Gardening in Tucson
and Southeastern Arizona

Growing Tomatoes

Disease Resistance
Tomato tags have a sequence of capital letters after the variety name such as VFNTA. Each letter stands for resistance to a different disease or pest. V = verticillium wilt, F = Fusarium wilt, N = nematodes, T = tobacco mosaic virus, A = alternaria stem rot canker. Heirloom varieties tend to have less disease resistance.

Heat Tolerance
The pollen of most tomato plants becomes sterile at 90° F and tomato production often ceases during hot periods. Pollen of some varieties, however, remains fertile at higher temperatures and those plants set fruit over a longer period of time in the American southwest. Some produce tomatoes the entire summer.
Tomato varieties setting fruit above 90°F in Arizona include Azteca, Homestead 24, Prescott and Super Sioux. See Aravaipa Heirlooms for a list of heat-tolerant varieties.

Days to Maturity
Early ripening tomato varieties may produce fruit in as little as 50 days after transplant. In general, the larger the size of the fruit, the longer the days to maturity. Early Girl, a popular variety, is one of the quickest to ripen. Cherry-sized tomatoes are able to ripen quickly even with lower sunlight levels in spring and autumn. Varieties bred for early production, however, sometimes have less flavor. Beefsteak varieties can take up to 85 days.

Determinate and Indeterminate
Determinate tomato plants grow to a certain size and then quit. They produce most of their crop at one time and are suitable for canning. Indeterminate plants are true vines and continue to grow. Their crop is produced over the entire season.

Use Raised Garden Beds or Large Containers
Having a raised garden bed provides better moisture control. The soil and roots of growing plants will not get waterlogged during heavy rains. Complaints that raised garden beds overheat in the southwest can be addressed by providing afternoon shade to the garden and sides of the raised bed. Raised beds also make it easier to leach excess salts out of Arizona soil through rain and watering.
A seven- to ten-gallon container can also be used, providing it is white or always shaded so the roots are not overheated. Drill four holes at the bottom of the sides for drainage unless there is a bottom hole raised off the ground. Do not use a saucer for the container unless it is placed upside-down or does not hold water.

Rotating Crops
Tomato plants should be rotated yearly. Divide your garden into four equal-sized sections. No garden section should have nightshade family members (tomato, eggplant, chili or pepper, tomatillo, potato, petunia, datura) more than once every four years. This will prevent the accumulation of diseases in the soil that attack nightshades.

Soil Preparation
Tomato plants can tolerate a soil pH range of 6.0 (acidic) to 7.5 (slightly alkaline). For information on soil preparation and soil pH, see Soil Preparation.

Spacing
Space tomato plants at least 24" apart and give them 6 square feet of space each. Planting closer together than 24" promotes fungal diseases. For shallow, raised garden beds, a 36" separation will provide more room for roots to grow without competing. Some varieties, especially indeterminates, will spread sideways even with a tomato cage and need at least 36" separation.

Planting Early
Some gardeners set out seedlings a month or more before the last frost date to get an early start on tomato production. They place a commercial product such as Wall O' Water or a ring of one-liter soda bottles (water-filled) around their seedlings when frost is predicted. This makes sense if you are growing popular varieties that are not heat tolerant. These varieties cease production during the hottest times of summer, then start producing again in the fall.
There are some heat tolerant varieties that produce all summer in Arizona if placed under 50% shade in temperatures over 90°F. They can be planted a week or two before the official end of frost date with less worry about freezing. They still may have to be protected from frost, but the frost danger interval is smaller, and the last frost is often weeks before the official end date.

Planting for Fall
In Southeastern Arizona, gardeners often grow vegetables fall to spring, then stop during the hot summer. Tomatoes, however, are sun-loving plants and do best if planted in the spring. Indeterminate cherry-sized tomatoes are more likely to ripen well into fall. For those getting a late start on planting tomatoes, July 31 is the most prudent last planting date for a fall crop of early-ripening tomato seedlings.

Transplanting Potted Plants
Put tomato seedlings in the ground when the danger of frost has nearly passed. Be prepared to protect from late frosts. Remove the entire pot, whether peat moss or plastic. Peat moss pots retard root growth, regardless of the claim that roots can grow through them.

Place tomato seedlings so that the top of the potted soil is level with the garden bed soil. This achieves the fastest, strongest growth. Some nurseries include instructions to place 2/3 of the seedling stem below ground. That will cause the seedlings to go into shock for the first month, and barely grow, but they will eventually recover and catch up to the others.

Support
Tomato plants do best with support if they need to be grown in confined spaces. Small wire tomato cages tend to fall over in strong windstorms. Large, heavy, cylindrical cages made of heavy field fence or concrete reinforcing mesh are best, but they still may have to be staked.
Concrete reinforcing mesh can also be used like a fence. Attach it, or field fencing, to steel T-posts. Tie vines loosely along the mesh. Check the ties periodically to ensure stems are not being constricted.

Remove All Flowers for the First Four to Six Weeks
Prevent the plant from growing tomatoes by cutting off all flowers and fruit for the first four to six weeks after transplant. This will force it to put its energy into growing roots, leaves and stems. When you do allow flowering and fruiting, the plant will be large enough to grow more tomatoes. The total fruit yield for the year will be greater.

Sun and Shade
The tomato, like any summer fruit or vegetable plant, needs sun. How much sun depends on geographic location, temperature and time of year. In the American southwest, 6-8 hours of full sun plus 50-60% shade in the afternoon is recommended when temperatures are under 90°F. Growing tomatoes in partial shade reduces water loss and sun scald and cools the plants so that they will be more likely to set fruit in the hottest part of summer. Latticework, sun shade cloth suspended 8' high, or tall plants can be used to shade the garden.
It should be noted that too much shade in cool temperatures can increase the likelihood of fungal diseases in susceptible varieties. On the other hand, when temperatures are over 90°F, provide 50% shade all day for the first two months of summer to give tomato plants a better chance of setting fruit.
By the first of September, the sun will be lower in the sky and there will be fewer hours of daylight. Provide more sun by removing overhead shade. By the first of October, eliminate shade altogether.

Watering
Water in the morning at the same time every day with the same amount of water. Check to make sure that 24 hours later, the top one or two inches of soil is dry.
If you are using containers or raised garden beds, and it rains, water as though there was no rain, or reduce water by no more than 25%. Water will reach the depth required and excess water will drain off.
Avoid watering in the late afternoon or evening because this encourages fungus infections in the soil. Also avoid getting water on leaves because this promotes leaf fungal infections in susceptible varieties, especially when mornings are cool.

Inspecting for Insects
Be sure to inspect closely for any insect activity. Pick off any insects found. Hornworms can look exactly like rolled-up, green leaves from the side. Inspect plants at midnight or very early morning for hornworms which feed on the outer plant at night and move to the interior of the plant during daylight. Aphids can be removed by jets of water.
One exception — leave the red Pipevine Swallowtail caterpillars alone. If they are on your garden cage or tomato support, they are merely looking for a place to pupate and will be a green or yellow chrysalis by the next morning. They don't eat human food plants anyway.

Pests and Plant Placement
When all plants of a given species are planted in the same place, insect pests have an easy time finding them and making a general feast of the neighborhood. This is called mono-cropping and should be avoided in the vegetable garden.
To confuse insect pests, place a variety of plant species throughout the garden in seeming random fashion. It will not be truly random, because we must know which plants go well together and which plants do not. Placing plants in a vegetable garden is almost like seating guests at a dinner table.

Good Companions
Surround tomatoes with companion plants that ward off pests and attract beneficial insects. Basil and marigold are great companions for tomatoes. The aroma of Sweet Basil repels hornworm moths. One basil plant is needed for each tomato plant. Marigold flowers attract hoverflies, which prey on aphids. Marigold roots kill nematodes in the soil, especially when planted the season before tomatoes are introduced. Other good companions of tomatoes are onion, sage, and native flowering plants, such as sunflowers, that attract pollinating insects. Some native flowering plants may have to be grown in pots or separate garden beds to avoid interference with the tomatoes. Geraniums grown in pots are used as trap crops, luring aphids and leafhoppers away from other plants. In one report, geraniums are said to repel tomato fruitworm moths, although this needs to be verified.

Sweet Basil is a warm weather plant. Seedlings can be damaged by nighttime temperatures under 50°F and must be planted a few weeks later than tomatoes. Once basil has been in the ground four months, it tolerates temperatures in the low 40's. Sweet Basil has the strongest aroma and best hornworm moth deterrence value of the basils.

Bad Companions
Plants that do harm to tomatoes should not be grown in their vicinity. Some bad companions release chemical compounds into the soil that harm tomato plant growth or fruiting. Others vigorously compete for the same nutrients. Beans, dill, fennel, kohlrabi and potato are bad companions. Corn is also a bad companion because corn and tomatoes both share a common parasite: the corn earworm is also known as the tomato fruitworm. For those reasons, tomatoes should not be planted where bad companions grow or grew the previous year.

All Foliage, No Fruit
This occurs when the plant has been given too much nitrogen. See Fertilization recommendations above. Ammonium sulfate may have been used to acidify the soil. Ammonium sulfate is a high nitrogen fertilizer and should not be used with tomatoes. Finally, do not rotate tomato plants into a garden bed where nitrogen-fixing legumes have been grown the year before.

Many Flowers, No Fruit
This normally happens when the tomato variety cannot set fruit in high temperatures. At times the blossoms on top of the plant will fail, but those in the shady middle, bottom or north side will set fruit.
If there is no pollinator, place blooming flowers nearby to attract bees. Pollination by hand is easy. Just tap the stem behind the blossom and it will self-pollinate, assuming that the temperature is suitable.

Dark Brown Spot on Bottom
A dark brown, leathery spot on the bottom of a tomato is called Blossom End Rot. The cause is inconsistent watering, insufficient water, or cutting roots during weeding. Any of these will result in poor calcium ion distribution inside the plant. Water every day at the same time with the same amount of water. At least 6-8 cups of water is needed per plant. Be sure to mulch the soil around the plant to retain moisture. This is not a calcium deficiency in the soil but rather a problem with the transport of calcium within the plant. American desert southwest soils have sufficient calcium. Some tomato varieties are more prone to blossom end rot than others.

Split Skin and/or Scarring
Split skins and scarring are caused by irregular water amounts, especially too much water. Know how much water the plants are getting every day. An increase in daytime temperature can cause tomatoes to grow faster resulting in higher water needs. Sporadic heavy rains may cause split skins in spite of your best efforts. Assuming no rain, the top one to two inches of soil should be dry 24 hours after watering. Some tomato varieties are more prone to skin splits than others.

Fruit Ripen Too Small
Two possible causes:
[1] The plant is not mature enough to have the leaf area and root length required to support the number of tomato fruit in production.
[2] Too much competition from the roots of other nearby plants in a garden bed or not enough soil in the container. Containers should hold at least 7 gallons of soil. Tomatoes grown in raised garden beds one foot deep should have at least 6 square feet of space to themselves, excluding nearby Sweet Basil.
Cut back on the number of fruit allowed to develop at one time until the plant grows larger. Never remove leaves from the plant. They provide energy for growth and needed shade for the fruit.

Holes in Fruit
This discussion assumes that the tomato plants are protected from birds.
Small holes, often one-eighth to one-quarter inch in diameter, are caused by tomato fruitworms. Much larger holes are often caused by grasshoppers.
Fruitworm and other caterpillar eggs can be controlled with Trichogramma wasps.
An effective defense for grasshoppers is to spray green and ripening tomatoes with a garlic-pepper solution before they are eaten. The solution is easy to make. Place four Habanero peppers and four large garlic cloves in a blender and mince. Blend in two cups of water and let stand out of direct sunlight at room temperature for one day. Strain into a 24 ounce spray bottle with water added to fill. The solution must be re-sprayed after every rain.

Large Whitish Spot on Fruit
The whitish spot is called Sun Scald. Sometimes there will be a smaller center of wrinkled skin inside the whitish spot. This condition is caused by too much direct sun. The best solution is to give the tomato plant afternoon shade and avoid removing any leaves.

Yellow Shoulders or Patches on Fruit
Tomatoes with yellow shoulders or patches may also have a green area inside the yellow. This condition has two main causes.
[1] The soil is deficient in potassium. Add compost to the soil before planting or use tomato fertilizer according to directions.
[2] When leaves have dark spots that spread to stems and form cankers and the fruit has yellow blotches, this is spotted wilt virus spread by thrips. There is no effective treatment except to cut down the plant and discard it. Do not put it into a compost pile.

Fruit Do Not Ripen
Some tomato varieties take a long time for each fruit to ripen, up to 85 days. Know what is normal for your plant variety. If a plant has been ripening fruit but stops doing so many months after planting, the cause is less sunlight at the end of summer, especially with many tomatoes on the plant. To overcome this, remove all sources of shade and give the plant full sun if it does not already have it. Cut off all flowers if it is still producing them. Remove any small green tomatoes that have not achieved full size. Reduce daily water to 4 to 6 cups per plant. Cold temperatures can also slow ripening. When it gets too cold, the entire plant can be removed from the soil and placed in a secure, dry place for tomatoes still attached to the plant to partially ripen. Partly ripened tomatoes, with a blush of yellow, will finish ripening inside in a bowl containing apples or a banana.

Tomato Plant Small or Stops Growing
If tomato plants (excluding those grown in pots) are not over 3' tall after 8 weeks, and there are no disease symptoms, the soil is deficient in potassium. In poor fertility conditions, an indeterminate plant can even stop growing. Tomatoes are heavy potassium feeders. The solution is to add compost to the soil before planting, or use tomato fertilizer to raise nutrition levels in the soil.

Rust- Brown- or Bronze-Colored Leaves and Stems
Tomato Russet Mites cause bronze-colored stems and leaves starting from the ground up. These tiny mites can be seen under 16x magnification. Sulfur dust or wettable sulfur, sold by nurseries, can be used as a control. The insecticide / miticide Azatrol, derived from Neem seeds, is also recommended. The mites can destroy an entire tomato plant and its fruit quickly. See Tomato Russet Mite.

Yellow Leaves
Several factors cause yellow leaves.
Fungal diseases may be a problem, especially with heirloom tomatoes. See Texas Aggie Tomato Problem Solver. Cut off yellow leaves with scissors when they occur, and pick up leaf litter on the garden bed to reduce the spread of any fungus.
Insufficient water is a possible cause in hot weather. If leaves are limp in late afternoon, add more water in the morning. Overwatering can also be a problem if plants are not grown in raised garden beds or well-draining soil.
Insufficient nitrogen or iron in the soil will also cause yellow leaves. Add composted manure to the soil before planting, or use a tomato fertilizer according to directions.
Insects such as aphids and hornworms can damage leaves and stems, turning them yellow and even brown. Remove them when they are found.
Shade at the bottom or interior of the plant may also cause yellowing, especially with older leaves.

Purple-Reddish Leaves
Insufficient phosphorous in the soil results in reddish-purple stems and leaves. Add compost to the soil before planting or use tomato fertilizer.