Marin Master Gardeners
University of California
Marin Master Gardeners

How To Grow Great Tomatoes In Marin

Our focus is upon indeterminate varieties, those that grow continuously until killed by frost or disease. By contrast, determinate tomatoes grow to a fixed mature size and ripen their fruit over a short period of time – e.g., two to four weeks. The rules for growing determinate varieties are similar in most respects, but determinate tomatoes need less support, less pruning, and can grow in smaller containers.

THE RIGHT VARIETY FOR YOUR CLIMATE

The single most important step towards a great tomato harvest is to purchase the varieties suited to your climate – e.g., coastal cool or inland warm. The simplest way to make the right choice is to focus on the number of “Days to Maturity” listed on the plant label. Unfortunately, commercial growers do not provide a fixed definition of this term, except to say that it represents the required number of “warm and sunny days” from the date you plant to the date that you pick the first fruit.

However, there is a solution. Tomato plants thrive in daytime temperatures between 80-90 F and do poorly if nighttime temperatures drop below 50-55 F. Consider days that meet these conditions (80-90 F during the day and not too cold at night) to be “warm and sunny days” that count towards “Days to Maturity.”

Your task is to estimate how many calendar days must elapse in your climate to reach the number of “Days to Maturity” on the label. For example, if you live in a cool climate (e.g., parts of Mill Valley or West Marin), it may take 90 or more calendar days for 70-75 “warm and sunny days” to occur – by which time it may be September. As a result, you would want to choose varieties with 75 or fewer “Days to Maturity,” as opposed to other varieties that require 85-90 “Days to Maturity.”

PLANTING YOUR TOMATOES

Tomatoes prefer full sun, all day. Ideally, you should find a location where the sun shines on your plants 10-12 hours per day. At a minimum, tomatoes require six hours of full sun. Look for a location that meets this need.

Bury the stem up to the point at which the topmost cluster of leaves begins, removing any small leaves below that point. Do not touch the stem! Those hairs are potential roots. Separate each plant by 24-36 inches.

If you have grown your plants from seed or have purchased them directly from a greenhouse that has not introduced cold air to the growing process, be sure to “harden them off” before planting. Take up continued next page to a week to gradually introduce them to greater amounts of outdoor temperatures (e.g., six hours outdoors during the day and protected at night on the first day, moving to outdoors all day and all night on the last day of the period).

THE RIGHT SOIL

In Containers
Use nothing less than ten-gallon containers for indeterminate tomato varieties. Remove all – yes, all – of the old soil and use it for other garden vegetables, like beans and peas. Clean the container with soap and water, a bleach solution (1 tbsp / gallon of water), or isopropyl alcohol (70% or stronger). Replace the soil with a reputable, medium-textured, well-draining potting (not planting!) mix.

In Raised Beds
Add large amounts of compost or other nitrogen-bearing amendments to create a nutritious soil. Loosen and work the compost at least three to six inches into the soil prior to planting. If you add soil, use a reputable, well-draining mix designed for raised beds (not potting or planting mix!). Raised bed mixes are new but widely available. They are designed to avoid soil compaction deep in the bed that can hinder aeration and retain too much moisture.

Be sure to rotate your tomato beds to avoid soil-born diseases. The University of California recommends that homeowners avoid growing tomatoes or members of the nightshade family (Solanaceae) in the same location more than two years in a row.

In the Ground 
Follow the same advice as for plants in raised beds. Consider adding cardboard and paper bags as mulch around the plants. Lay it down and cover it with compost. It smothers weeds, holds moisture and eventually improves the soil.

THE RIGHT IRRIGATION

This one is tricky, because there are two critical rules that, at first, seem like they conflict, but they do not.

First, your soil should never become too wet (i.e., soggy) or too dry (i.e., cracking). This is why we recommend specific soil mixes. The right mix helps to pass excess moisture below the plant roots and holds needed moisture for a least a few days at a time.

Second, the amount of water that you apply declines over the life of the plant. Until “fruit set,” water regularly and deeply – no less than once each week. “Fruit set” refers to the point at which you begin to see tiny tomatoes emerging just below the withering flowers. Watering more frequently during this stage allows the stems and leaves to grow vigorously. After “fruit set,” use a three-inch rule. Do not let the soil dry out deeper than three inches. Poke your finger two to three inches into the soil to check. Water only if the soil is dry below two inches. This minimizes further vegetative plant growth, but allows sufficient water to reach the flowers and fruit. Do not over water at this stage or your fruit may crack or have less concentrated flavor.

Near the end of the harvest, if you stop watering altogether, the flavor in the remaining tomatoes will concentrate as the fruit ripens.

One last note. Never water the leaves! Use drip irrigation if possible.

THE RIGHT FERTILIZER & SUPPORT

Use a reputable tomato or vegetable fertilizer and follow the directions on the label, but do not start until flowering and fruit set are well under way. This is important. Earlier feeding can delay both flowering and fruit set! Homemade nitrogen sources can work well, but not too strong and not too often (e.g., every four to six weeks after flowering and fruit set have started).

Indeterminate tomatoes require tall (5-6’) trellises, cages or stakes. Tie each sagging stem to the supports at multiple places to prevent sharp bends that can restrict the flow of water and nutrients. 

THE RIGHT PRUNING

Unpruned plants will bear fruit, but pruning will significantly improve your yield and fruit size. The goal is a plant with two or more main stems, but not so many as to block sunlight to the interior. Also, allowing more sunlight and air into the interior helps to prevent certain plant disorders.

The number of main stems you choose will depend upon how tightly your support structure crowds the plant. Narrow cages may limit you to two main stems per plant. Wider cages may permit three to four main stems per plant. Once you have chosen the stems you want to keep, pinch out the others as well as any suckers that may emerge from the root area.

As your main stems grow, pinch and remove any new branches that form at leaf nodes that may crowd the center of the plant or grow in undesirable directions. How many of these new branches you remove is a balancing act. If you allow the plant to become too dense, less sunlight will flow to the flowers and fruit in the interior. By contrast, if you thin the plant too severely, you will have fewer fruit-bearing branches and fewer leaves to feed the fruit that forms.

PROBLEMS 

If you notice a problem, diagnose and deal with it immediately. Some tomato diseases can spread within and across plants rapidly. Know good bugs from bad. Know watering problems from disease problems. If you need help, call the MMG Help Desk (415-473-4204). Use chemicals as a last resort.

Every one of these tips is important, but we end where we began. If you select tomato plants that are suited to your location, you will take a giant step towards improving your tomato yield and flavor this summer.

By Allie Edwards Williams, Dana Tamura and Kirby Wilcox, Marin Master Gardeners

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