8 Problem Causing Mistakes Tomato Plant Growers Make!

Most tomato trouble starts before the first fruit ever forms. Seedlings shocked by the move outdoors, plants crammed too close together, a spot with too little sun: each one is easy to avoid once you know it’s a problem.

Here are ten common mistakes, with the fix for each, based on university extension advice. New to growing tomatoes? Start with our guide to basic tomato plant care.

Mistake #1 – Skipping the Hardening-Off Step

Tomato seedlings raised indoors or in a greenhouse have never dealt with wind, cold, or full sun.

According to Nebraska Extension, once outside they lose water faster from heat, sunlight, and wind, and strong outdoor sun can easily sunburn the leaves of plants that were not hardened off. Hardening off gives them time to toughen up before planting.

Here’s how to do it:

• Start 7 to 10 days before you plan to transplant. Nebraska Extension recommends hardening plants off gradually over that period. Clemson Extension says to start a week before transplanting, and UNH Extension says about ten days.
• On day one, set the seedlings in a shaded spot for 2 to 3 hours, sheltered from strong wind, then bring them back inside.
• Each day, add a few more hours outside and a little more sun.
• Watch the soil moisture. Nebraska Extension and Clemson say to cut back on watering gradually without letting the seedlings wilt, while UNH says seedlings dry out faster outdoors and need more frequent watering. Check them every day and don’t let them wilt.
• Skip the cold nights. UNH says to bring plants indoors at night and on days when temperatures fall below 60°F, because tomatoes can suffer chilling injury below that.

Count back from your planting date so the seedlings finish hardening off right when the weather is ready for them.

Mistake #2 – Not Doing Anything About the Soil

Tomatoes do best in soil prepared before planting. Clemson Extension recommends a well-drained site.

UF/IFAS suggests having your soil tested through your local Extension office and working in organic materials such as composted manure before planting, and Minnesota Extension suggests adding compost in spring or fall.

A soil test also tells you your pH. Extension guides give slightly different targets: UF/IFAS says 6.2 to 6.5, Clemson says 6.0 to 6.5, and UNH says 6.2 to 6.8. The goal is soil that is well amended with organic compost.

Related: Epsom Salt For Tomatoes: What It Actually Does (and When to Use It)

Mistake #3 – Planting Tomatoes Too Close

It’s tempting to squeeze in extra plants, but crowding works against you. Dense foliage holds humid air around the plants.

Clemson Extension notes in its pruning advice that higher humidity from heavy foliage can let leaf diseases and fruit rots spread more easily, and Minnesota Extension lists adequate spacing as one way to increase airflow and reduce early blight.

How far apart is far enough depends on how you support the plants. Iowa State Extension gives these spacings:

• Determinate tomatoes: 2 to 2½ feet apart.
• Indeterminate tomatoes on stakes: 1½ to 2 feet apart in the row.
• Indeterminate tomatoes in wire cages: 2½ to 3 feet apart. Our guide to building the ultimate tomato cage shows one way to support them.
• Indeterminate tomatoes left to sprawl: 3 to 4 feet apart.

Other guides run tighter for supported plants. UNH says 18 to 24 inches for staked or caged plants, Clemson says 24 inches, and UF/IFAS’s Florida vegetable planting guide lists 18 to 32 inches for supported tomatoes. For more detail, see how much space a tomato plant needs.

Whichever spacing you choose, support the plants. UF/IFAS recommends staking your plants to keep the fruit off the ground and save space, and you can also use tomato cages to trellis them.

Mistake #4 – Planting the Wrong Tomato Varieties

Pick a variety that fits your space and your season. Tomatoes come in two growth habits.

Determinate Tomatoes

Determinate, or bush-type, plants stay compact. Minnesota Extension says they stop growing and produce a set number of fruits that all ripen within a four- to six-week period, and that they do not need pruning, staking, or trellising.

UF/IFAS notes that most tomatoes can benefit from staking or trellising to help keep fruit off the ground, and that a determinate variety gives just a single crop. If your garden plot is small or you plan to grow tomatoes in containers, Minnesota says bush-type plants may be the only option.

Indeterminate Tomatoes

Indeterminate, or vining, plants keep growing. Minnesota Extension says they need support such as cages, stakes, or trellises and keep flowering and ripening fruit until the weather turns too cold. UF/IFAS adds that they have a large, sprawling growth habit, will require pruning and support, and produce fruit over a period of several months.

Two more things to match to your garden. First, disease resistance. UF/IFAS recommends choosing varieties resistant to known tomato diseases, including Verticillium wilt and Fusarium wilt, and says these qualities are usually noted on the label by short letter codes.

Clemson says Fusarium wilt can destroy an entire tomato crop, and that the letters F, FF, or FFF after a cultivar name indicate resistance to races 1, 2, and 3. Utah State University Extension says Fusarium and Verticillium wilt are both soilborne fungal diseases that can survive in the soil for many years, and that once a plant is infected there is no treatment or cure, so management should focus on prevention.

Second, heat. UF/IFAS says once average nighttime temperatures climb above about 80 degrees, large-fruited tomatoes keep flowering but stop setting fruit, while cherry and grape tomatoes typically perform well through Florida summers.

Mistake #5 – Planting Tomatoes in Shade

So, how much sun do tomatoes need? UF/IFAS says at least four to six hours a day, and its Florida vegetable gardening guide says to choose a garden site with at least six hours of direct sunlight daily.

Maryland Extension says full sun means direct light at least 6 hours a day, and that tomatoes prefer 8 to 10 hours. Colorado State University Extension says tomatoes need full sun for good fruit production.

Trees, roofed decks, and nearby houses can all cast shade, so watch the spot through the day before you plant. If you’re growing in lightweight containers, UF/IFAS notes you can move them around the yard to follow the sun.

Mistake #6 – Never Pruning Indeterminate Tomatoes

Indeterminate tomatoes need some pruning to stay manageable. The shoots to look for are called suckers, which grow in the angle where a leaf branch meets the main stem. Maryland Extension says moderate pruning increases fruit size, hastens ripening, and keeps your plants more manageable.

UNH says pruning can improve harvestable yields, prolong the harvest season, and help reduce some disease problems. According to Minnesota Extension, bush-type determinate plants do not need pruning.

How many stems to keep depends on who you ask. UF/IFAS says leave two to three main stems, Clemson says one or two, and Maryland says one to three for staked plants. Clemson says you can easily pinch off shoots by hand.

It also notes a trade-off: caged tomatoes pruned only once develop heavier foliage, and higher humidity may let foliar diseases and fruit rots spread more easily. When you prune tomato plants, keep leaves and airflow in mind.

We like these hand pruners.

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Clean your tools, too. Minnesota Extension says to wash and sanitize pruning tools after touching infected plants, so make sure you sterilize your pruning shears after cutting a plant that shows disease symptoms.

Mistake #7 – Not Ready for Early Blight

Early blight is one of the most common tomato diseases. Minnesota Extension says it can cause severe defoliation, and both Minnesota and UNH say the fungus overwinters in infected plant debris and soil. Minnesota adds that lower leaves get infected when they touch contaminated soil, including when raindrops splash soil onto them.

Here’s how to spot it. Small dark spots form first on older leaves near the ground. Larger spots have target-like concentric rings, and the tissue around the spots often turns yellow. Severely infected leaves turn brown and fall off.

What helps, according to Minnesota Extension, unless noted:

• Rotate. Wait two years before planting tomatoes or peppers in the same location. Clemson recommends choosing a site where tomatoes, peppers, eggplant, or potatoes haven’t been grown for at least three years to help avoid soil-borne diseases and nematodes.
• Cover the soil under the plants with mulch, water at the base of each plant, and stake or trellis for airflow.
• Prune the bottom leaves. This can prevent spores from splashing up from the soil onto leaves.
• Remove spotted leaves. It is okay to remove up to one-third of the plant’s leaves if you catch the disease early, but not more than that.
• Look for resistant varieties. Resistance does not mean you will never see early blight, but the damage will be less severe.

As for spraying, Minnesota Extension says most home gardeners don’t need to treat tomatoes with a fungicide, because tomato plants can tolerate a lot of early blight without producing fewer tomatoes.

Related: What Causes Blossom Drop in Tomato Plants?

Mistake #8 – Overfertilizing Your Tomato Plants

Tomatoes do need nutrients, but more isn’t better. Minnesota Extension says too much nitrogen fertilization leads to bushy, leafy plants that are slow to bear fruit, and UNH says excess nitrogen can produce lush, vigorous foliage but little fruit.

UF/IFAS adds that excess nitrogen can also contribute to blossom-end rot because extra leaf growth redirects water and calcium from developing fruit to the leaves.

Let a soil test guide you. Clemson calls a soil test the best way to determine what the crop needs, and Minnesota says you may not need extra fertilizer, depending on how much organic matter you apply. When you do feed, compare the best fertilizer for tomatoes and follow the label.

Mistake #9 – Watering Unevenly

Erratic watering causes trouble, and blossom end rot is the classic result. UF/IFAS explains that calcium travels up the plant in water, so erratic watering also creates an erratic calcium supply to developing fruit, which may be missing when needed.

Maryland Extension describes blossom end rot as caused by a calcium shortage in enlarging fruits and lists inconsistent watering as a factor that encourages it. Once a fruit has it, UF/IFAS says it can’t be fixed, so the plan is prevention.

Here are the tips that matter most:

• Aim for steady moisture. UF/IFAS recommends about one to two inches of water per week, with a heavy soaking once a week preferred to several light sprinklings, and Maryland says to water deeply and regularly during dry periods. Try a consistent watering schedule with soaker hoses and learn about deep watering tomatoes.
• Mulch. UF/IFAS says mulch helps the soil retain moisture, and Maryland says to keep plants well mulched and watered throughout the growing season.
• Water the soil, not the leaves. Minnesota Extension says to water at the base of each plant with drip irrigation, a soaker hose, or careful hand watering, and that wet leaves, especially in the evening, can cause leaf diseases.
• Don’t overdo it. Minnesota Extension warns that plants may decline and die when their roots stay too wet, and UNH says too much water at once can cause ripening fruit to split. Here’s why tomatoes crack.

Mistake #10 – Hanging Tomatoes Upside Down Without a Plan

Upside-down planters look simple, but Colorado State University Extension points out several catches. A planter filled with damp soil and a large tomato plant can weigh 50 pounds, so the hardware must be strong enough to hold it. Trees, roofed decks, and nearby houses cast shade, and the planter itself can shade a young plant.

Choose the variety and container with care. Colorado State says cherry and other small-fruited tomatoes are better suited to upside-down growing, and that a small rooting volume (one brand’s planter called for two pounds of soil) would not support a large tomato plant in its hot, windy climate.

Buckets are preferred over thin, breathable plastic planters, which dried out so quickly that watering even once a day was not enough. Colorado State also notes that in Colorado’s windy weather, the weight of the stems can pull them over or break them.

Colorado State’s bottom line: upside-down planters are an option if you can’t grow tomatoes right side up for reasons of space or sunlight. Otherwise, it is easier to grow them right side up.

Get these ten right and you’ll sidestep most of the planting-stage trouble that holds tomatoes back. For more on a bigger harvest, see our guide to high-yield tomatoes. Which mistake has tripped you up the most?