The Best Ways to Harvest Hydroponic Lettuce
Lettuce grows very well via hydroponic production. When it comes to harvesting those tasty greens, growers have a couple of options based on the type of lettuce, as Chris Bond explains.
Lettuces are one of the crops most suitable for hydroponic production. Its quick turnaround and popularity make it a great plant and easy choice for both new hobbyists as well as seasoned commercial growers. Hydroponic lettuce can start to be harvested just a few weeks after seeding, depending on variety. There are several ways to harvest hydroponic lettuce which makes it a very versatile crop.
It can fit a variety of applications, whether growing for your own salads or for market production.
While there are hundreds of species of leafy greens, most fall under one of two categories: head lettuce or leaf lettuce. As their designation implies, one type is harvested as a whole plant, and the other is harvested leaf by leaf. Each has its own unique methods of harvest.
Whole-Head Lettuce Harvesting
With whole-head lettuce harvesting, the plant is harvested fully and a new one will need to be seeded. Depending on the end user, this can be the whole plant including roots, which is how they are often harvested commercially or for market. If intended for quicker consumption, it may be the whole head cut just above the roots, and then the roots will need to be disposed of separately. When this is done, it is common to have to cut or remove some of the bottom leaves near the base of the plant that may not be suitable for eating either because of appearance (for commercial growers) or it they are too limp or damaged for use.
Harvesting the whole head with the roots is referred to as a living harvest. There are many benefits to this harvest method if the lettuce is not intended for immediate consumption. The harvests are cleaner because there is no soil. If the roots are too long, they can be trimmed or wrapped around the stem of the head. Leaving the roots intact allows for a much longer storage period. This can be as long as four weeks when stored in appropriate temperature and humidity conditions. The reason is likely self-evident; the roots will continue to feed the lettuce plant for an extended period, thereby extending its shelf life.
The whole-head lettuce harvesting method is preferred by commercial or market growers because it is more efficient than leaf by leaf harvests and the heads stay fresher for longer. Less time and labor are needed when harvesting lettuce by the head too. In some cases, though, leaf lettuce can be more profitable to harvest, even though it takes more resources to harvest.
Harvesting Leaf Lettuce
Most hydroponic lettuce growers raise some type of loose-leaf variety and leave whole-head production for the field. The benefits to growing leaf lettuce are many. You can harvest the same plant repeatedly until it has spent all its stored energy. Leaf-type lettuces are harvestable sooner than head-types and can be harvested over longer periods of time. They also typically take up less room in a hydroponic system than head-type lettuces, so more volume or more variety can be grown at one time.
This method of harvesting keeps the plant alive and encourages continued production of harvestable leaves. The how-to of harvesting hydroponic leaf lettuce is very easy. It can be started as soon as there are mature leaves that are large enough to harvest. It is generally considered best practice to harvest the older (bottom) leaves first. Shears are better for harvesting the plant versus pinching off with fingers. This will reduce the amount of surface area on the plant that needs to heal and the amount of energy the plant needs to devote to it instead of towards pushing out new growth. It is a wise idea to sterilize your pruning shears between harvests and, if there is any evidence of plant disease, to do so between crops or even plants.
Never harvest more than half to three-quarters of any one plant in a single harvest, so that there are still leaves left to photosynthesize. This can be done every few days until the plant sends up a flower stem and begins to form a flower. This is called bolting. It is a sign your plant is done with being harvested and is devoting its remaining energies to setting seed to carry its genetics onto the next generation. In most species of lettuces, leaves develop a bitter taste once the plant begins to bolt. It is best to remove these plants from the hydro system and start anew. Lettuce is a cool-weather crop and as such will bolt when the temperature gets too high or the hours of daylight are too long. It does best in temperatures below 70°F (21°C). To slow down the onset of bolting, a shade cloth should be put up if grown in sunny areas or the grow lights should be adjusted if grown using artificial lighting.
Eventually, all lettuce plants that are not harvested using the whole-head method will bolt, but by reducing the amount of light shining on your crop, it will slow it down to prolong harvests.
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For continual loose-leaf harvests, several plants should be seeded at regular intervals. The amount and frequency will be dictated by space constraints as well as how much lettuce is wanted. However, it is possible to harvest fresh lettuce leaves every week by seeding new plants every two to four weeks and alternate the harvest on the mature plants.
Another method of harvesting leaf lettuce is similar to head lettuce in that the entire plant is harvested, but unlike head lettuce, the roots remain in the hydro system. Not all types of lettuce are suitable for this type of harvest, but many are. Butterhead and romaine types do typically grow back well after the initial harvest, but many other varieties also do well. Those that do are sometimes referred to as cut-and-come again lettuces. With this method, the lettuce is cut down to about one inch above the roots. So long as the root system is healthy, and the hydro system is circulating enough nutrients, the plant will regenerate.
Cut-and-come again types will have a second full flush of leaves, though not always quite as full as before the first harvest. Once they have achieved harvestable size, they can be cut again, down to about one inch. A third flush of growth should follow. It should be expected to not be as full as the first or even the second but should produce enough leaves of sufficient size for a third harvest. Not many types will generate enough for a fourth or even fifth harvest, and it does not always make fiscal sense to keep nurturing them along past their prime when younger plants will provide greater harvests.
To successfully employ this method of harvesting and to have a continual supply of lettuce, it is necessary for staggered seeding and harvests, just like with the loose-leaf harvests. Unlike the leaf-by-leaf harvest which can be done every couple of days, the cut-and-come again method requires at least a week or two in between harvests depending on variety.
Regardless of harvesting method for leaf lettuce, it will need to be consumed within a few days of harvest before it wilts and is unusable. Because so much of the makeup of lettuce is water, it is not possible to freeze it successfully and long-term storage is not an option either. Freshly harvested leaf lettuce can be stored in a refrigerator for one to two weeks on average. If it starts to get limp soon after harvest or while being stored, it can be rinsed with ice water or submerged in cold water. Then, the excess water should be shaken off and the lettuce leaves can be placed in a resealable bag until they are ready to be taken to market or consumed.
Best Varieties for Hydroponic Harvests
Varieties to choose for hydroponic growing are overwhelmingly loose-leaf types. There are dozens of different species with ruffled leaves, smooth leaves, round leaves, lobed leaves (oakleaf types), and many other shapes. Colors vary widely as well, spanning almost the full spectrum of the rainbow.
Lettuces are not the only greens that can be grown hydroponically. There are dozens of other types of salad green plants that can be grown hydroponically with varying degrees of success and varying skill levels. Consider greens like kale, collards, arugula, spinach, mizuna, or Swiss chard for variety. Microgreens can also be grown and harvested hydroponically. They can be harvested by cutting stems, or taken out in clumps, or roots and all like head lettuce.
Several herb varieties are appropriate for hydroponic production as well. Some growers will seed a variety of greens and herbs in the same block or cluster and harvest these bunches as one clump, providing great variety for consumers as well as for home-consumption. Whatever type you choose, lettuces are an easy crop to grow and harvest in a hydroponic system.
Written by Chris Bond | Certified Permaculture Designer, Nursery Technician, Nursery Professional
Chris Bond’s research interests are with sustainable agriculture, biological pest control, and alternative growing methods. He is a certified permaculture designer and certified nursery technician in Ohio and a certified nursery professional in New York, where he got his start in growing.