One of the many challenges facing tomato (Solanum lycopersicum L.) and kale (Brassica oleracea) greenhouse growers is the management of edema (oedema). Often described as intumescence or enations, edema is an abiotic-induced complication that develops at different levels of intensity depending on the cultivar and, in some cases, between individual plants.

Symptoms

Edema is often described as a blister or callus-like formation that forms predominately on the bottom of leaves. This swelling is the result of the epidermal layer on the leaf expanding due to cellular elongation, which eventually burst as cells reach their limits of expansion

The resulting damage on the bottom-side of the leaf can be characterized by dark brown, tan, or even yellow necrotic tissue on the surface of the leaf that expands past the original location of the blister as a scab-like injury. On the upper side, necrotic spots mark the location of the ruptured intumescence below, while the surrounding tissue is generally chlorotic. Tissue affected by edema is brittle, with the leaf structure cracking under gentle pressure by the fingers. Extensive edema can severely decrease the leaf’s photosynthetic capability and lead to senescence.

Causes

The cause of edema can be linked to a variety of environmental conditions. The long-standing suggested cause of intumescence is the buildup of excess moisture in the root medium paired with conditions favorable to low transpiration (i.e. high humidity).

In a study conducted by Kansas State in 20091, it was concluded that the water content of the root zone may influence the development of edema in tomatoes. Other research has pointed to light quality (lack of ultraviolet light) being a driving force behind the development of edema in Northern greenhouse tomato production.

Due to largely overcast, low light conditions during winter months, most of the lighting provided to tomato crops in greenhouse operations are supplied by high pressure sodium (HPS) fixtures. While these units produce significant amounts of light in the yellow/red spectrum (550 – 650 nm), they lack in short wavelength energy (300 - 400 nm). This shortwave energy is also lost in greenhouses constructed with glass glazing which has relatively low transmissivity of UV light as compared with polyethylene glazing.

In the same study by Kansas State, it was found that UVB helps prevent the formation of intumescence. In another experiment, conducted at the University of Arizona2, using small doses of end-of-day far red (EOD-FR) and a 75% blue light ratio led to a decrease of intumescence injury from 62-70% of leaves being symptomatic to only 5%. Work at Michigan State University3, found dramatically reduced edema symptoms when light with 50:50 Blue:Red ratio was used as compared with 100% Red light.

Edema of kale

In kale, sudden increases in humidity can cause edema, often coinciding with outdoor weather like rain. Edema can be especially impactful on kale crops as the superficial appearance of leaves affects their marketability. Maintaining a low relative humidity within greenhouse environments, and changing management strategies to better cope with outdoor weather will help prevent edema in kale. This may include increasing shading within a greenhouse to reduce use of cooling pads to help manage humidity. Changes to irrigation timing and application or spacing plants further apart may also help to reduce humidity.

Solutions

Controlling the impact of edema should be done on a case-by-case basis for each operation and based on likely causes.

As mentioned before, the severity of intumescence on tomatoes is largely dependent on the susceptibility of the cultivar and individual plants. If the crop is experiencing major leaf senescence and poor yields, it may justify the cost to invest in preventative measures. Most modern lighting fixtures with digital ballasts can run both high pressure sodium and metal halide (MH) bulbs. For older magnetic ballasts, conversion bulbs can be purchased that also allow you to utilize MH bulbs in an HPS fixture. While somewhat less efficient and shorter lived, the benefit of metal halide is it provides more of a “full-spectrum” light output, with significantly shorter wavelength energy. This strategy combining HPS and metal halide could reduce the onset of edema-like symptoms. While a considerably costlier investment, LED fixtures offer the greatest flexibility in light quality. As prices decrease and efficiency increases, LED technology may offer the solution to improved yields through the reduction in intumescence frequency.

For many other growers, the prevalence of edema may not justify the investment of retrofitting an entire greenhouse.

It may also be beneficial to experiment with a variety of cultivars to screen for edema-like symptoms or work with a seed distributor on selecting cultivars that are less sensitive to edema.

No matter the situation, it is advantageous to develop an action threshold to determine at what level of prevalence should you address the issue of edema in your operation.

Editor's note: This article originally ran as an e-GRO alert.

Photo courtesy of Christopher J. Currey

Basil is the most popular culinary herb grown hydroponically or in controlled environments. For greenhouse production, basil is most productive from the late spring to the early fall. During the later fall, winter and early spring, basil productivity decreases and growers are left wishing they had more production to harvest and sell. Let’s explore how to keep sweet basil productive throughout the year.

Temperature

Basil is a warm-growing plant (Fig. 1) During the fall, winter and spring, cooler outside air temperatures combined with less radiant energy from sunlight will lead to cooler temperatures than during the summer. The optimal temperature for basil is around the mid- to upper- 80°s F and shoot growth is greatest at this temperature. Maintaining greenhouses at that warm average temperature is not feasible. However, it demonstrates how basil is clearly more productive in warm temperatures. It is very tempting to grow cooler during the winter, but try to remember warmer air temperatures will lead to greater yields.

Additionally, don’t drop temperatures too cool during the night, either. Basil is sensitive to temperatures below 55° F and while cool night temperatures are also more cost efficient, they can also restrict growth. If multiple zones are available in the greenhouse, basil can be grown warmer in a separate area as opposed to being grouped with leafy crops such as lettuce, spinach, kale and culinary herbs including dill, parsley and cilantro that can be grown at cooler air temperatures.

Light

While basil growth will slow down in cooler temperatures, diminished light will also negatively affect yield. Basil is a high-light crop, with growth maximized at a daily light integral (DLI) of around 29 mol·m–2·d–1. Depending on geographic location, the outdoor DLI will not even be this high in the fall, winter and spring, much less inside the greenhouse where light is often 30% to 50% lower than outside. While cooler temperatures will slow down the rate of crop growth, low light limits yield for basil crops. Minimizing overhead superstructure and equipment, and cleaning glazing materials will help maximize transmission of sunlight inside the greenhouse. But supplemental lighting is the only way to increase the amount of photosynthetic light available to basil.

Targeting basil over other culinary herbs in a greenhouse with any available supplemental lighting will be beneficial. But while nearly all culinary herbs grown in a greenhouse benefit from supplemental lighting in the winter, the relative increase in yields from lighting basil is greater and will make for a better return on investment than other culinary herbs.

Nutrition

One of the natural instincts most commercial growers have is understanding when to boost mineral nutrients to boost plant growth. While that may work for some crops, it is not an effective strategy for hydroponically grown sweet basil. Whether grown at an EC of 0.5 mS/cm or 4.0 mS/cm, there is no discernable effect on plant growth or fresh weight. Basil is efficient with mineral nutrients and, as a result, providing more nutrients will not increase yields. While increasing fertilizer won’t boost growth, be sure to watch for magnesium deficiencies to maintain leaf greenness and marketability.

Planting density

One of the easiest opportunities to boost yields during the cooler, low-light months is by increasing planting density by reducing the space between plants. By increasing the planting density and producing more plants per unit area, yield over your growing space can increase. For instance, if basil is currently on a 6- or 8-inch center, change to a 3- to 4-inch center. There will be an increase in costs for substrate and seed because more seedlings will be grown for transplant. However, this should be more than compensated for by the increased yield and subsequent revenue.

Take-home message

Sweet basil is the most popular culinary herb grown hydroponically. For greenhouse production, the late spring, summer, and early fall can be very productive for basil. However, basil yields in the late fall, winter and early spring can be improved through better management of air temperature, light and plant spacing.

The author is an extension specialist and professor at Iowa State University. ccurrey@iastate.edu

Photo courtesy of Lufa Farms

Lufa Farms opens new rooftop greenhouse in Montreal

Building on a new garden trend, a greenhouse atop a Montreal warehouse growing eggplants and tomatoes to meet demand for locally sourced foods has set a record as the largest in the world.

It’s not an obvious choice of location to cultivate organic vegetables — in the heart of Canada’s second-largest city — but Lufa Farms on Wednesday inaugurated a facility that spans 160,000 square feet (15,000 square meters), or about the size of three football fields.

>> Read the full story here. 

Photo courtesy of Vertical Harvest

Vertical Harvest to open new farm in Maine

According to story from News Center Maine, vertical farming company Vertical Harvest is expanding to Westbrook, Maine, with a 70,000-square-foot vertical farm.

“We want to grow as much food as possible. We want to employ as many people as possible. We want to do both year-round. That’s what gave us the idea to go up. So at its heart, we want to be able for these farms to serve the community in which it sits, so that means being in the heart of our urban centers,” said co-founder Nona Yehia, in the article.

>> Read the full story here. 

AmericanHort, HRI announce new recommended terms for disease resistance claims

AmericanHort and its research foundation, the Horticultural Research Institute (HRI), has started an “industry-initiated movement” to begin a market-wide standardization for terms appearing on plant material being released to the market. The tags will reference specific diseases and/or abiotic stresses.

The goal is ultimately to help customers understand different traits and characteristics — drought, cold exposure, etc. — so that customers have more reasonable expectations for performance once they purchase plants. Another term they hope to clarify is resistance, as customers often treat resistance and tolerance as the same. The reason they are recommending these changes now, is due to an expected influx of new plant material into the market over the next few years.

>> Read the full story here.

The weather is starting to turn into fall here in Ohio and after enjoying a bountiful summer garden (sometimes a little too bountiful), I’m starting to harvest the last of the veggies from my garden. It’s always a little bit sad to take a last trip out and grab the remaining few tomatoes and peppers on the vine. I’ll miss the fresh-off-the-vine taste.

Like many, I delved into growing my own a little more than usual this year after COVID hit. I had more time to explore new heirloom tomatoes, hybrid hot peppers and teeny-tiny eggplants I had never heard of before. And I’m not ready to go back to the same old boring options I’ve had in the past.

In the upcoming winter months, I’ll be looking at ways to continue experimenting with local produce, even if it doesn’t come straight from my own backyard. A big driver in my search is the great recipes I’ve found on social media like Pinterest and Instagram. By looking at what folks around me are finding and cooking, I’ve discovered new recipes as well as different spots to buy locally grown greenhouse produce.

Recipes ideas are a great way to market your offerings and inspire potential customers to pull the trigger and try your produce. Even my local chain grocery store has started sending out newsletters with specials and recipe ideas to inspire shoppers to try something new.

And it works. A beautiful photo of a delicious dish is a sure-fire way to get me thinking about putting in an order. Plus, it gets people excited about adding more veggies into their diets.

For example, over the Labor Day weekend, I saw so many more vegetable-based dishes than I have in previous years when it had just been burgers, chicken, hot dogs and other things I grew up eating off the grill. I’ve got to say a colorful vegetable kabob is a lot better looking than a half-burned hamburger.

I’ll be interested to see if that same trend carries over into the holidays, when food takes over social media again and we all start scrolling through to find ideas for our next big celebration.