Lettuce Season Extension Trials

Final Report for FNC99-286

Project Type: Farmer/Rancher
Funds awarded in 1999: $3,450.00
Projected End Date: 12/31/2002
Region: North Central
State: Indiana
Project Coordinator:
Dale Rhoads
Rhoads Farm
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Project Information


Our farm is located on ten acres of steeply sloped ground in Brown County Indiana. When we purchased the property twenty years ago we had been organically growing vegetables on a flat land farm that we did not own. Moving to this land ended our farming. However for personal reasons in 1995 we started farming here. We recognized that there was some significant in land use as we began to utilize what could be considered non-farm land. A careful consideration of what kinds of crops to grow on what degree of slope, soil improvements and soil erosion practices hallmark our farm. We currently have approximately three acres under cultivation, but the major cash crop activity occurs on one acre. We grow salad greens, lettuces and specialty tomatoes. Another acre is in Asian Pears. The remaining cultivated areas are in fruit trees and family gardens. The other seven acres is steeply sloped woodlands. Although we are a family owned and family labor farm, we often have various young people temporarily involved in our farm activities. Along with the above named sustainable practices we have always grown organically and have been certified for three years. We plant winter rye cover crops every year. We make sparing use of mowing and leave most flowering weeds in order to cultivate native pollinators, (this is very important with the recent honeybee disease problems). We make yearly contributions of manure and compost to all our growing areas.

In terms of this grant project we had for several years prior to the project unsuccessfully attempted to significantly lengthen our growing season of lettuces. Through this grant process we did significantly lengthen our season and defined the boundaries of just how far it can be extended in the different seasons.

Over wintering of Head Lettuces:
We have discontinued over wintering in the field because of severe weeding problems and low harvest weight. We have done a number of over wintering trials using a variety of techniques and none of them have proven cost effective for us. Perhaps the most sophisticated in the field trials were done with lettuces planted in holes on 10” by 10” centers in black landscaping cloth and covered with a floating row cover. Even in this situation we suffered from excessive weeding difficulties.

In our area of the country we have chickweed and a henbane/loose strife like weed that grows best in exactly the season when it is hard to get into the field. The plants are also prolific seeders making the elimination of these plants from the soil in large areas impossible. In a study done in England there was over 1 million chickweed seeds found in a 1000 sq foot area. These plants grow best in slightly cooler weather than lettuces, making their biggest growth spurts right after cultivation is difficult in the fall and right before cultivation is possible in the spring. What these weeds tend to do is overgrow the lettuces in this time limiting harvest weights. It is possible to carefully hand weed around each lettuce plant to control the weeds, but for us the labor is excessive for the return.

Over wintering of head lettuces in an unheated greenhouse does provide marketable crops, however it is not a very cost effective use of greenhouse space. It is possible to control the chickweed problem experienced in the field due to drier and warmer soil conditions that allow weeding in the weed’s peak growing season. But over wintered crops of head lettuces in the greenhouse all mature at the same date, regardless of the fall planting date. Even though the lettuces are maturing in relatively short daylight days in spring, they tend to transform from growing to bolting about twice as fast as regular season’s lettuces. If a farmer had unused greenhouse space and a ready market that could handle the entire crop, this could be an option worth considering. A local farm near us does just this, as they concentrate on seedling sales in spring and harvest their lettuce crops just before starting some of the warmer season seedlings like tomato plants.

We are now using and recommending serious lettuce producers to use the next explained technique for the earliest spring lettuce crops.

Starting Very Early Crops Outdoors:
The technique that we now use to get the earliest crops in the spring is to use a new weight of row covers available that provides up to 15 degrees frost protection. These covers are similar to a very loosely woven ploy tarp that allows both moisture and air to pass through them eliminating the need for venting and watering. These covers, which are sometimes marketed as greenhouse covers, have multiple years of service. We are anticipating seven plus years of service on ours. The more readily available lightweight spun covers only last us one, perhaps two seasons if we are very careful with handling them. Our main problem with the lightweight covers is that they tear in high winds and rot where they have contact with the ground.

The heavy weight covers do have several disadvantages when compared with the lightweight covers. These problems are they demand sturdy hoops to keep the cover off the crop and they catch the wind and sail. By placing these covers over sturdy hoops with heavy weights along the edges we have had good success. We made our hoops out of recycled 1/2” galvanized electrical conduit bent into half circles on a pipe bender. We place 12-18” of each end of the hoops in the ground, spanning a growing area of 3-5’ depending on what we need and how much we bend or straighten the hoops. Originally we used large covers of 60’ by 100’, but the wind whip was too severe. We now use covers of 15’ by 100’ with an 8-10’ strip of protected soil under each 15’ wide cover.

We use two methods of planting with these covers. Both methods require catching a late winter/early spring tilling time. While we generally plant winter rye as a cover crop on all our fields, we sometimes will leave the areas for our earliest crops without a cover crop. It is our experience that there is some time in late winter when the soil is perfect for tillage, even if soil temperatures are too low to plant. The first method is to plant when soil temperatures are around 45-50 degrees. We know that to plant and place the covers after planting will boost the soil temperatures right up to the 55 degree lettuce germination range. The second method is to place the cover on the soil first and then when 55 degrees is reached then plant. We prefer the method of only handling the cover once, but have had to replant several times when germination rates were insufficient.

We have found in most years for it to be possible to do these techniques in mid-February in most years. These crops will quickly catch up to and surpass over wintered lettuces in growth and it is easier to manage the weeds. This can hasten the spring harvest of typical lettuce plantings by 4-6 weeks. Generally the covers are left on for at least a month, but we have left them on until harvest on cold years. We use this technique for several weeks in our succession plantings until the temperature stabilize to allow regular season unprotected plantings.

Regular Spring/Early Summer Lettuce Season:
This is the standard lettuces season that usually begins with planting in late March/early April as soon as the ground temperatures reach 55 degrees. Harvesting of these crops usually begins around June 1. Conventional lettuce growing techniques generally allow for 3-4 weeks of succession plantings. This time the last harvests to be around July 1st. After this date bolting and other heat related problems stop lettuces harvests.

Summertime Production of Lettuces and Salad Greens:
Our early experimentation at growing lettuces in summer conditions proved successful enough to call for more extensive and more clearly documented trials using technologies that lend more grower control. We made use of different forms of shading and heavy watering schedules. Some of the shading strategies that we used were: interplanting with corn and tomato crops, planting under fruit trees or near heavily shaded wooded areas. Although there was some success in over coming summer heat, results were often widely diverse due to the hard to control nature of these shaded areas.

For the next several years we experimented with shade cloths and mulching techniques. We used shade cloths with 30 and 40% shade and found no discernable difference between the two. We also used white and black shade cloths and again could not measure any difference in heat or difference in plant growth. Basically a white or black shade cloth, in 30-40% shade gives a temperature difference at the hottest point of the day of 7 degrees. In July this can make a significant temperature difference allowing late crops to come to maturity that would otherwise bolt. Part of this difference, which will be repeated several times, is that shade cloths reduce the loss of soil moisture, which seems to have dramatic differences in soil temperature. We have found that the use of shade in June and early July for early summer salad greens significantly increases germination rates. Also plantings under shade in July and August does significantly affect the germination rate of lettuce plantings in these months that will be harvested in September and October.

Along with the shade cloth experiments, trials were run on different mulching techniques. In these trials we tired a number of different mulches and mulching strategy. Some of the mulches that we tried were straw, sawdust, leaves and landscaping cloth.

These mulching techniques did add a significant cooling factor to the soil, but some of the techniques increased air temperature measured 4” off the ground, adversely affecting the crops. It was also found that adding of organic mulches inhibited succession plantings, weeding techniques and added to cleaning time of harvest. It was also found that one of our most effective temperature reducing techniques, watering during the hottest part of the day, resulted in excessive disease problems when combined with mulch. In our trials heavy watering produced better results than mulching, although mulching can have its place on some farm systems or with certain long season crops (like chard). These results lead us to the conclusion that for the commercial production of salad products, mulching is not cost effective.

Another set of trials for summertime production was measuring the differences between drip irrigation and sprinklers. It was found that while the drip irrigation consumed less water, the more conventional sprinklers add a significant amount of cooling beyond what the drip watering produces. But overhead watering has its limitations as a cooling technique due to over watering which leads to greater disease problems.

The results of the above described trials that took place over a 4-5 year period lead us to the current strategy of using shade cloths with water misting. These two techniques reduce air and soil temperature 10-12 degrees. But there are many days when the misting cannot be run due to excessive moisture which in turn leads to excessive disease problems.

It should be noted that the true hot weather lettuce experts are the Israeli’s. Our research showed that they make use of an aluminized 30-40% shade cloth and drip irrigation to grow hot season lettuce crops. The aluminum shade cloths are made up of thin (<1/4”) strips of aluminum foil bond with poly string and are the best performing shade cloths in reducing summer heat. But these shade cloths cost twice as much as conventional shade cloths, are harder to obtain and have relatively short lifespan of only 3-4 years as compared to the 15-20 years of use of conventional shade cloths. The reason the Israeli’s use drip irrigation is due to water shortages. This is a good example of how different systems can be mixed to produce the best results for different climates. For the hot dry deserts of Israel with cool nights, aluminum shade cloth with drip irrigation is the best solution. On our farm not having mulch to aid soil drying after heavy rains, shade cloths to moderate soil and air temperatures and watering through misting when soil moisture allows is the best solution. For other farms further north and in drier climates, shade cloth and mulching may offer the best results.

Our current strategy is growing lettuces and salad greens through the month of July. We do our last planting of head lettuces around the beginning of June and the last salad greens plantings by June 15th. This allows these crops to be harvested by midsummer. Although we have grown salad greens all through the month of August, the harvest weights are ¼ of what we get in other months due to slow rates of re-growth. The rate of bolting is also several times greater. These two factors have made the growing of salad greens in August not profitable for us as it forces the tending of 3-4 times the amount of space to produce the same income as other seasons. It is in August that the combination of long days, hot weather and high humidity percentages all come together in ways that we cannot offset as we can in other months. For instance, in July we can run the misting cooling devices 2-3 times as much as we can in August allowing good harvests to occur. This misting can be done then because humidity levels are generally lower than in August.

Even if we are not harvesting in August, we are planting both salad greens and lettuces under shade and misting for the fall harvest, which begins in September. This is covered in more detail in the next section. It is the month of July in which we do not plant head lettuce. In May we begin increasing the size of our salad greens plantings. By the end of May we plant twice as much area in salad greens in order to obtain the same harvest weight as the spring season. This is because in summer heat the growth rate of salad greens slows down. During the month of July we plant no salad greens crops. In August that we will begin planting salad greens, aiming for early September harvests.

Two of the greens that we like to have in the greens side of our salad greens mix are arugula and tatsoi. In the heat of summer they begin to suffer from excessive damage from flea beetles. In past years we have used floating row covers, organic pesticides and plant vacuuming to attempt to control these pests. Levels of control were never satisfactory and/or too costly with either labor or materials. The solution we now use is to customers preferred a more “lettucy” mix anyway. By the month of July (planted in June) our salad mix is 90% lettuce. The plantings we make for salad greens in August are similar. It is not until plantings in September that we begin to move back towards a 30% greens/70% lettuce salad mix. During cooler months the flea beetle control strategies then work well, if needed at all.

For several years we tried all the different kinds of lettuces for salad greens that are called summer crisps. These lettuces show less bolting tendencies, but suffer from similar pest pressures of flea beetles. We no longer use those lettuces in warm seasons for this reason.

As I have covered a lot of material in this section I want to do a quick summary. We stop planting our regular salad mix in late April and go to more lettuces. We continue to plant salad greens until mid-May, early June. We will harvest these plantings through late July/early August depending on how well they hold up, weeds, bolting etc… we being planting salad greens again in August under shade and mist for all plantings.

We plant head lettuces through May, timing for harvests to be done by August. We put shade over both salad greens and head lettuces around June 15th depending on heat and other factors, like moisture etc…in July when temperatures are consistently above 80 degrees we begin misting during the heat of the day. We mist as much as possible with the limitations being excessive soil moisture/lettuce disease problems. We plant head lettuce in August for late September harvests.

The cost of these shade/misting devices is roughly equal to one weeks harvest the size of areas covered. These devices offer many years of service.

Early Fall Lettuce Crops:
An early fall lettuce crop is a crop that is planted in the month of July, grows through August and is mature in September. As should be obvious, growing an early fall/late summer lettuce crop is pushing the limits of growing this cool season/low light crop. Although we have experimented with various techniques for modifying the microclimate for growing in this season, we no longer attempt this on our farm. The best way to illustrate the results of this season is to give the examples of summer of 2000 and 2001. In each of these years, these crops were grown under shade and misting.

The summer of 2000 was one of the coolest summers on record. We delayed putting up shade that year for about a month compared to other years. Rarely did high temperatures exceed the mid-80s. Even under shade only 50% of the lettuces survived. Harvest weight was about 50% smaller than regular season heads (1/2 lbs/head or smaller). In summer of 2001, temperatures were above average. Crops in this season grown under both shade and misting were about 40% in size and survival. In our estimation for our farm, the labor outweighs the return.

Fall Lettuce Crop:
A fall crop of lettuce is planted in August and is ready to harvest by the end of September. This is one of our favorite crops to grow due to less weed pressures and no succession plantings. We plant the entire crop at once even though we will sell it week by week rather than the weekly succession plantings we make at the rest of the year. The lettuces rather than bolting after the normal number of days to harvest, continue to grow larger and larger. We have sold heads of lettuce that weigh over 2 lbs apiece in this season. We harvest until repeated days of night time lows around 20 degrees kills the plants. When plants start to receive significant damage, we sell off everything that is left before it is killed.

Around the first of August we make a large single planting of head lettuce, more than what we anticipate being able to sell in the months of October and November. Because of the lower germination rates and higher attrition rates due to hot humid conditions we plant seeds about twice at thick as we would in other seasons. Any excess plants that survive are hoed out during cultivation in September.

These densely planted crops are started under shade and mist. We remove shade and misting when daytime high temperatures remain under 70-75 degrees, generally in early September. This is one of our more interesting crops in terms of it starts under shade/mist in +90 degree temperatures and finishes its life under heavy row covers protecting it from 25 degree frosts. In 1999 we sold lettuce until December 15th. In 2000 our last lettuce sales were December 3rd. Length of sales each winter is determined by the onset of cold weather.

Winter in Unheated Greenhouse:
By growing outside under heavy row covers, such as described under late fall lettuce crops, it allows our unheated greenhouse to remain in tomatoes through September. On the first of October we plant the greenhouse in salad greens that will grow through October and be ready for harvest the first of November when the outside fields being to slow down. We experience good re-growth rates through November and into December. At the onset of consistent night temperatures below freezing we begin putting lightweight row covers over the greens in the unheated greenhouse. This treatment allows the salad greens in the greenhouse to maintain good re-growth rates through December or when lows are consistently below 20 degrees. Around December 21st growth rates drop dramatically. Our records show that between 1/5 to 1/6 of the spring time growth rate occurs. This translates into needing 5 or 6 times the growing space as would be needed to produce the same summertime weight of salad greens. Of course the selection of what is grown in this winter time season is important and has been thoroughly tested. We grow the more winter hardy of the lettuces. Growth rates remain at this slow rate until about February 15th. Depending on the outside temperatures growth rates begin to pick up dramatically. Generally by March 15th we are in full production in the greenhouse, growing and harvesting at rates similar to May for outside production. Over wintering of the greens in the greenhouse gives a dramatic jump of the spring time harvest, which is not planted in outside fields until around February 15th to March 1st and is not ready to harvest until April 1st. This is about the time that the over winter greens in the greenhouse are closing out their life cycle. By this time we are growing outside and the greenhouse is cleared out for an early tomato crop. Although we have tried to heavy weight of row covers in the greenhouse, we have found that the lightweight spun poly covers provide sufficient protection from cold, are easier to manage in the greenhouse and have a significant longer lifespan in the greenhouse than when used outside. For example in winter 2000 after a month of temperatures below 10 degrees, the soil temperatures in the greenhouse under the light row covers was in the low 40’s at night after three days of clouds. Another example is in the warm 1998-99 winter we harvested all winter except for the month of January.

A very excellent booklet on growing salad greens all winter long is Farming the Backside of the Calendar. The author is Eliot Coleman of the popular book Four Season Harvests. In this booklet he outlines the adaptation of personal season extension techniques to commercial application. On his farm they make use of similar greenhouse system with the addition of a heated greenhouse for growing lettuces. They grow the very hardy greens like claytonia, kales, and minutina that our market will not accept in substantial quantities in a salad mix. These very cold hardy salad greens grow in colder weather than lettuces and lower light conditions. During the coldest months of the year his greenhouses are not so much growing areas as holding areas for already mature plants waiting for their turn to be harvested.

The nature of these trials was to increase the growing season of certain seasonal products. The majority of these trials took place on 8000 square feet or about 1/5 of an acre. Last year we made approximately $16,000 off that 1/5 acre. When we first started with these trials we could grow for about 3 months of the year. Last year we grew for 9 months. In other words each month of the year that can be farmed, about $1,800 can be made off that 1/5 acre. Farming it three months of the year yields $5,400. Using the technologies that we have developed, partially through this grant enable that land to be farmed for 9 months, yielding $16,000/year.

The bottom like is: before grant - $5,400/year and after grant - $16,000/year

In the back of Eliot Coleman’s Farming the Backside of the Calendar he goes into somewhat lengthy calculations about the use of plastic greenhouses to grow local salad green as opposed to trucking those greens from California. Whether his figures are only justification or factual could be debatable, but they conclude that a major environmental impact is concluded on the side of local growers. While one area of our studies is also in the winter production of salad greens, hence the use of greenhouse film, other areas of our studies also makes use of plastic climate control devices.

First of all let me state that in each of these areas of season extension the dollar cost of the materials used is about equal to one weeks of harvest profit off that area covered. By making environmental comparisons similar to Coleman’s based on greenhouse film over shipping of this product from California, growers using these technologies should have a positive impact on the level of petrol-chemicals used for these salad foods.

How does one measure the social impacts of a food that normally appeared in the local stores for three months of the year, now appearing for nine months? How does one measure the impacts of a hillside farm that produces more income than 100 acres of adjacent flat farmland? How does one measure the social impact of many people stopping by just to ask “What are you doing?”

In Indiana there is a strong county extension system. Through this grant I have made acquaintance with the major vegetable researchers at Purdue University and the county extension agents located in the three major vegetable growing areas of the state. There was an effort to have a presentation of these trials placed on the agenda of three different winter agricultural meetings. However none of those presentations came through due to various reasons. Supposedly two of the sites want to try and fit it into the 2002 program.

There was also supposed to be a poster made of these trials for viewing at these meetings and through a glitch in the system they did not get made. Supposedly this is to happen next year also. These posters are to refer interested parties to a local website where complete trials data will be available.

There were three local field days associated with these trials, one for other farmers and two for consumers. The field day for farmers was poorly attended; the field days for customers had nice attendance.

Partial results for these trials are available at a website through the Indiana Commissioner of Agriculture. This is supposed to be updated in the near future with the completion of the manual. There is an additional website through the university Ag network that is supposed to enter these trials in the near future also.

Recently a county extension agent who had a local farmer wanted to extend his salad greens season contacted me. This let me know that my name is in the network now. Any season extension inquiries that enter the Purdue network will for the next several years be sent my way.

I am currently considering publication of the results of these trials through several sustainable and organic farming magazines. Frankly I am a little disgruntled at the outreach results, but am realistic about it too. At this point I know for certain that one other farmer is using what we worked on in this grant.


Participation Summary
Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.