by Bill and Mary Weaver
You can grow culinary herbs in your greenhouse for the holidays and beyond, and have your herb crop harvested in time to start growing spring plugs for bedding plants. These herbs may make a good alternative for poinsettia growers.
“Poinsettia sales are pretty much flat,” noted Dr. Christopher Currey of Iowa State University Extension, speaking at “Cultivate ‘14 in Columbus, Ohio. “Greenhouse herbs may also make a profitable replacement for greenhouse tomatoes. The herbs have the advantage of a shorter production cycle than greenhouse-grown tomatoes.
“Culinary herbs are definitely a value-added product, and greenhouse grown, they will be clean and can be sold several ways: cut and packed in clamshells; as living herbs in plastic sleeves (when grown hydroponically); or as potted herbs that the customer can grow and re-harvest.”
In popularity, herbs fall into six groups, according to Currey. Basil and parsley are the most popular. In the second group, you’ll find chives, dill and mint. Thyme, rosemary and French tarragon fall in the third most popular group. Oregano and sage stand fourth. Fifth in popularity are sweet marjoram and French sorrel. The last of the frequently grown culinary herbs, in terms of probable sales, is cilantro. One-stop shopping attracts buyers. If you have all the herbs your buyer will want, he’ll only need to make one phone call to place his whole herb order.
Greenhouse-grown mature greens and herbs make a nice combination, as herbs generally need a similar infrastructure and growing conditions to mature greens. Greenhouse herbs will yield more handsomely if they’re given a bit more warmth. The grower will need to balance the cost of fuel against the higher production of leafy mass that warmer temperatures bring.
“An important concept for hydroponic herb production is that leaves unfold at a rate dependent on temperature,” Currey continued. “Below 50 degrees as a base temperature, few or no new leaves may unfold. Peak production of most herbs is reached at about 72 degrees. Above 83 degrees, most herb plants will be experiencing stress. Dr. Currey recommends 60 to 70 degrees F as the optimal temperature range for both reasonable fuel use and good leaf production.
Light levels also matter. To increase your harvestable mass, try maximizing growth with high-pressure sodium lamps. LED lights may also have potential for the culinary herb grower, but their particular wave lengths may impact the plants’ production of volatile oils. Too much supplemental lighting may backfire, however, by inducing flowering in some herbs, such as basil and dill, when they are given too long a photoperiod. “We’re not growing these plants for flowers,” Currey continued. “Watch the plants’ growth, and harvest just before the flower buds form.”
Some herbs have many different cultivars. “There are 35 types of basil alone in our cultivar trials,” commented Currey. Unfortunately, most cultivars have been bred for growing conditions in the field, rather than in the greenhouse. To determine which cultivars might grow best in your environment, talk to other growers, seed companies and particularly to potential customers. Depending on where you are located and what your markets are requesting, some specific species and genetics may work better than others. Two specialty basils that are becoming more popular are holy basil and Thai basil.
Will annual or perennial herbs work best for you? In general, annual herbs are propagated from seed, and as a broad stereotype, have a faster rate of growth and of regeneration after cutting, but annuals also tend to bolt and/or “tire out” more quickly. Perennial culinary herbs are typically propagated vegetatively, mostly from stem tips, but also from root cuttings and rhizomes. Herbs tend not to be variety-protected, and this allows you the freedom to grow your own stock plants and propagate them yourself. Perennials tend to have a slower rate of regeneration and tend to have a larger root mass. Don’t use a 4-inch channel for perennial hydroponic herbs if you’re using nutrient film. The root mass will clog a 4-inch channel.
“We’ve found nutrient film moves through a 9-inch wide, 6-inch deep channel satisfactorily,” stated Currey. It may be simpler to use a raft or deep flow culture, planting the seed in Styrofoam channels, with the roots hanging down in the nutrient solution.
If you use media beds or troughs, you can use perlite as a substrate. “It’s not uncommon to have almost a raised bed-type system, with the media bed or trough filled with peat. This may be a comfortable way for a grower to work into a raft system in the future.”
Should you plan to harvest just once, or several times? There are pluses on both sides. A single harvest has higher substrate and seed cost. “You’ll get 2 to 3 times the yield per plant, with lower seed and substrate costs, with multiple harvests.
“However, that said,” continued Currey, “I’m a cautious man. I’m nervous when I see long woody stems with a tuft of leaves on top. If disease starts, there aren’t many chemicals you can use in a greenhouse. I am a fan of the single harvest system. Get the plants cut and out to prevent disease development. The second cutting will not yield as much biomass as the first cut, although the physiology of the second cutting of basil, I’ve been told anecdotally, stands up a little better for marketing than the first cutting.”
There isn’t a lot of information available about optimal pH and EC for individual herbs. We can arbitrarily categorize mint and parsley as doing best at a lower pH (5.4 to 5.8), basil at a moderate pH (5.8 to 6.2) and rosemary at 6.2 to 6.6. The big question is, how many nutrient tanks do you have?
“An Iowa grower I know has one nutrient tank,” said Currey, “and he shoots for the middle of the pH range. With such broad plant diversity, you won’t be able to arrange for ideal growing conditions for any one plant. The same applies to EC (electrical conductivity). When in doubt, I like to say, ‘It depends.’”
As seedlings mature, you can cut EC to one third, and you can cut it even lower right after harvest, because the plants don’t have lots of foliage. If you grow perennials, understand that they grow more slowly with low EC. With higher light levels, you need higher EC to get transpiration levels up. The most useful thing you can do with EC is to provide a general range with the most overlap among the plants you’re growing.
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