Tomato, Outdoor

Introduction

Tomato (Solanum lycopersicum) is a warm season crop in the nightshade family (Solanaceae), along with pepper, eggplant, and potato. Tobacco, petunia, nicotiana, and several important weed species are also solanaceous. Tomato is native to South America, with its use as a food crop originating in Mexico. It grows best on well-drained soils that are high in organic matter. It is frost-sensitive and should be transplanted into fields once the soil has warmed to 60ºF. 

Types and Varieties

There are thousands of tomato cultivars available, and varieties may be selected based on market preferences for shape, color, and flavor, as well as for ease of harvest and storage, and for tolerance or resistance to the many diseases that affect this crop. 

Indeterminate varieties produce stems that will continue to grow until killed by frost. They perform best when staked or trellised. Side shoots and suckers arising from the base of the plants should be pruned off weekly and the main stem should be secured to the stake or trellis. Indeterminate cherry, grape, and some plum/'saladette' vines are often more vigorous than indeterminate slicing tomato vines.

Determinate varieties have stems that grow long enough to produce 2-3 flower clusters and then stop. It is necessary to allow the suckers to grow to produce more flowers and fruit per plant. Determinate varieties are well-suited for ground, cage, or basket weave culture.

Several tomato varieties can be classified as “vigorous determinates”, and are important commercial production varieties. They produce a higher percentage of grade A fruits when they are staked. Under good conditions, they can produce 15-20 lbs per plant, but require more space.

Varieties that are later maturing are of higher market quality than the earlier varieties, but early varieties are important for attracting customers early in the season. Cherry tomatoes are appealing throughout the production year, as are “cocktail” and “saladette” types. Interest in the less juicy paste/Roma-type fruits is usually during late summer to early fall.

Hybrid Varieties. Most varieties used by commercial growers are hybrids, generally labeled "F1", which are crosses produced by controlled pollination between 2 different varieties to select for the desirable characteristics of each. Seed produced by F1 hybrids are not genetically stable and tend not to grow true to type if seed is saved and replanted. Plants produced from hybrid seed tend to be more productive and vigorous, and may be bred for resistance to specific diseases or environmental conditions like heat.

Heirloom/Open-Pollinated varieties are very popular in the marketplace. A diverse array of varieties are available and many growers create their own market identities by reliably producing them. Most heirloom varieties carry little or no disease resistance. The fungal pathogens Verticillium, Fusarium, Alternaria, and Septoria can be particularly problematic. Cultural practices like crop rotation and good sanitation are essential in the production of heirloom varieties.

"Hybrid heirlooms” (sometimes referred to as hylooms) are now common in the marketplace. These have the appearance and flavor of heirlooms, more resistance to diseases and have less variability in fruit size and shape. 

Soil Fertility

Apply lime according to soil test results to maintain soil pH at 6.5-6.8. Maintain high calcium. Base saturation for calcium should be 65-80%. Use calcitic lime or gypsum if necessary.

When growing plants on plastic mulch, the amount of nitrogen (N) fertilizer to be sidedressed can be reduced, and most of it can be applied when laying the plastic, since leaching is minimized. Leaf tissue testing can be an important tool to monitor the nutrient status of your plants. If testing is done at the right time (early to mid-bloom), additional nutrients, most importantly N and K, can be supplemented. While these can be sidedressed along the edge of the plastic mulch, nutrients can be applied more effectively in soluble form through drip irrigation installed under the plastic. Liquid suspensions of organic nutrients can be applied this way as well. If this method is used, apply no more than 10 lbs per week of actual nitrogen fertilizer per acre.

A 30-ton tomato yield removes about 200 pounds of N per acre while a 15-ton yield removes about 100 pounds of N per acre. On the "Plant Nutrient Recommendations" table, the N recommendation for outdoor tomatoes is calculated based on a yield goal of 22 tons per acre. Do not apply more N fertilizer than is required to achieve your realistic yield goal. Excess N can reduce yield. Use a high P liquid starter fertilizer at transplanting, especially with cool soil conditions. Less N fertilizer will be needed on some soils high in organic matter, or if manure or legume sod was plowed down (see Table 1 and Table 2). When sidedressing N, the nitrate forms (such as calcium nitrate) are preferred over the ammonium or urea forms.

Maintaining adequate calcium (Ca) in the soil is essential. Blossom end rot is a physiological disorder in which cells die early in the fruit maturation process. This disorder is more likely to occur when there are low Ca levels in the soil, but is no longer thought to be solely caused by Ca deficiency. Instead, blossom end rot and other physiological disorders such as yellow shoulders and internal white tissue can be caused by a number of stresses including high salinity, drought, high light intensity, rapid growth, irregular or under watering, and the weather. Adequate, regular irrigation is essential throughout fruiting. Other strategies to reduce physiological disorder occurrence include the use of shade cloth when temperatures are routinely over 85°F, soil and plant tissue testing to ensure adequate nutrient availability and plant nutrition, and good pruning to limit excessive vegetative growth.

Plant Nutrient Recommendations According to Soil Test Results for Outdoor Tomato

Planting

Tomatoes are transplanted in New England due to the short growing season. Early fruit production requires quality transplants. Adequate spacing produces short, stocky plants with good root systems, whereas crowding produces tall, spindly plants. One ounce of tomato seed will produce about 7,400 plants.

Sow tomato seed in an open flat in germination mix and maintain uniform moisture and bottom heat at 75ºF until emergence. Transplant young seedlings into 2-4" cells or pots when they have 2-3 true leaves. Choice of pot size depends on the number of weeks before anticipated field transplant date. For earliest production, some growers finish their transplants in 6" (or even larger) pots. 

Grow transplants at 70-75°F day and 60-65ºF night temperatures. Night temperatures in the greenhouse that fall below 60°F may result in irregular fruit (catfacing) on the first few clusters. Five to 8 weeks from seed are required to produce field-ready transplants, depending on the temperature at which the plants are grown and the size desired. Avoid the temptation to start the plants too early; holding them for too long will reduce yield. Harden only slightly, by reducing water and nutrients, and ambient temperature, if possible but not below 60°F. Small plants that have been slightly hardened, or not hardened at all, will outperform larger, over-hardened plants.

Transplants should be given a complete nutrient solution supplemented with trace elements (the latter especially if artificial mixes are used) at weekly intervals. Alternatively, they can be fed whenever they are watered with a dilute nutrient solution. The advantage to the latter system is that the fertilizer is supplied more in relation to the plant's needs: more on bright, warm days, less on cool, cloudy days. In either case, follow directions on the fertilizer label for amounts to feed. Use a soluble fertilizer and be careful of salt buildup. It is advisable to use a fertilizer in which most of the nitrogen is in the nitrate form rather than ammonium or urea.

Field Culture

Many will grow tomatoes on black plastic mulch, which has several benefits. It warms the soil, promotes early production, conserves water, reduces nitrogen fertilizer use by reducing leaching losses, and facilitates weed control. Plastic mulch also keeps most of the fruit off the soil and reduces soil splash onto the plant. One of the primary disadvantages of using plastic mulch is the labor and cost involved in managing it at the end of the growing season. After use, the plastic must be carefully removed from the field, often a time-consuming process, and then properly disposed of or recycled, which can present logistical and environmental challenges. Biodegradable mulches have the advantage of being able to be tilled into the soil at the end of the season, instead of being disposed of in a landfill. Most organic certifiers do not allow for use of biodegradable mulches, or require them to be removed at the end of the season like regular plastic—check with your certifier before using. Plastic should be laid tightly over the beds to conduct heat to the soil more efficiently and to avoid depressions where hot spots or puddles can form. Many growers use plastic mulch in conjunction with raised beds. This warms the soil more quickly. Prior to laying plastic mulch, soil moisture should be at or near field capacity.

Trickle irrigation is a very efficient watering method used under black plastic mulch. Consistent soil moisture reduces or eliminates problems with blossom end rot and cracking. Using a fertilizer injector in conjunction with drip irrigation allows for nutrients to be sidedressed more easily. Row covers over wire hoops are used by some growers for faster early season growth. They do not provide significant frost protection, but they do speed growth. Ventilation is usually needed on warm, sunny days. Do not allow temperatures under covers to exceed 90°F. (See Plastic Mulch and Row Covers, and High Tunnels.)

Trellising/Staking

Staking or trellising certain varieties may advance production by 7-10 days. Early tomatoes usually bring higher prices, but this has to be weighed against the labor costs of trellising. Fruit quality and plant health will be enhanced by keeping fruit off of the ground and allowing air movement into the plant canopy, reducing the incidence of anthracnose on fruit and foliar disease spread. Harvest efficiency is improved with trellising and you can use a tighter spacing, further improving yields. There are two main approaches to trellising. The basket weave which is more common with a bush-type or determinate plant, and the overhead or hanging string system which is more common with an indeterminate plant. For trellising, plants are supported by weather-resistant or polypropylene twine, which does not stretch or sag. Avoid using treated baling twine. Tomato twine for this purpose is available in 3 or 4-pound boxes.

With the basket weave, sometimes called stake and weave or Florida weave, wooden stakes 4-5' long and 1" square, or similar lengths of rebar, are driven 1' into the ground between every other plant, sometimes every third plant. Stronger hardwood or metal T-posts are often used at the end of the rows for extra support. Weather-resistant twine is then tied to the end stake and run down one side of the row, wrapping the twine around each stake. Most growers weave the twine back and forth between plants. The process is then repeated on the other side of the row. The stringing operation is repeated 3-4 times, with the first being 8-10" above the ground when the plants are 12-15" tall. Subsequent strings are added just before the plants begin to fall over. A home-made stringing tool or wand should be used for convenience. It is simply a length of metal or plastic conduit. The twine is fed through the conduit, which acts as an extension of the worker's arm. Most growers remove all the bottom side shoots up to, but not including, the one below the first flower cluster. After this, no pruning is done. When using this system with taller or more vigorous plants, you can trim the tops once they reach the tops of the stakes. If the row becomes too top heavy the system can fall over. There are many variations of this system. NOTE: birds cannot perch on rebar, resulting in a cleaner crop at harvest than if using wooden stakes. 

With the overhead or hanging string system, plants can be pruned to 1 or 2 leaders. For two leaders, keep the lateral branch (sucker) just below the first cluster. Two leaders yields more fruit per plant than a single leader, but pruning is more time-consuming, and each plant requires more space, with fewer transplants needed. Pruning of suckers and lower leaves should be done frequently for fully indeterminate varieties. Shoots are most easily removed when they are a few inches long. To reduce disease, do not prune during wet weather or if bacterial canker is present.

For this system, a 9 or 11 wire is held up by posts 5-6’ above the ground, spaced 20-30' apart in the row. Metal fence posts or U-Posts help to support the wire, with sturdy wooden posts at the end. Another option is the "A" trellis, a double row 18-24" apart, which uses fewer posts and less wire; fruit tends to be more shaded, and sun scald is less of a problem. Note that each plant may weigh 30 lbs. or more at peak fruit set, so be sure that it is secured firmly and can support the weight.

With either system, the lower end of the twine is tied loosely around the base of the stem. Do not tie too tightly, as it can cut into the plant. As the plants grow, the string spirals around the main stem, or you can attach the leader as it grows using a tomato clip. Pruning is usually done at the same time.

Ground Culture and Basket Weave

Flat ground culture is a system without support, usually done at field scale only with determinate types, often with processing tomatoes. This is a low cost low labor system, with minimal pruning. Mulching with straw or plastic will help keep fruit clean and reduce rot and soil contact. This system benefits from low input costs, but results in higher disease and pest challenges with more challenging harvest as fruit is hidden in the foliage. Ground culture is most suitable for dry climates.  

Spacing

Plant spacing will vary according to cultivar and type of culture. Frequent roadways may be necessary to drive spray or harvest equipment between blocks of rows.

• Staked: 5' between rows, with 12-18" apart in rows when pruning to a single stem or 18-24" apart when pruning to 2 stems. 
• Basket Weave: 5-6' between rows, 18-24" between plants.
• Ground: For determinate varieties, leave 4-6' between rows, and set plants 12-24" apart within the row. For vigorous determinate and indeterminate varieties, set plants 2-3' apart in the row with 5-7' between rows. Remember that there will be significant losses due to fruit rots if indeterminate varieties are left unstaked.

High Tunnels

High tunnels allow for planting up to 4 weeks earlier than in the open field. Harvest is earlier and can yield up to 4 times as much as field-grown tomatoes. High tunnels keep rain off the foliage and fruit, resulting in fewer foliar diseases and rain check (a russeting of the fruit). Sides are rolled up during warm weather, but should be lowered when temperatures fall below 60ºF. See High Tunnels and Tomato (Greenhouse and High Tunnel) sections for more information.

Irrigation

If there are no restrictions, tomatoes develop a deep root system. When irrigating tomatoes grown on black plastic, sufficient water must be applied so that lateral water movement can take place under the plastic to provide adequate moisture to the root system. Watering should be deep and spread out across the bed. This can be accomplished by running at least 2 lines of drip per bed. Once fruit begin to enlarge, tomatoes require at least 1" of water per week depending on temperature, wind, and relative humidity. (See also Trickle or Drip Irrigation in the Irrigation section).

Harvest and Storage

The six stages of tomato ripening are 1. Green Mature Stage, 2. Breaker Stage, 3. Turning Stage, 4. Pink Stage, 5. Light Red Stage, 6. Red and Final Stage. Since most wholesale tomatoes are marketed in the New England area, it is recommended that those tomatoes be harvested at the breaker to turning stage. Tomatoes harvested in the green stage do not promote an image of high quality. Once the fruit reaches the breaker stage, a skin is formed between the fruit and stem, cutting off nutrients to the fruit. At this stage the tomato will have its vine ripened flavor. As tomatoes remain on the vine, they are subject to cracking, sunscald and other disorders. There may be reduced fruit injury when harvesting around breaker stage and ripening off the vine. 

Jointless varieties have no joint on the fruit stem. When picking, the stem separates completely from the fruit. These varieties work well for bulk packaging because stemless fruit do not puncture other fruits in the box. With jointed varieties, the stem usually breaks at the joint, leaving a small stem attached to the fruit. This makes for an attractive retail item but requires special handling and more boxes, since fruits cannot be layered on top of each other.

Tomatoes are susceptible to chilling injury if stored at temperatures below 50ºF for more than 24 hours. Continual exposure to these temperatures will prevent normal ripening even after temperatures are elevated. Store tomatoes at 55ºF or above, depending upon how long they must be stored. The speed of color development will increase up to 75ºF. Temperatures above 80ºF will inhibit red color development. For best eating quality, ripening and color development should take place between 65 and 70ºF. Late in the season when night temperatures routinely drop below 50ºF, tomatoes should be harvested in the breaker stage and ripened at room temperature.