Caren White is a Master Gardener and instructor at Home Gardeners School. She has been associated with Rutgers Gardens for over a decade.
Tomatoes (Solanum lycopersicum) 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 Spanish 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.
What is the Difference Between San Marzano Tomatoes and 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 and less acidic.
Aside from their looks and their taste, San Marzano tomatoes grow like other tomatoes. They are indeterminate, which means that they 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 and fruit. 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 to Grow San Marzano Tomatoes From Seed
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. A balanced fertilizer is good for foliage. At this point, you want the plants to grow foliage, not fruit.
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. Place your tomato cages or sturdy supports around your plants two to three weeks after planting.
How to Grow San Marzano Tomatoes From Purchased Plants
When you buy San Marzano tomato plants from a nursery, you plant them the same way you would plant seedlings that you grew yourself. Dig a hole that is deeper than the container your plant is in. The hole should be deep enough so that the first set of leaves on the plant will be flush with the top of the soil. Tap the bottom of the container and gently remove the plant. Place it in the hole and fill the soil around it burying the plant up to the first set of leaves.
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. Keep the mulch four inches away from the stems. Don’t let the mulch touch the stems of the tomatoes. This could result in disease and insect infestation. Since these plants will be larger than seedlings that you raised yourself, place your tomato cages or sturdy supports around them when you plant them.
How to Ferilize San Marzano Tomatoes
You will need to fertilize your plants to encourage maximum fruit production now that they are growing in your garden. Use a low nitrogen fertilizer such as 8-32-16 or 6-24-24. Too much nitrogen will result in a lot of foliage, but very few fruit. Mix a slow release fertilizer in the bottom of the planting hole when you plant your tomatoes. Fertilize again when you see the fruit start to develop using a water soluble fertilizer. You can continue applying the water soluble fertilizer every week until the fruit ripens.
How to Harvest San Marzano 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 from which the fruit hangs close to the tomatoes.
How to Store San Marzano 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
Question: 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!
Answer: 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!
Question: Can San Marzano Tomatoes be grown in containers?
Answer: Yes, provided you use very large containers. The plants are 4 to 6 feet tall which means that the root mass supporting a plant of that size is quite large. Many gardeners grow tomatoes successfully in 5-gallon containers. A general rule of thumb when growing tomatoes in containers are that the bigger, the better container.
Question: Are San Marzanos indeterminate?
Answer: 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
Caren White (author) on March 07, 2017:
Thanks! I'm a big seedsaver of heirloom flowers myself.
Bob Ewing from New Brunswick on March 07, 2017:
I collect heritage tomato seeds, plant some in my greenhouse and save the seeds. Good information here.