Growing San Marzano Tomato Plants
If you’ve ever tasted authentic Italian tomato sauce, chances are it was made from San Marzano tomatoes. This plum tomato is prized for its sweet, robust flavor and low acidity, and is a popular choice for gardeners interested in preserving tomatoes for sauce. In this article, you’ll learn how to grow San Marzano tomatoes from seed to harvest and with tips on how to preserve this world-famous sauce-tomato.
San Marzano Tomato
Solanum lycopersicum
- Sweet and robust
- Bright red color
- Indeterminate Paste Tomato
- A part of the Solanaceae family
- Exposure: Full sun
- Temp Preference: Warmer (70–85°F), frost-sensitive annual
- Soil Preference: Well-draining, loose (sandy loam), slightly acidic (pH: 6.2 to 6.8)
- Garden: Greenhouse, plot, raised bed
- Germination Rate: 96%
80-90 Days to Maturity (from transplant)
San Marzano tomatoes, known scientifically as Solanum lycopersicum, are indeterminate paste tomatoes, meaning they produce fruit throughout the entire growing season rather than all at once. This variety reaches maturation in 80-90 days and produces elongated fruits, typically 3 inches long, with thick, meaty flesh and few seeds. These heat-loving plants flourish in sunny, warm conditions and reward growers with a steady harvest perfect for canning, roasting, or simmering into slow-cooked pasta sauces.
How to germinate San Marzano seeds
Like most tomato varieties, San Marzano tomato seeds need warmth, light, and consistent moisture to germinate successfully. Start your San Marzano seeds 6-8 weeks before your last frost date.
San Marzano tomato seeds
How to germinate San Marzano seeds
- Use a quality seed-starting mix—light, airy, and well-draining.
- Sow seeds ¼ inch deep and lightly cover with soil.
- Keep soil temperatures between 70-85°F for best germination.
- Maintain consistent moisture using a humidity dome or plastic wrap.
- Expect sprouting in 7-10 days with a 96% germination rate under ideal conditions.
When to transplant San Marzano plants
Transplant San Marzano tomato seedlings after all danger of frost has passed and nighttime temperatures stay above 55°F. Select a sunny location that receives at least 6-8 hours of full sun each day.
How to transplant San Marzano tomato plants:
- Prepare loose, well-draining soil rich in organic matter with a slightly acidic pH between 6.2 and 6.8.
- Dig deep enough to bury two-thirds of the plant, including part of the stem. This encourages additional root growth from the buried nodes, creating a stronger, more resilient plant.
- Space plants 24–36 inches apart in rows 3 feet apart.
- Add compost or a water-soluble fertilizer high in phosphorus to promote root development.
- Mulch around the base of the plant to retain moisture and regulate temperature.
Sturdy cages or trellises are a must—these Marzano tomato plants can grow upward of 5 feet tall and become heavy with fruit.
How to prune San Marzano plants
Pruning is key to growing tomato plants successfully. Proper pruning enhances air circulation, reduces disease risk, and directs energy toward fruit production. Unpruned vines can become unruly and should be pruned after the plant reaches a height of 18-24 inches. The small shoots that grow in the junction between the main stem and leaf branch are known as suckers and should be thoughtfully removed.
Indeterminate tomato varieties will keep growing and producing all season, so if every sucker is allowed to grow, the plant can become overcrowded. By removing the suckers on the lower half of the plant, you can help your plants focus on producing larger, better-quality fruits, rather than excessive foliage.
Pruning can be as simple as using your thumb and forefinger to pinch off the branch sucker or garden scissors. These suckers can be placed in a jar of water, and they will start growing roots rather quickly, making excellent clones that can be planted or given away.
Harvesting San Marzano tomatoes
Knowing when to harvest San Marzano tomatoes ensures peak flavor. These tomatoes are ready about 80–90 days after transplant, when the fruits turn a deep, glossy red and feel firm but slightly soft to the touch.
Tips for harvesting San Marzano tomatoes
- Pick in the cool morning hours for the best flavor and texture.
- Use pruning shears or twist gently to avoid damaging the vine.
- Handle fruits carefully; thin skins can bruise easily.
- Store at room temperature and avoid refrigeration to preserve sweetness.
- Regular picking encourages continuous production throughout the entire season.
Canning Tomatoes
San Marzano tomatoes are the ideal choice for canning, providing long-term preservation. These tomatoes can be canned whole or turned into a tasty sauce before being water-bathed. Once canned, they remain shelf-stable for 18 months.
The trick to safely canning tomatoes is achieving the correct acidity. Bacteria like botulism cannot grow in acidic environments. While tomatoes are naturally acidic, we have bred them to be sweeter, which means you need to add citric acid or lemon juice to your tomatoes to achieve the correct pH level of 4.2.
How to safely acidify tomato sauce for canning
- Add bottled lemon juice: 2 tablespoons per quart (1 tablespoon per pint).
- Or use citric acid: ½ teaspoon per quart (¼ teaspoon per pint).
- Always use bottled lemon juice — fresh can vary in acidity.
Start with clean tomatoes, clean glass jars with new lids, and a water bath canner. You can remove the skin by blanching your tomatoes: boil them for 30 seconds, then drop them into an ice bath. At that point, the skins come right off. Don’t forget to remove the core. Place tomatoes in sterile quart jars, leaving ½ inch of headspace. Wipe the mouth of the jar, tighten the lids in place, and boil in a canner for 85 minutes.
Common issues growing San Marzano tomatoes
Even the best growers face challenges when growing tomatoes. The most common problems tomato growers encounter are blossom end rot, flower drop, and cracked or split fruits. Understanding what causes these issues can help you avoid the problem or resolve it if it occurs.
Blossom End Rot
This is one of the biggest problems tomato growers face, and the solutions usually offered are to add more calcium. Blossom end rot is indeed caused by calcium deficiency, but this is typically caused by lock-out from inconsistent watering rather than a true lack of calcium.
Instead of crushing eggshells and sprinkling them around your plant (which doesn’t provide calcium immediately anyway), make sure your soil is consistently moist but not soggy. Consider adding automated drip tape irrigation and straw mulch to help maintain adequate moisture levels.
Flower Drop or Lack Fruit Set
Despite loving the heat, tomato flowers can be finicky when temperatures are above 90°F. When temperatures are consistently high, the flowers can dry out and fall before setting fruit. This happens because high heat interferes with pollination; the pollen can become nonviable, preventing fertilization from occurring.
If you’re experiencing prolonged bouts of high temps and your tomato flowers are dropping, consider adding shade cloth. A simple row cover can help reduce heat, or even companion planting with taller plants, such as cannabis, to provide shade. Be patient, once temperatures drop back into the ideal range (70-85), flowering and fruit set will resume naturally.
Cracked or Split Fruits
When your tomato skins crack or split, this is caused by internal swelling from water. If this occurs due to rain, there’s not much you can do, but if it happens after you water your plants, consider reducing the amount of water you’re giving them and increasing the frequency. Slow and low at more frequent intervals is optimal for tomatoes.
FAQ
Are San Marzanos hard to grow?
San Marzanos are considered moderately easy to grow. They require some attention with regular feeding and generally perform better in warmer climates.
What’s the best tomato for making sauce?
San Marzanos are the gold standard thanks to their low acidity and sweet flavor.
What’s the best fertilizer for growing San Marzanos?
These tomatoes do well with loose, well-draining soil with a slightly acidic pH range between 6.2 and 6.8. Look for water-soluble fertilizer with lots of phosphorus to aid in root development.
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About the Author: Alexandria Irons
