How To Grow San Marzano Tomatoes

San Marzano is, without doubt, one of the best tomatoes for making sauces. This Variety has a strong sweet taste with very few seeds. San Marzano Tomatoes are an indeterminate variety with a long growing season. Due to this variety being open-pollinated the seeds can be kept from season to season without any issues. If you are looking for a suitable variety to grow to make sauces from this is an excellent variety to grow year on year.

You can freeze and can these tomatoes and enjoy the taste all year round! Trust me nothing is better than a home cooked meal on a cold winters day, This is especially true when it’s using your own homegrown tomatoes making those warm summer memories come flooding back.

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san marzano tomatoes
San Marzano

Grow San Marzano Tomatoes From Seed

Growing San Marzanos from seed takes a bit more effort but is worth doing as you can get more plants for your money. Depending on where you live in the world start sowing your tomato seeds late winter if you are planting in a greenhouse or early spring if you are growing them outdoors. I grow my tomatoes in compostable cardboard pots. I find the plastic reusable pots are always splitting and I like to do my bit for the environment. To Grow from seed start with small seed pots or cell trays 1″ diameter.

Make sure to label the seeds including the variety and planting date

Lightly fill your seed trays or pots with potting compost try to avoid compacting the dirt too much a lighter mix will help the roots develop. I put 1 seed per pot and lightly cover with approx 1 cm of compost. Water well and leave to stand in a warm sunny place like a window sill or greenhouse. Water daily and you will expect to see tiny seedlings within 7 to 14 days.

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Potting On San Marzano Tomatoes

Potting on is the process of moving the plants to bigger and bigger pots until its large enough to be planted out in its final growing space. Why do we do this? Transplanting a plant to a larger pot as it grows stimulates root growth and is an opportunity to plant it deeper each time for even more root growth. This potting on process does shock the plant and halt growth so only do this 2 to 3 times.

You know its time to pot on when you start to see roots poking out the bottom of your pots. The time this takes varies with the amount of water, light, and warmth so check every week. Here is the potting on process I follow:

StagePot Size
First Stage (seeds)1″
Second Stage (First true leaf)3″
Pre-Planting Out6″

Planting Out San Marzano Tomatoes

Tomatoes need full sun so start to plan where you are going to put them. The San Marzano plants may not look big right now but give them a spacing of around 24″. This is to allow adequate space to grow out and enough space to airflow between plants to help avoid diseases.

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I’m fortunate enough to have a large greenhouse but you can grow San Marzano outside in warmer climates (Europe and US zones 5 to 10). In fact, I split half my tomatoes between the greenhouse and the rest outside. It’s always interesting year on year that you will have inconsistent results but if you are looking for a safe bet grow them in the greenhouse.

If you do want to grow San Marzano tomatoes outside they need to be hardened off first.

Why And How To Harden Off Tomatoes

Once you have your tomatoes in large 6″ pots and you want to grow tomatoes outside you need to Harden off the plants first. This is the process of gradually getting the plant used to growing outside. I do this by putting my tomato plants outside in their pots on a table on a sunny day then bringing them back inside during the late afternoon.

Take care not to leave young plants out in weather extremes like wind and rain as they can easily become damaged.

This process takes 7 to 10 days and hardens up the stems when you can finally leave outside overnight. You can tell when this process is complete as the plant will appear less leggy, have a darker green color, and will appear stronger.

Supporting San Marzano Tomatoes

There are lots of ways to support tomatoes so much so I wrote a separate article on it. San Marzano tomatoes are indeterminate this means they grow and grow until you pinch the growing tip. You need to provide support for the tomato to grow up and tie the vines to the support. For outside I find a piece of a wood trellis or tomato cage is perfect for this. I tend to avoid single canes for San Marzano tomatoes because the weight can cause them to topple over. If I am growing in the greenhouse I use thick wire supports that I can tie the tomatoes too and pinch the tops once It gets to the roof.

How To Fertilise San Marzano Tomatoes

Fertilizing tomatoes is very important as San Marzano plants need a lot of nutrients so fertilizing to keep them going is key. I start fertilizing as soon as a have 3 or 4 sets of tomato flowers. I use a seaweed extract formulated for tomatoes and feed little and often once a week. This keeps the tomatoes growing and gives it that boost it needs to produce a heavy crop.

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How To Prune San Marzano Tomatoes

I go on about this a lot because it’s an important step not to be missed. You need to pinch out the suckers on a regular basis. This prevents the plant from putting all its growth into producing more fruit instead of vines. I recommend you read the article but here is a brief overview.

  1. Follow the main step from the ground upwards
  2. You will find a shoot that goes off to form flowers
  3. Between this Y shoot, another shoot will appear this needs to be pinched out with your fingers
san marzano tomatoes
Pinch Out Suckers

When to Pick San Marzano Tomatoes

Mature San Marzano tomatoes are approximately four inches long and oval in shape. Within 75 to 80 days of sowing these tomato seeds, the fruits will mature and be ready to harvest and serve.

So, when to pick San Marzano Tomatotes? Well, you can begin harvesting these tomatoes when they are large and plump, but still a touch green and yellowish in color. Take note that, like other fruits, tomatoes continue to ripen after being harvested. You may also wait until the tomato is completely ripe before harvesting it.

Pests And Common Problems

One thing you need to do as well as routinely check for suckers is keen an eye out for pests. Your tomatoes are like gold and all the garden bugs want it. Slugs, Snails, and a variety of diseases can ruin a plant easily. I have an article about common pests and diseases and how to fix them.

Harvesting San Marzano Tomatoes

Seed to harvest can vary but around 80 days is when you need to start looking for color changes in your tomatoes. San Marzano tomatoes will not ripen all at once, this is a good thing as it gives you time to preserve them. Don’t pick them until you see at least 80% of the fruit turning pink/red.

How Many San Marzano Tomatoes You Can Get from a Plant?

The fruits are oval in form, ranging in length from 1 to 3 inches and weighing up to 3 ounces. Usually, a single San Marzano plant can produce approximately 100 tomatoes.

Preserving San Marzano Tomatoes

Now the fun can really begin! these tomatoes are so versatile, salads, eating raw, lots of options but best of call caning them for the winter. Preserve your tomatoes the Italian way with my favorite recipe.

Tips to Grow San Marzano Tomatoes

Here are some tips on how to start your own San Marzano tomatoes:

Select the Rigth Container

If you’re going to grow them in a container, go with the largest one you can find. These tomato plants have a robust root system to match their vitality. Small pots will simply promote root cramping and reduced tomato output. Grow these plants in 20-gallon pots to get the most tomato yield out of them. 

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Soil Requirement

For San Marzano tomatoes, add compost and manure to the soil. This breaks down in the soil and helps your tomatoes. Nitrogen encourages plum tomatoes to develop quickly and large. So, if you’re searching for a way to boost nitrogen levels in your soil, mixing compost with coffee beans is a fantastic way to go.

pH of The Soil

San Marzano tomatoes should have a pH level of about 6.8 in the soil. Tomatoes thrive in acidic soil with a pH of 6.0-6.8, therefore, before planting, it is recommended to use a soil pH tester. 

Provide the Plant Required Support

Despite the fact that these plants will not grow as large as other indeterminate kinds, they will still require the assistance of a tomato cage. For the greatest results, use a cage that is at least 5 feet tall.

Provide a Lot of Light

The San Marzano tomato type, like all other tomato plants, requires at least 6–8 hours of direct sunshine every day. While they will take up a little less space, the yields and fruits will be smaller.


Tomato plants prefer to be kept wet at all times. Don’t let their soil fully dry out. Compost tea can be used to water them. To water plants properly, use this tea, pondwater, well water, or rainfall.

Fertilizer for San Marzano Tomatoes

Use a fertilizer that has a 5-10-10 N-P-K ratio. Avoid using high-nitrogen fertilizers, which can result in luxuriant plants that produce little or no fruit. For tomatoes grown in pots, use a water-soluble fertilizer.

Temperature and Humidity 

These tomatoes thrive in temperatures ranging from 50 to 95 degrees Fahrenheit, with soil temperatures of at least 60 degrees. Seedlings and young plants should not be transplanted into too cold soil since the shock will impede their growth.

Are San Marzano Tomatoes Indeterminate

Yes! San Marzano tomato plants are indeterminate!

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