Growing Tomatoes

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Variety Selection

There are over 2,000 cultivars of tomatoes grown worldwide.  Ask neighbors, local gardeners, and garden center staff about local favorites.

Hybrid tomatoes are popular in the United States to reduce problems with Verticillium and Fusarium wilt, common soil-borne pathogens.  Early hybrids were developed for their yields and disease resistance.  Flavor became a driving factor in the breeding of newer hybrids.  Some gardeners prefer to trade off the disease protection of hybrids for the rich tomatoey flavors of heirloom varieties.

For early production, Early Girl is a popular variety with mid-size fruits. 
Celebrity, Big Boy, and Better Boy are examples of popular main season varieties.  Many gardeners prefer the rich tomato flavor of heirloom Brandywine or the large beefsteak types.  Pear tomatoes and yellow types are gaining popularity.

Cherry and the new grape-type tomatoes are popular for salads and snacking.  Many, but not all, have small size vines suitable for container gardening.  [Figure 1]

Sweet 100 Tomato
Figure 1.  Sweet 100 is the most popular home garden tomato. On a large vine, it produces hundreds of sweet, cherry sized fruits with very tender skins.


Requiring less time to cook down, paste types such as Roma and its descendents are preferred for making salsa, chili sauce, and other tomato products.  Be aware that paste types and standard varieties are not directly interchangeable in recipes.

Where the growing season is short, select Early Girl and other varieties that will mature in 50 days or less.  In many mountain communities, tomatoes may only be successfully grown in a structure or adjacent to the south side of a building to provide frost protection and warmer growing temperatures.

Whatever type you prefer, VFN resistant hybrid varieties are recommended.  The abbreviation VFN indicates resistance to Verticillium wilt, Fusarium wilt, and nematodes.  Verticillium and Fusarium wilts are common soil- borne fungal diseases.  Nematodes are not an issue in Colorado due to cold soil temperatures.  Researchers have found multiple strains of Verticillium and Fusarium, so if you are having problems with these diseases, try other VFN varieties.

Vine types – There are two types of vines: indeterminant and determinant.  Most popular home garden varieties are indeterminant.  The vine keeps growing through the growing season, extending fruit production until frost kills the vine.  Plant size is typically large.  Determinant types are common in commercial production as vine growth stops when flowering begins; plants will typically be moderate in size.  Determinant types put on a large single crop.  They may be suitable for container planting where trellises are not possible.

Planting

Planting Time

Tomato plants need night temperature above 32°F and daytime temperatures above 60°F.  They are readily killed by a light frost.  A week of cool daytime temperatures (below 55°F) will stunt plants, reducing yields.

With these warm temperature requirements, planting time along the Colorado Front Range is typically late May.  Do not plant tomatoes out into a cold spell and make sure soil temperatures are warm.

To get a head start on the season, gardeners use a variety of frost protection techniques.  The Wall-of-Water® provides protection into the mid teens, or lower.  Cool soil temperatures also inhibit early growth.  When using a Wall-of-Water, also use black plastic mulch to help warm the soil.  Be cautious in filling the Wall-of-Water not to splash water around, as a wet soil will be slow to dry and warm in the spring.  For additional information on using plastic mulch, refer to the CMG GardenNotes #715, Mulches for the Vegetable Garden.  [Figure 2]

Tomatoes in Wall-of-Water
Figure 2.  Wall-of-Water protects individual plants down to the mid-teens.  Notice that black plastic mulch was also used to warm the soil.  Cool soil temperatures are also a growth-limiting factor with early plantings.

 

Selecting the Ideal Plants

The ideal tomato transplant is dark grass green and six to eight inches tall.  The stem is about pencil size in diameter and the plant has not been pruned or cut back.  Transplants are hardened-off (growth rate slows so the plant is more tolerant of the move the greenhouse environment to the bright, windy outdoors) by withholding water and/or nutrients or by exposure to cooler temperature.

Plant leggy transplants horizontally – When gardeners are shopping for transplants in the warm greenhouse conditions of May, tomato plants quickly grow from ideal size to tall and leggy.  The white bumps along the leggy tomato stem are roots beginning to form. 

Plant these taller leggy transplants horizontally.  Dig a trench a two to three inches deep.  Place the plant horizontal with only the top two to three sets of leaves showing above the soil.  Pinch off other lower leaves below the soil line before planting.  These leggy plants readily root out along the stem in the warm soil near the surface, supporting rapid growth.  [Figure 3]

Plant leggy tomato horizontal
Figure 3.  Plant tall leggy tomatoes horizontal in a shallow furrow.


Space and Trellis Plants

To minimize Early Blight, space and trellis plants to allow for good air circulation and promote rapid drying.  Trellised tomatoes are easier to pick and less preferred by tomato psyllid insects.  Trellising eliminates problems with fruit rotting where they touch the ground.

The minimal spacing for trellised tomatoes is two feet apart in a hedgerow.  Research has demonstrated that crowding plants will not increase yields, but will increase disease problems.

Cages

The American Society for Horticultural Science suggests a trellis two feet in diameter by four to five feet tall.  It is easy to make from a six and half-foot length of concrete reinforcing mesh.  Cut off the bottom ring of wire so the cages can be pushed into the ground.  When a branch sticks out of the cage, simple tuck it back in. [Figure 4]

For the smaller-vined, determinant types, two cages may be made from a six and half-foot length, cutting the height in half.  Cages will be 2 feet diameter but only three feet tall.

Commercially available cages are too small for most popular tomato varieties grown on good soils.

Tomato with cage and black plastic mulch
Figure 4.  Tomatoes planted in a raised bed with black plastic mulch and cages made from concrete reinforcing mesh. Cages are six feet around, two feet across and five feet tall.  On improved soils, tomato vines will loosely fill the cage, allowing for good air circulation and easy picking.


Tender transplants are rather sensitive to cool winds.  Wrapping the cages with a plastic sheet or newspapers to provide wind protection for the first week helps plants acclimate. 

Wind screen around tomato cage
Figure 5.  Wrapping the tomato cage with plastic or newspapers protects tender plants from cold winds.



Single pole trellis – Some gardeners prefer to trellis tomatoes on a single pole or stake.  To do this, prune plants to a single trunk by removing all side shoots.  This requires constant removal of side shoots.

Tomato trellised to single pole
Figure 6.  Tomatoes trellised to a single pole.



Fan trellis – Another method, which produces larger fruit, is to trellis to a three-trunk, fan shape, removing all other side shoots.  This requires a sturdy frame to support the weight of the vine and fruit.

Tomato fan trelllisFigure 7.  Tomatoes trellised into a fan shape.


Mulching

As with any crop, a surface mulch is recommended to conserve soil moisture and manage weeds.  Mulching helps reduce the splashing of Early Blight fungal spores from the soil onto the leaves.  It also helps stabilize soil moisture levels, reducing the incidence of blossom end rot.

Black plastic mulch is popular for tomatoes, warming the soil and pushing production two to three weeks earlier.  When using black plastic mulch, crops must be planted early so plant growth covers and shades the plastic before summer heat sets in.  For details, refer to the CMG GardenNotes #715, Mulches for the Vegetable Garden.

Irrigation

Avoid overhead sprinkling on tomatoes.  Fungal spores are easily water-splashed from one leaf to another, and water on the leaves creates a favorable environment for disease development.  Watering in the morning, allowing plants to dry before nighttime, may also be helpful.

For details on garden irrigation, including drip irrigation, refer to the CMG GardenNotes #714, Irrigating the Vegetable Garden.

Fertilization

Tomatoes have a low nitrogen requirement.  Under high nitrogen conditions, vines grow excessively large at the expense of fruit production.  More correctly stated, tomatoes are a fussy nitrogen feeder.  On soils low in organic matter, tomatoes typically run out of nitrogen in mid-summer, reducing yields and predisposing the plants to Early Blight.

At transplanting, apply one to three applications (depending on soil organic content) of a water-soluble, “plant starter” fertilizer.  This includes any of the water-soluble products like MiracleGro, Peters, RapidGro, Schults, etc.  Transplants would have been “hardened off” (growth slowed) in the greenhouse. Water-soluble fertilizers stimulate renewed growth.

If the weather turns cold late spring after tomatoes are out (that is a week with daytime temperatures below 55°F), use water-soluble fertilizers to stimulate growth when warm temperatures return.  A week with daytime temperatures below 55°F stunts tomato growth, reducing yields.

Mid-summer – On low organic matter soils, tomatoes typically run out of nitrogen in mid to late summer.  Yellowing of the foliage, starting with lower leaves, is the typical symptom of nitrogen stress.  Low nitrogen in the plant allows Early Blight disease to spread like wildfire.  Keeping nitrogen levels up in mid to late summer is a primary means of Early Blight control and significantly improves yields.

Fertilize tomatoes lightly as the first fruits reach two-inches in diameter.  Water-soluble fertilizers (such as MiracleGro, RapidGro, and Peters) used according to label directions make a good summer fertilizer supplement.  Make applications every two to four weeks, depending on soil organic content.

If using a dry granular fertilizer, apply 21-0-0 (ammonium sulfate) at the rate of one level tablespoon per plant.  Sprinkle the granular fertilizer in a wide circle 12 to 20 inches out from the plant, and water in.  Dry granular fertilizers can easily kill tomatoes if over-applied.

For additional details on soil management and fertilization, refer to CMG GardenNotes #711, Vegetable Garden Soil Management and Fertilization.

Pollination and Summer Temperatures

Tomato pollination is temperature dependant.  If nighttime temperatures drop below 55°F, pollen fails to develop and flowers that open the following morning will not set fruit.  Cool nights often interfere with fruit set for early tomatoes and in higher elevations.  Blossom set sprays help set fruit even with cool nights.

If the daytime temperature reaches 90°F by 10 a.m., blossoms that opened that morning abort.  Blossom set sprays are not effective under high temperatures.

In July and August along the Colorado Front Range, night temperatures have a 50/50 probability of staying above 55°F any given night.  In unusually warm seasons, tomato fruit set may be unusually high.  When poor soil conditions and/or watering problems limit plant growth potential, fruit may ripen while small.  With good soil tilth and water conditions, fruit size may be unusually large.

Garden Sanitation

Control weeds.  Common weeds harbor many garden insect and disease problems.  Volunteer potatoes and tomatoes could be a source of Early Blight infection.  [Figure 8]

Tomato Early Blilght
Figure 8.  Early blight leaf spots (Photograph from USDA)



For Early Blight management, some references suggest removing lower leaves showing symptoms.  Symptoms start as tiny black spots on lower leaves.  Spots enlarge to light and dark target-like rings.  Leaves yellow and the disease progresses from lower leaves up the plant. 

If removing lower leaves, focus on leaves with the tiny black spots.  Removing just the lower yellow leaves will not be adequate.  Wash hands with soap and water immediately after touching diseased leaves to prevent spreading spores to other plants. Avoid working with the plants when they are wet.

Another disease, tobacco mosaic virus (TMV) can readily spread from tobacco smoke residues on the clothing and hands to tomatoes.  Prevent TMV infections by washing hands after smoking or handling tobacco products.

Rotation

Since the common tomato diseases (Early Blight, Verticillium and Fusarium wilt) are soil borne, crop rotation is an effective management tool.  However, this may not be practical in most home garden situations, particularly since rotation allows no tomatoes, peppers, potatoes, eggplants, vine crops (cucumbers, squash, pumpkins, and melons), strawberries, or raspberries in the same growing area for at least four years.   In a garden bed, moving the tomatoes a few rows to the left or right is not an effective rotation.

Fall Clean Up

Remove all tomatoes and potato debris in the fall.  Dispose of debris in municipal trash or by burial.  Do not compost unless the compost heats to at least 145°F and the pile is turned occasionally.  Most home compost piles do not heat adequately to kill pathogens.

Common Disorders

Information on common tomato disorders is found in the following CSU Extension publications:

Ripening Fruit at the End of the Season

To speed fruit ripening in the fall, hold back slightly on watering.

Ripening Fruit Indoors 

With the forecast of a light frost, tomatoes may be protected by covering.  If heavy frost is forecast and covering is not practical, harvest fruit before the frost event and carry indoors.

Pick ripening fruit and green tomatoes with a glossy green appearance that have reached about three-fourths of their full size.  Remove stems.  Wash fruit under a stream of water and allow to air dry on a clean towel.  Save only blemish-free fruits for ripening indoors.

As for humidity, fruit shrivel if it is too low.  If the humidity is too high, fruit mold.  A gardener will have to learn by trial and error what works for their home.

Some gardeners simply hang the whole plant upside down in a dark cool barn or basement to let the fruits ripen gradually.  In Colorado’s dry climate, fruit tend to shrivel from the low humidity.

Other options include placing tomatoes one or two layers deep in a covered box for ripening.  Some people find better success by individually wrapping fruit in newspaper or wax paper and placing them in a covered box.  Placing a few fruit together in a vegetable storage bag has been effective for others.  For higher humidity, place tomatoes up to two layers deep in a blanching pan or strainer inside of a covered pan with some water in the bottom.  Make sure the fruit does not touch the water.

Ethylene gas produced by ripening tomatoes is a ripening hormone.  To speed the ripening process, place a ripe tomato in the container with the fruit.  To slow the ripening of green tomatoes, routinely remove ripening fruit from the container.

Green fruit will ripen in about two weeks at 65°F to 70°F, and in about three to four weeks at 55°F.  Storage below 50°F will give fruit a bland, off-flavor. Ripe tomatoes may be stored in the refrigerator for a few days.


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Authors: David Whiting with Carol O'Meara and Carl Wilson; Colorado State University Extension. Photographs and line drawings by David Whiting.

  • CMG GardenNotes are available online at www.cmg.colostate.edu
  • Colorado Master Gardener/Colorado Gardener Certificate Training is made possible by a grant from the Colorado Garden Show, Inc.
  • Colorado State University, U.S. Department of Agriculture, and Colorado counties cooperating
  • Extension programs are available to all without discrimination.
  • Colorado Master Gardener LogoNo endorsements of products mentioned is intended nor is criticism implied of products not mentioned.
  • Copyright. 2003-12. Colorado Master Gardener Program, Colorado State University Extension. All Rights Reserved. CMG GardenNotes may be reproduced without change or additions, for nonprofit educational use.

Revised January 2012

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