How To Grow San Marzano Tomatoes

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How To Grow San Marzano Tomatoes

How To Grow San Marzano TomatoesHow To Grow San Marzano Tomatoes

The following instructions are just one way to grow San Marzano tomatoes. There are many different techniques and rules that gardeners follow that come from their own experience, access to convenient supplies, nationality and ethnic origins, economic factors, growing zones and weather patterns.

This article is aimed towards beginners and will show you how to grow San Marzano tomatoes in a container which is one of the easiest ways to get started with your new hobby. Whenever possible, we will make these directions easy to follow. The trend in recent decades is to over-complicate gardening in general, and growing tomatoes in particular. I prefer to keep it as simple as possible.

Let’s get started:

There is a low probability that the tomato starters (tomato plants 6-8 weeks old and ready to transplant) at your local nursery, farm supply store, or big chain hardware store are going to offer San Marzano tomatoes. Despite their popularity with Foodies and cooks, in the gardening world, it’s not a commonly grown cultivar. Tomato starters that are usually the most popular include beefsteak, Celebrity, Big Boy, Better Boy, romas and Earl Girl, to name a few. Since you can’t count on a local seller to offer them, you will have to start your San Marzanos from seed.

1. Know your hardiness zone and the last day of frost:

Tomatoes are usually transplanted when they are 6-8 weeks old. To know when to plant your seeds, you need to know when the last day of frost is for the area you live in. If you don’t know, try doing a Google search for “last day frost ——-” and fill in the blank with the nearest major city. Be aware of the fact that this is only an estimate based on averages over a long period. There is no guarantee and you could still get a surprise late frost. In the area where I live, the last day of frost is April 1st. To play it safe, I hold back from planting for two more weeks or on tax day, April 15th. This date then tells me when to start my seeds by counting backward 6 to 8 weeks, which is Feb 15 to March 1st.

You will also need to know the Growing Zone or Hardiness Zone for your zip code which can be found here, or with a more exact map here.

There are many ways to start tomato seeds but we’ll show you this one since it’s easy and the germination rate is 96 percent. The simplest way to start your seeds is with peat pellets and greenhouse domes like the two pictured below. These are sold about anywhere that seeds and vegetable gardening supplies are sold.

peat pellet

 

In the first image, the peat pellet as it comes compressed. The one on the right has been hydrated with water and expands on its own.

Four hydrated peat pellets in their tray with San Marzano seeds visible in the top two. I use the tip of my pocket knife to pull back the soil 1/4 inch. One seed in each pellet is enough.

 

pellets

The peat pellets in their tray. Trays come in many different sizes.

The clear plastic top acts as a greenhouse by keeping the seeds/pellets warmer than room temperature. Condensation will also build up on the inside. The “climate” inside is “warm & muggy” even if you are starting in Feb.
In 7 to 10 days, your tomato seeds will sprout. Germination rate with peat pellets is pretty good, and it’s only necessary to plant one seed in each pellet.

The greenhouse tray must then sit in your window that get’s the most light. If you live in the Northern states that don’t get enough Sun during the first 8 weeks, you may need to get a grow light. However, if you are just a beginner, you may not want to invest that much money at first.

seed tomatoes germination

LEFT: The seedlings pictured here in front of the window are approx. 2 weeks old. I like to let them grow a bit too long in their pellets so I can plant them DEEP. Planting your tomatoes deep now and when transplanted outside is very important for root development.

Center/Right: Seedlings are transplanted into peat pots or other containers that are 3 to 4 inches wide at the top. You must make 2 small holes in the bottom to let the water run out. Place your starters in the window and water about every other day until they are six weeks old.

2. Hardening off your San Marzano tomato plants:

All tomato plants started indoors (and not in a greenhouse) need to adjust slowly to outside temperatures and breezes before transplanting. This is called “hardening off.” This is usually done April or early May when temperatures are suitable. On Sunny with a low breeze, place your tomato pots outside in the Sun for a few hours during the warmest part of the day. Keep an eye on them so they don’t get damaged by the wind or temperatures. If it’s too breezy, use a windbreak or take the plant back inside, and let it get Sun and breeze from an open window. You can also run a fan on them while they are inside for about 1 hour per day. This will help make the stems stronger and thicker.

3. A common problem at this stage

A common problem at this stage is tomato starters that are too “leggy” -as in the main stem is tall but the foliage seems sparse in comparison to its height. This is caused by not enough Sunlight on your young plants (if you have a top-notch greenhouse or a grow light, you will probably not encounter this problem). Tomatoes need lots and lots of Sun from the first day they sprout and you need to get more Sunlight on your starter if you can. This condition is why I like to transplant my starters at 6 weeks instead of 8, and 6 weeks is also recommended by Seed Savers Exchange, one of the best gardening organizations in the world. As discussed above, I plant my seeds on March 1st so they will be 6 weeks old on April 15th when I transplant them.

4. Soil Preparation

With an eye toward transplanting the San Marzano tomato starter outside on April 15th (which is suitable for my growing zone & may be different than yours), the next step is to prepare our container. Since most San Marzano’s are indeterminate plants (vines, over 6ft tall, and produce fruit all season long), it’s advisable to grow them in a container 10 gallons or more. A 5-gallon bucket is undersized for an indeterminate tomato, and better suited for a determinate cultivar. At the bottom of your 10 gallons + container, drill 2 holes, on opposite ends, using a 3/8 inch drill bit, to allow water to flow out.


The trend in gardening these days seems to be to over-complicate the soil mixture. While this might be fine for an experienced gardener, beginners should keep it simple. To this end, keep your mixture at 1/3 organic matter, and 2/3rds top quality organic soil sold in 1 to 1.5 cubic foot bags under the brand names of Scott’s, Miracle-Gro, and Schultz. This soil is of higher quality than regular, plain topsoil which is priced much cheaper. If you do use regular topsoil, use 1/2 organic matter, and 1/2 regular topsoil. Suitable organic matter, also sold in 1 to 1.5 cubic foot bags, includes Peat moss, Manure, and Compost of various origins.

Besides the organic amendments and soil, you’ll also need to mix in 1 to 2 cups of garden lime for a 10-gallon container. Agricultural lime is calcium carbonate which will be necessary later on to prevent blossom rot, also called bottom end rot.

Blossom Rot can be prevented with powdered agricultural limeBlossom end rot is caused by a lack of calcium getting to the fruit. It is much easier to prevent blossom end rot when preparing your soil than it is to fix it after it spoils your tomatoes. Instead of garden lime, you can also use crushed eggshells.

Combine and mix the soil and amendments together thoroughly.

5. Ideal ph Soil levels 

Ideal ph Soil levels for tomatoes are between 5.8 to 7. Inexpensive ph meters are sold in most garden stores. You can raise ph levels with wood ash or agricultural lime, and lower it with organic matter.

6. Transplant your 6 weeks old San Marzano tomato 

Transplant your 6 weeks old San Marzano tomato starter when nighttime temperatures are above 55 degrees and a week or two after the last frost for your growing zone. Dig a hole in the center of your container with a hand size garden shovel.

tomatoes containerToss in a teaspoon tomato or vegetable fertilizer pellets (if desired), or non-fresh horse manure, and a few crushed eggshells into the hole that you’ve made. Carefully separate the tomato from the container and place it into the hole. (If you are using peat pots, like the one at right, they can break apart easily and are biodegradable and you can leave the pieces in the hole.

Watch the weather for the next couple of weeks and if temperatures look like they will drop below 55 degrees, cover your tomato or move the container inside your garage or shed until the next day.

7. A tomato cage or support system 

Will have to be put in place immediately after you transplant your San Marzano starter. It’s better to put it in now when the tomato roots are small, then later on when the roots cover more area and you have to slide it over the tomato plant. Since most San Marzano tomatoes are indeterminate, the simple tomato cages seen are going to be too small. Indeterminate means vines, not a small bush determinate, and vines can easily grow 6 to 8 feet tall or taller. There are many different styles of cages and stakes that you can use, but make sure you get one that is tall enough and strong enough to support a tall tomato plant.

8. Lots of Sunlight

Tomatoes thrive on 3 things: Water, organic matter and Sunlight. After you have transplanted your starter into a 10+gallon size container and installed your stake or cage, get your tomato plant in the area of the yard where it will get the most Sunlight for at least 8 hours a day, and 12 hours a day is fine too. The nice thing about growing in containers is you can move your plant around if you have to.

9. Container grown tomatoes will need to be watered 

Container grown tomatoes will need to be watered more than those grown in the ground since their root system is more susceptible to heat and dehydration from the Sun hitting the outside of the container. When it comes to watering container-grown tomatoes, keep these 3 rules in mind:

  1. Always water at the base, directly on the soil, and never on the leaves or upper stem.
  2. Water gently and not with high pressure as this could damage the roots and splash soil-based diseases onto the bottom level of leaves.
  3. Water once a day, in the morning, and until the container fills to the top with water and it’s running out of the holes in the bottom – then stop.Moisture Meter

10. Mulch and monitor the moisture when the weather is hotter

Mulch basically means a layer of organic matter (pine needles, wood chips), inorganic matter (plastic, landscape fabric) or rocks which keep the roots and soil cool, and prevents soil-based diseases from splashing upwards. Mulch also slows down the evaporation rate and keeps the moisture in the soil longer. Personally, I prefer to use white quartz river rock as this has kept my soil moist for 24 hours and in temperatures in the 80s when the outside temp was 100 degrees.


Moisture meters like the one shown above help monitor your plant’s watering needs. A meat thermometer with at least a 5-inch stem stuck into the soil will help you monitor the temperatures for your tomato’s roots. When temperatures get above 90 degrees, fruit production begins to drop off dramatically. It is the temperature of the roots, and not the leaves and stems, that matters more.

11. Fertilizer, Compost Tea & Rainwater

After a few weeks, your San Marzano tomato plant should be doing pretty good on its own. It is recommended that you fertilize your tomato plant once a week with tomato fertilizer or vegetable fertilizer. Look for some at your garden supply store. You can also make a compost tea by allowing organic matter, either compost or manure, to sit and mix for a while in a bucket of water. I prefer 5-gallon buckets filled with rainwater. (By the way, rainwater is always 10x better than garden hose water). Compost tea is more organic and environmentally friendly. If rain is in the forecast, place as many buckets and containers outside to catch as much rainwater as possible and use them. Pond water and well water is good too.

12. Pruning

When your San Marzano tomato starts to get about 2 to 3 feet tall, you will need to prune it in order to send the sugars and nutrients to the places you want and keep them away from areas where they will do no good.

And finally, if all goes well, your San Marzano tomato plant should be producing ripe fruit in 78 to 85 days after you transplanted it. Maturation dates are always based on when the plant is transplanted, and not when the seeds are started.


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