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Greenhouse tips for hoophouse growers

publication date: Feb 1, 2014

By Andrew Mefferd

overheadAs I’ve visited many heated, hydroponic tomato greenhouses in recent years, I’ve noted several techniques that would benefit soil-grown tomatoes in unheated hoophouses. Many hoophouses are put up by growers who are more familiar with open field growing and may not employ the full range of management strategies available to make the most of protected cropping space. In this two-part series of articles, I will talk about four techniques that I think could be used profitably in an unheated hoophouse.

These techniques may involve increasing the amount of initial cost and labor that goes into a crop in order to multiply productivity. I know labor is always short on farms and many people may question why they would put more labor than they are already using into growing a crop. But protected growing space is precious real estate, and there are a lot of techniques that can help you get more out of yours. As long as you’re getting more productivity or quality out of a GH/HH space than the value of the labor you’re investing, you’re coming out ahead. Investing $5 more in labor to make $10 more is a good investment.

Variety selection
It’s winter time and you’re looking at your seed catalogs. You notice there is a greenhouse tomato section in the seed catalog, and maybe your favorite varieties are not in it. Why not just go ahead and plant your favorite field varieties in your hoophouse or greenhouse?

Well, a lot of people do, which is why I make the point that growers could benefit from at least trying some dedicated greenhouse varieties if they haven’t already. From visiting with growers I know that there are a lot of varieties (Big Beef, Early Girl, and New Girl come to mind) that are commonly grown in hoophouses though they were not bred for that. And, yes, they may do pretty well in a hoophouse. They may keep up with the yield of a greenhouse variety for a while. But over the course of a season a greenhouse variety will usually outperform a field variety.

One of the reasons is disease resistance. There are a bunch of diseases that are pretty rare in the field but common under cover, such as leaf mold and tomato powdery mildew, so even if the field variety is keeping up through the early part of the season, it’s going to start falling behind if it comes down with one of these diseases. 
The other reason GH varieties will usually do better in protected agriculture is because they have been bred and carefully selected to do just that. The GH varieties that get commercialized are picked because they have performed well under the conditions including higher temperatures and higher plant densities of the GH. 

A good analogy for this would be horses.  We have many different breeds of horses each bred for a specific task. If you had a thoroughbred, you wouldn’t take it into the woods logging, would you? And likewise you wouldn’t enter your draft horse into a race. Sometimes it’s hard to see the difference without putting a few draft horses in to race against the thoroughbreds. So my advice is that if you’ve never tried dedicated greenhouse varieties, try a few plants or a row of one against your standard varieties this year. 

I’m not saying you should not grow non-greenhouse varieties in your hoophouse. I usually grow about half a house of red greenhouse hybrids, and half a house of various heirlooms every year. I see the difference when I get a third of the yield out of the heirlooms than I do out of the greenhouse varieties. But that’s a choice I’m making with eyes open, knowing what kind of yield I’m missing out on. I can count on the heirlooms getting leaf mold and stopping production late in the season when the greenhouse varieties are still chugging along.

grafted plantsIf you only try one technique from this article, I would say the most important one is grafting. Most of the big greenhouses are already grafting their tomatoes, but when I go to smaller farms it’s hit or miss whether they’re using grafted plants. Based on my experience on my own farm and doing variety trials on rootstocks at Johnny’s Selected Seeds, I would say that grafting to a good rootstock variety like the ones that are currently on the market has the potential to add 30-50% to your tomato yields without compromising quality one bit.

It’s worth noting that you can graft other greenhouse fruiting vining crops as well, like eggplant, cucumbers and peppers, but our results with those crops have been less clear cut. You can graft eggplant and peppers onto tomato rootstocks, and cucumbers onto other cucurbit species rootstocks. But developing good rootstock/scion combinations in those crops is trickier than in tomatoes, so that’s why grafting those crops is not as common as with tomatoes. But it’s definitely worth looking at and I’m sure we’ll see more work on rootstocks for those crops in the years to come.
Grafting is the kind of technique where your mileage will vary depending on the conditions. It helps more where the conditions are less optimized…like in unheated, soil grown hoophouses! Larger hydroponic growers usually only get about 10 percent yield increase from grafting because they have already optimized most of the other growing conditions like temperature, fertility, humidity, etc. Most of the large greenhouses are grafting since that extra 10 percent is on top of an already very high yield, it is the icing on the cake. The increased profit on that extra 10 percent is still much more than the cost of using grafted plants.

Smaller growers generally have even more to gain from grafting. It is a generalization that obviously doesn’t apply to everyone but many smaller HH and GH growers have not optimized production conditions to the extent that larger growers have. My own HH is a good example of this. Living in Maine as I do, every single night in my unheated hoophouses, even during the summer, is cooler than ideal, and many days during July and August the temperature is hotter than it should be. I grow in soil, so fertility for a long season crop is tricky, it’s hard to tell how much of what nutrient is available at any given time, unlike in hydroponics where you know exactly what the plant is getting because they are getting a specific mix designed for the crop. Grafting is helping me overcome the production difficulties that I face to get a 30-50 percent yield increase from grafting. Let’s break it down and look at why.

Interspecific hybrid vigor
I like to think of tomato rootstocks as the mules of the vegetable world. Mules are the cross of a horse and a donkey, two different species that produce offspring that has advantages over either of its parent species. One of the reasons mules can be better than their purebred parents is due to hybrid vigor. Hybrid vigor, whether in plants or in animals, is generally more pronounced when the two parents of the hybrid are more distantly related.

My friend Donn Hewes says that mules have the toughness of a donkey in the size of a horse, and that’s basically what a grafted tomato plant is. It has the toughness of a wild tomato with the culinary qualities of your favorite tomato, plus interspecific hybrid vigor.

For example, if you cross two tomato plants both of the same species Solanum lycoperswhite mulchicum, you may notice some increased vigor in the progeny from the hybrid.  However, if you were to cross two tomatoes of different species, say S. lycopersicum and S. pimpinellifolium, you are likely to see a much greater boost in vigor than your cross of two S. lycopersicums. This type of cross of two different species is called an interspecific cross, between two different species. It is a natural way to get a huge vigor boost without turning to genetic modification or other more modern tricks. 
Disease resistance
The technique of grafting is as much a plant breeding trick as it is a plant husbandry trick. What I mean by that is being able to make two plants into one frees the plant breeder’s hand to work on the traits for each part of the plant independently from the other. Traditionally, one of the challenges for plant breeders was to give us a plant that has it all: high yields, good flavor, good disease resistance, etc. But it’s hard to have everything we want because plant breeding is a balancing act and frequently adding to one area takes away from another one. For example, a breeder may start out with a great-tasting tomato that is hard to grow because it has no disease resistance. So the breeder crosses the good-tasting tomato with a disease-resistant one and most of the time, the flavor suffers in the process. This progression taken to the extreme can be found at your local grocery store, especially in winter. The grocery store tomato that we all know and loathe is an example of breeding that has focused on everything but flavor.

The beauty of grafting is that it frees the scion breeder to focus on the traits that would make a great tomato above the graft union (leaves and fruit) and the rootstock breeder to focus on what would make a great tomato below the graft union (mainly roots). The rootstock breeder can focus on just giving the plant a really great root system with lots of soilborne disease resistances, and forget about flavor, appearance, foliar disease resistance, etc. The scion breeder can forget about the root system and focus on fruit quality, foliar disease resistance, plant habit, etc. In fact, if you grow out most rootstock varieties they produce fruit that takes a long time to ripen, is not attractive and doesn’t taste good. You would never want to use a rootstock ungrafted as a tomato plant but they do make great roots.

One of the common misconceptions about grafting is that the disease resistances from the rootstock extend up into the scion. This is not true. Each plant’s own disease package applies only to its side of the graft union. For example, if you grafted a top variety that was susceptible to leaf mold onto a rootstock that was resistant to leaf mold, the top of the plant above the graft union would still be just as susceptible to leaf mold as if it had never been grafted.

That’s why when you look at the resistance packages for rootstocks, they mostly stick to soilborne diseases. This is still a huge advantage, though, since it allows you to grow top varieties in soil you know to be infested with diseases that they cannot resist.  The resistant rootstock acts as a blocker, keeping the disease in the soil from getting up into the top variety that is not resistant. That is why it is so important to plant grafted plants with the graft union above the soil line and keep the plants from sprawling on the ground, to prevent the scion variety from rooting in the ground.

If the plants are planted deeply, as is commonly done with tomato plants to develop adventitious roots off the stem, the top non-resistant variety will root into the infected ground and become infected itself. When grafting, it is advantageous to make the graft union as high up the stem of the rootstock as possible. If the graft union is just barely above the soil line, there is a good chance that the stem will sag and come into contact with the soil or soil will be thrown up against the stem causing the scion to root down into the soil. 
Stress resistance
The last group of benefits that rootstocks can confer on plants is the ability to thrive in conditions outside the ideal, which cause stress on the plant. Because rootstocks incorporate the genetics of wild, tougher ancestors, they can make our cultivated varieties more resistant to stress.

Especially in unheated hoophouses, plants are often outside of the ideal high or low temperature, causing thermal stress on the plant. Rootstocks are selected for resilience in the face of thermal stress, and in this sense rootstocks can do some of the work in a HH that climate control would do in a heated GH. They broaden the temperature range in which plants will thrive.

Another type of stress that rootstocks are being selected to resist is water stress - drought, flooding and salinity. This means that plants will be less impacted by any one of these hardships, making the crop more forgiving for us to grow.  Rootstocks are being developed specifically to deal with excess salinity, which could be useful wherever water is scarce and salts tend to build up, in places as diverse as Israel or Texas.

Rootstocks are also bred to resist chemical stresses, including pH being too high or low and nutrient stresses, too much or too little of essential nutrients. Wild species tend to invest more biomass into root production, so once again we see how grafting results in a superior plant where the rootstock does what it does best (root production) and the top variety does what it does best (fruit production). This superior root production includes both root density and rhizosphere exploration, which basically means that the root system is bigger and more aggressive about growing to seek out what it needs in the soil. 

A bigger, more aggressive root system, in turn, can help the plant deal with both previously mentioned stresses. A bigger “pump” to draw from can help the plant through times when there might not be enough water, which may also help it through thermal stress by being able to pump more water. In times of excessive heat this may keep the plant from wilting simply by being able to keep up with the plant’s water demand for longer. 

I have seen the cumulative effects of all these advantages in my own hoophouses. In 2012, we had a very hot summer with many days in July and August in the high 90s and low 100s. I kept thinking that I was going to see some blossom drop, drop in production or fruit quality due to these excessively high temperatures. 2012 ended up being my best production year so far, and I think some of that success was due to the resilience imparted to my plants by their rootstock.

All of the tips in this article mean increasing up-front costs, be they seed cost of greenhouse varieties, propagation costs of starting more plants or grafting, or labor costs of lowering and leaning the plants.  But any of these strategies that you may choose to do have the potential to increase the yield and profitability of the crop over the course of the season.  Which is one of the ways that greenhouse growers achieve such high yields—by investing a bit more money and labor in the beginning to achieve an even greater return on the structure over the course of the season.  You may not choose to do all of these things, but it is worth considering whether they may be able to help your 2014 crops.

Lowering and leaning
How do you grow a 30 foot tomato plant in a 9 foot space?  Lowering and leaning!  If you find your plants outgrowing the space they’re in before the growing season is over, you might want to consider lowering and leaning.

Lowering and leaning is a trellising technique that was developed to be able to grow tomato plants in long-season greenhouses where the plant would outgrow the vertical space several times over.  The basic idea is that instead of tying a string to a pipe or other solid object overhead, you attach a spool of greenhouse twine to a wire so that when the tomato outgrows its space you can lower it down a foot or two and give it some more space to grow.  The leaning part of the two-step process comes in because after it is lowered, you move the spool down the wire so the plant’s stem doesn’t just coil up below the plant and possibly break.  Leaning keeps the stem at a gentle angle to avoid breaking.
There are specially designed spools for this process, the most common being the tomahook but there are also rollerhooks and other devices that accomplish the same purpose.  One other option is the Qlipr system I wrote about in the April 2013 GFM.  Most of these devices require a wire instead of a pipe or other object overhead to hook onto, because of the size of the hook.  But it may be possible to put very sturdy loops of wire everywhere a plant is supposed to be anchored in order to retrofit an existing pipe-trellis system.

Where I live in central Maine, the length of my growing season puts me right on the edge of needing to lower and lean.  So I’d say for the people in the continental US who are farther south than I am (most of you), it’s probably worth considering this technique.  The top wire in my hoophouses is nine feet off the ground.  In a year like 2012 when it was hot and the plants grew very fast, the plants grew up to the wire quickly.  We ended up lowering and leaning many times and I think it was beneficial to have that extra space.  In years like last year when we had cooler weather, overall poor growing conditions and the plants didn’t grow as much it may not be worth it.  I had installed tomahooks but never ended up lowering and leaning the plants because they grew so much less than the previous year, they were only just hitting the top wire when they needed to be topped at the end of the season.

This can be done with cucumbers and eggplant as well.  Though you have to start leaning the eggplant to get it slanting in the direction you want it to go in before you ever lower it since the stem is so much more brittle than a tomato.  Peppers can be trellised, but they are too brittle to lower and lean.

Planting density
Another possible yield increasing strategy is how many plants you put in a given space.  I think planting density is worth thinking about because I go into so many hoophouses that have, say, five or six single rows across a 30’ wide hoophouse.  At this spacing the airflow is great but there could be a lot more plants in there!  One way to look at it is if you go into your hoophouse and there is a lot of sunlight hitting the ground, that sunlight could be put to use growing plants.  In my own hoophouse, I have emulated greenhouse spacing, with 5 double rows across the 30’ house.  I use a spacing for cukes and tomatoes with the double row two feet apart with four feet between double rows.  And I’ve seen even tighter spacing used successfully.  This has brought the amount of space used by each plant from nearly 7 square feet per plant in the first scenario to 3.5 square feet per plant in my current setup.  The sheer number of plants in there has increased yield accordingly.

There is very little light hitting the floor in my hoophouse by the time the plants have hit the top wire.  You might think the plants shading each other would reduce yield, but plants don’t have an unlimited capacity for photosynthesis, and on a good sunny day, the plants will absorb as much light as possible even with a little shading.  The thing I worried about the most when going to this heavy of a planting density was increased disease pressure with so much less airflow.  And that is one possibility but with a good leaf pruning schedule to open up airflow down low and remove diseased leaves (see next month’s article), I have kept my disease levels manageable and greatly increased yield.

I’d like to revisit the planting scheme that I mentioned in the March 2013 issue of GFM. That article discussed the use of a double row of tomatoes instead of single rows to greatly increase planting density.  It is worth mentioning that instead of planting a double row of tomatoes in a bed, you can put the same number of plants in a single row.  You then split the canopy into a double row by having two trellis wires or poles above the bed.  The advantage of planting a single row at double density and training the plants to two overhead wires (making a double row out of a single row) is that weed barrier fabric can be laid right up to the single row of stems, leaving very little HH real estate uncovered, leading to very little weeding. This method has worked well for me, and is more similar to how hydroponic tomatoes are grown in a single channel and then trained to two overhead wires.

Some prefer to actually plant two rows to trellis up to two overhead wires. This allows for more air flow around the stems, and in hydroponic systems where people grow in containers like 5 gallon buckets, the buckets cannot be spaced closely enough together in a single row to accommodate all the stems. The disadvantage of planting two actual rows in the ground is that weeds can pop up in the space between the rows, unless you lay a skinny strip of weed barrier between the rows.  And those weeds are hard to reach between the rows.

Andrew Mefferd co-owns One Drop Farm in Cornville, Maine, and is a senior trial technician at Johnny’s Selected Seeds Research Farm. Contact him at

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