. They grow four to six feet tall and require staking with something sturdy that can bear the weight of the plants. San Marzano tomatoes are not hybrids so you can save the seed for next year. Tomatoes are self-pollinating so the seed will always be the same as the parent plant.

How do I grow them?

You will need to start your seeds indoors six to eight weeks before your last frost date. Plant the seeds ¼ -inch deep then mist the soil to moisten it. Heat is critical to grow tomatoes. Keep your seeds on a heat mat between 70⁰F and 80⁰F. 75⁰F is the ideal temperature. Germination should occur within seven to ten days.

Transplant each seedling into its own 4-inch pot when they develop their first set of true leaves. When transplanting, plant each seedling in soil up to that first set of true leaves. New roots will form along the buried stem. Fertilize the seedlings every seven to ten days with 10-10-10 fertilizer.

One to two weeks after your last frost, you can transplant your seedlings outdoors in your sunny garden when nighttime temperatures stay consistently above 50⁰F. Just like you did when moving them to their own pots, bury the seedlings in the ground up to the first set of leaves. More roots will develop out the buried stem. The more roots you have, the larger the plants will grow and the more fruit they will bear. Space your plants 36 inches apart to allow them room to grow without crowding which can lead to disease because of poor air circulation between the plants.

Lay a layer of mulch two inches deep around your tomato plants to keep the soil cool and moist and to prevent weeds from growing. Don’t let the mulch touch the stems of the tomatoes. Keep it four inches away from the stems. Place your tomato cages or sturdy supports around your plants two to three weeks after planting.

When can I start harvesting tomatoes?

San Marzano tomato plants reach maturity and begin to bear fruit within 85 to 90 days after seed germination. The fruit will be a deep red in color and firm to the touch. Gently grab the fruit and twist it until the tomato pulls free from the stem. Alternatively, you can use scissors or pruners and cut the stems close to the tomatoes.

Store the harvested fruit indoors at room temperature or outdoors in a shady location. Don’t refrigerate it because temperatures below 55⁰F cause the fruit to lose flavor. Tomatoes will store longer if you leave the stems and caps in place until you are ready to use them. For best flavor, use the tomatoes within seven days of harvest.

Questions & Answers

  • Is it possible to prune the San Marzano tomato plant? We had them staked but went away for an extended vacation, and when we returned they had grown as if hit with a gamma ray!

    You can prune your plant, but it won't help. San Marzano tomatoes are indeterminate which means that they will continue to grow until they are killed by the frost. In my garden, I allow them to grow as big as they want, because more plant means more tomatoes!

  • Are San Marzanos indeterminate?

    Yes, they are indeterminate, i.e., will continue to grow and set fruit until killed by the frost. They grow four to six feet tall, and require staking with something sturdy that can bear the weight of the plants.

© 2017 Caren White

Comments

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    • OldRoses profile imageAUTHOR

      Caren White 

      20 months ago from Franklin Park, NJ

      Thanks! I'm a big seedsaver of heirloom flowers myself.

    • Bob Ewing profile image

      Bob Ewing 

      20 months ago from New Brunswick

      I collect heritage tomato seeds, plant some in my greenhouse and save the seeds. Good information here.

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