How to Plant, Grow, and Care for ‘San Marzano’ Tomatoes

‘San Marzano’ tomatoes are a culinary delight. Considered to be a top – even the best – paste tomato, this tomato carries a robust, complex flavor in a plum tomato form.

Originally from southern Italy and prized for use in sauces, This variety brings heirloom characteristics of vigorous growth and exceptional flavor to the garden. Even if we don’t live in the shadow of Mount Vesuvius, with fertile soils and boundless sunshine, we can imbue a little of the Italian countryside in our own gardens and kitchens by growing ‘San Marzano’ heirloom tomatoes.

What are ‘San Marzano’ Tomatoes?

These Italian heirlooms are famous for their exceptional flavor in pastes and sauces. These plum, or paste, tomatoes feature a complex, rich flavor perfect for preserving, canning, and fresh eating. 

True ‘San Marzano’ tomatoes originate in the Campania region of southern Italy, in the volcanically rich soils of Mount Vesuvius. Italian-grown ‘San Marzano’ tomatoes are said to be sweeter and less acidic than those grown in other regions. 

These superior paste tomatoes are a prized export protected under an official governmental convention, the Denominazione d’Origine Protetta, or Protected Designation of Origin. This designation protects significant foods and foodways of Italian origin. 

The designation means certifying jarred and canned tomatoes with an official seal. The tomatoes must be strains of the original heirlooms, grown outdoors, harvested by hand in the summer, and jarred halved or whole (you won’t find true San Marzanos as fresh produce, chopped, or diced in cans). The cans are recognizable in grocery stores and markets and are pricy because of their specialty.

Tomatoes grown outside the region are sometimes called “San Marzano-style” tomatoes. For our own taste of the Italian countryside, we can grow this specialty heirloom in our gardens. 

Characteristics

This variety yields loads of long, narrow, pointed plum tomatoes, growing in clusters of six to eight tomatoes each. Fruits mature 70-90 days after transplanting. These are large for paste tomatoes, with each oblong fruit weighing about five to six ounces and growing three to four inches long. ‘San Marzanos’ are sweet in flavor and have low acidity. The bright red tomatoes have less water and seed content than other plums, making them meaty and thick – ideal for pasting.

In addition to their delicious flavor, these plants are vigorous indeterminate growers with good disease resistance. Plants resist fusarium and verticillium wilts, which are common (and often fatal) fungal issues for tomatoes.

‘San Marzano’ is an indeterminate tomato with long vines that reach six feet or longer. Plants grow all season and produce fruit from late summer through frost. As productive fruiters, they need a large tomato cage or staking/trellising for best growth and support.

Native Area

‘San Marzano’ is named for its town of cultivation, San Marzano sul Sarno, in the Salerno province of Campania south of Naples. This region of southern Italy features abundant sunshine, mild temperatures, and fertile soils.

Its ancestors are small tomatoes originating in coastal South America, with a narrow growing range bordered by the Andes Mountains from Ecuador to Chile. Humans cultivated this small-fruited plant, spreading its range throughout Central and South America. The fruits that became the modern tomato began in cultivation some 7,000 years ago.

Explorers and colonists from Spain brought domesticated tomatoes to Europe in the 15th and 16th centuries. They rose in popularity in cuisine in the 17th and 18th centuries.

Planting

Tomatoes require warm air and soil temperatures for best growth. These heirlooms are open-pollinated and grow easily from seed. Start seeds indoors four to six weeks before your final frost date for best results. Transplant seedlings outdoors one to two weeks after the last frost or as nighttime temperatures exceed 55°F (13°C).

When choosing a garden location for tomatoes, pick a spot where you haven’t grown other nightshades in the past year. Tomatoes benefit from an annual crop rotation. As a nightshade, they’re susceptible to diseases from other nightshades like eggplants, peppers, potatoes, and tomatillos. Rotating crops helps avoid lingering soil-borne diseases.

In the ground, space plants two- to three feet apart to allow for long vines and good air circulation. For row plantings, space rows three to four feet apart. 

Plant tomato seedlings deeply to promote a robust root system, with two-thirds of the stem in the ground and one-third exposed. The stem has a special plant adaptation  – its hairs and nodes set roots for more vigorous plants and higher water and nutrient absorption.

The sprawling vines need sturdy support to keep fruits and stems upright. A five-foot-tall by two-and-a-half-feet-wide tomato cage put in place at planting suffices. Or, opt for trellising or staking to support the fruiting vines.

To grow in containers, opt for a 10-gallon or larger pot with good drainage. Install the vine support structure at the time of planting.

Transplanting 

Seedlings growing indoors benefit from a hardening-off period before being transplanted into the garden. As frost passes and temperatures warm, gradually expose seedlings to outdoor garden conditions. 

Move plants outside to experience a few hours of sunlight and natural breezes (but not winds). Gradually move the plants to conditions mirroring their new garden location. Do this over a week to 10 days to allow seedlings to acclimate to outside growing conditions.

How to Grow

‘San Marzano’ tomatoes are prolific growers and easy-care tomato plants. Give them plenty of sunlight, consistent water, and good air circulation for the best health and vigor.

Light

Growth is best in full sun. The optimum amount of sunlight is six to eight hours daily.

In hot growing regions, provide afternoon protection from intense, direct sunlight. Morning sun and dappled afternoon light are ideal to prevent fruit and leaf burn.

Water

Provide one to two inches of water per week for consistent moisture. Two deep soakings per week are generally enough when conditions are mild. As summer temperatures rise, increase watering sessions as needed to maintain even soil moisture.  

Fluctuations in watering stress tomato plants and lead to pest and disease issues. Regular water is best, and a soil touch test to feel for dryness helps avoid overly dry or wet soils.

Another measure to prevent disease when it comes to tomato watering is to avoid splashing the leaves when feasible. Watering at the soil level and base of the plant allows water to penetrate the roots without spreading fungus or bacteria among the foliage. If your garden relies on sprinklers or overhead sprays, water in the early morning so leaves dry in the sun.

Regularly check the moisture level of plants growing in containers. Pots dry out quickly in the summer heat and warm, drying winds. Use a well-draining potting mix suited for vegetables and blend with compost to retain moisture.

Soil

‘San Marzano’ tomatoes prefer organically rich soils. Slightly acidic soils with a pH between 6.0 and 6.8 are best. Above all, well-draining soils are critical to tomato health. 

At planting, add one-third compost to two-thirds high-quality organic soil mix. For native soils, add a more generous heap of compost, up to one-half of the composition.

Temperature and Humidity

‘San Marzano’ are frost-sensitive plants that thrive and produce best in the heat of summer. Seeds germinate with temperatures between 70-90°F (21-32°C). Seedlings grow outside with air temperatures above 55°F (13°C) and soil temperatures above 60°F (16°C). Warm temperatures between 70-80°F (21-27°C) are prime for growth.

Tomatoes thrive in hot, humid summers, but plenty of air circulation around plants is necessary to counteract humid conditions and prevent pests and diseases. 

Fertilizing

This productive fruiting annual benefits from nutrients to flower and fruit vigorously. Compost and fertile soils set tomatoes up for success, and fertilizer applications keep them thriving.

Apply a low-grade fertilizer like 5-5-5 or 5-10-10 at planting. Organic options of fish emulsion, kelp, and seaweed at planting and during growth add the necessary enrichment for growing, flowering, and fruiting.

For flowering and fruiting, opt for a fertilizer higher in phosphorous (the “P” in the N-P-K fertilizer rate) and lower in nitrogen (N).  A 6-8-12, 8-32-16, or 6-24-24 ratio is usual for tomatoes in the flowering phase. Organic tomato-specific fertilizers will have these formulations.

Overfertilizing and too much nitrogen lead to stress and disease issues. They also create leafy plants with few blooms or fruits. 

Tomatoes sometimes need calcium enrichment, indicated by yellowing leaves and blossom end rot. In many of these cases there is enough calcium in the soil, but not enough water for the nutrient exchange that funnels calcium to your plants. Test your soil before planting, and amend as needed. Then water regularly after planting to provide proper nutrient content for luscious tomatoes.

Maintenance

Pruning isn’t necessary with caged tomatoes, but pinching off low offshoots helps direct nutrients to the upper parts of the plant, especially with staked or trellised plants. Pinch off suckers as they appear to inhibit detracting growth and increase airflow around the base of the plant.

Mulch after the sun has had time to warm plant roots for growth. Mulching too early and at planting can inhibit the sun’s warmth from reaching the surrounding soil. When stems reach 18-24 inches tall, layer two to three inches of weed-free straw around plants to help with moisture retention, weed suppression, and soil temperature regulation.

Propagation

These are open-pollinated heirlooms, meaning they’ll come true to type from seed unless they’ve cross-pollinated with another nearby tomato variety. If you hope to preserve the true heirloom for seed collection in the home garden, provide 35 feet between homegrown tomato varieties or enjoy your own hybrid the following season from cross-pollinated selections.

Growing From Seed

Sow seeds in early spring after the last frost in mild climates or indoors four to six weeks before the anticipated final frost date in cold climates. Tomato seeds germinate quickly, usually in five to ten days.

To sow seeds indoors, plant them ¼ inch deep in a lightweight potting medium in a tray, cell, or small pot with drainage. Place the seeds in a warm spot with temperatures between 70-90°F (21-32°C) for germination. Keep the potting media evenly moist with regular misting.

When sprouts emerge, place the seedlings in a sunny location like a windowsill. When two to three sets of true leaves appear, step seedlings up from trays and cell packs to a small pot. Seedlings are ready to harden off outdoors when they reach six inches tall and wide.

Common Problems

Fortunately, ‘San Marzano’ tomatoes aren’t overly bothered by pests. They’re also disease-resistant to the typical tomato fungal problems of fusarium and verticillium wilts. 

This cultivar is still susceptible to common tomato problems. The best way to prevent tomato pests and diseases is to maintain optimal cultural requirements, especially consistent watering, air circulation, soil health, and crop rotation. Companion plants for tomatoes, like basil, marigolds, and dill, attract beneficial insects and promote plant health.

Pests

Regular scouting to spot pests early is the best control against quick damage to ‘Sungold’ tomatoes. Use organic and food-safe pest controls regarding food crops, and follow label instructions to avoid impacting healthy plants, flowers, and pollinators.

Aphids sometimes visit ‘San Marzano’ tomatoes. These are common sap-sucking garden insects that don’t often cause a severe threat but can weaken and spread diseases among plants. They also leave behind a sticky honeydew that can lead to black, sooty mold. 

If you notice the insect or see signs of curled leaves or stunted growth, spray plants with a stream of water early in the day to knock them off stems. A simple horticultural soap or Neem oil treats infestations. However, these affect pollinators and beneficial insects. Use them with caution.

Predatory insects feed on aphids, so planting pollen and nectar-rich plants nearby benefits vegetable crops, increasing biodiversity and natural pest control.

Diseases

Paste tomatoes are susceptible to blossom end rot, which is more of a physiological disorder than a disease. As with pests, the best disease control is prevention through cultural conditions.

Blossom End Rot

Blossom end rot likely results from fluctuations in watering and the plant’s inability to absorb nutrients, including calcium. It is evident when a developing tomato suddenly becomes brown and rotted at the base—what a sad sight.

Paste tomatoes like ‘San Marzano’ may experience blossom end rot due to their lower moisture content. Provide regular irrigation, especially during dry spells, for consistent moisture. Add calcium to the surrounding soil through kelp, seaweed, and calcium additives.

The specific fruits experiencing blossom end rot won’t reverse, so cutting off the damaged tomato is best. However, future tomatoes won’t necessarily be affected if you provide even moisture, compost-rich soils, low-nitrogen fertilizer, and calcium.

Leaf spots, Anthracnose, and blights are also common tomato diseases. These are best controlled through pruning away diseased plant parts, and tossing heavily affected plants. Again, crop rotation and pest control through planting a biodiverse garden that attracts beneficials will also prevent many diseases.

Frequently Asked Questions