Growing Strawberries Indoors

Should You Be Producing Strawberries Indoors?

Strawberries are a crop for which there is an annual hole in production. Grocers and restaurants often can’t get enough high-quality strawberries in November, December, and January (or June, July, and August if you live Down Under). There aren’t a lot of cultivars of strawberries that set fruit in the waning days of summer because of their daylight requirements. Local field-grown fruit suffer unpredictable and deteriorating weather, and an early frost cancels out a late crop. Consumers pay a premium for strawberries flown in from the other side of the world. Greenhouse-grown strawberries offer superior sugar levels, color, and flavor in cold-weather months, particularly if they can be produced locally — that is, at least on the same continent on which they are consumed.

But greenhouse production of strawberries not only fills consumer demand, it also helps growers maintain their cash flow. If you already have greenhouses, strawberry production is a logical way to increase your revenues and lengthen your season. But before you take the plunge and devote a greenhouse you already have set up to strawberry production, there are some particularly important considerations that aren’t obvious to most new growers. We’ll tell you about them in this article.

Let’s start by reviewing a very obvious principle.

Indoor production is more predictable than field production.

Growing plants indoors gives you an opportunity to control light and temperature. You will not eliminate pests and diseases, but your efforts will be limited and concentrated so that you will get better results with non-toxic approaches. You can consider ergonomics in setting up your growing trays so harvest no longer requires back breaking labor. You can eliminate losses caused by drought and freezes.

You don’t have to invest in hydroponics to grow strawberries indoors, but hydroponic production is usually more cost effective — if you are fully informed of variations in nutrient requirements during the life cycle of your plant. You can deliver the exact nutrients your plants need at the precise times they need them. Just keep two general rules in mind.

The first rule of profitable hydroponic greenhouse production of strawberries is…

Start where you are.

Most growers of strawberries in greenhouse will use buckets or a gutter system specifically designed for strawberry production, but strawberries don’t require containers specifically designed for them. You can grow strawberries in hanging baskets. If you are already growing perennial plants in 3-liter pressed fit pots, you can grow really nice strawberries in the same system.

Your greenhouse doesn’t have to be kept at tropical temperatures. Strawberries like it cool. It’s OK to keep strawberries at 45° F/ 7° C at night. They prefer a temperature around 75° F/ 23° C during the day. A minimum-heat house is good enough.

More control over temperature is always better. You can always improve temperature control. But if minimum-heat is what you have, there’s no reason not to try strawberries. Cool nighttime temperatures while your plants are fruiting maintains fruit quality.

In fact, some nighttime cooling is essential to strawberry quality. As University of Arizona strawberry specialist Mark Kroggel puts it,

“If greenhouses cannot be cooled to 59ºF or lower at night, fruit quality is going to be drastically affected. Primarily the acidity will be too high, the Brix (sugar content) will be too low and the surface of the fruit will be off. The strawberry starts to get mealy or soft. The texture, sweetness and acidity are all affected by the temperature.”

It is also important to provide strawberries with the right amount of light. Strawberries need light to make sugar. The rule of thumb is that strawberries need minimum daily light integral (DLI) of 12 moles per square meter per day.

The concept of a daily light integral or DLI is like a rain gauge, only for sunlight. The intensity of sunlight varies from time to time during the day. That’s why a measurement of overhead light in lux, like the lux put out by a sun lamp, isn’t useful for measuring light provided to plants. Foot-candles that are used to measure the adequacy of illumination for people aren’t helpful for measuring light for strawberries.

DLI measures the amount of chlorophyll (in moles) that is activated by photosynthetically active radiation (sunlight in the range of 400 to 700 nanometers, which roughly corresponds to visible light, although not all visible light activates chlorophyll) in a 24-hour period. There are meters that give you a 24-hour reading and also an instant reading if your plants get the same level of light all day every day.

If you live south or north of about 35° N latitude, your strawberries will get all the light they need on a sunny day even in the middle of winter. On the other hand, 30 moles per square meter per day and higher is too much light. In the Southwestern United States, your plants will start getting too much light about the middle of March, and you will have to shade them. The ability to measure light and to provide supplemental light or shade as necessary is essential to good production.

So could you grow strawberries with existing light in your greenhouse? Chances are that you won’t have to make major modifications to take on this new cash crop, Most growers who already have greenhouses will be able to provide light and temperature control, or at least light and temperature measurement that allows them to compensate for limitaitons in their existing structures, for good strawberry production. But there are other considerations.

And the second rule of profitable hydroponic greenhouse of production of strawberries is…

Never stop learning about your plants.

Be ready for predictable decisions facing all indoor strawberry producers. You need to know which cultivars to plant. You will need to know how to plant them. You need to know how to feed them. You will have to deal with pests and diseases. And you will need to know when to harvest your fruit.

One article can’t tell you everything you need to know about indoor strawberry production, but we can at least make sure you know the right questions to be asking. Let’s start with selecting the cultivars of strawberries you need to plant.

Which cultivars do you plant?

For beginning indoor strawberry producers, the best advice about choosing cultivars is to try as many different cultivars as possible.

Growers have a fantastic range of cultivars from which to choose to plant. But when you are growing strawberries in a greenhouse, the best cultivar for the highest yielding, highest quality crop really is a local question. Growers have to experiment with cultivars to know what grows best.

A good place to start is with some of the cultivars that are standards for field production in mild, predictable California climates, such as Albion, Portola, and San Andreas. These are ever-bearing types well suited to year-round production, but all types don’t work equally well in all conditions. (It’s OK not to use ever-bearing varieties, but you need to understand that they won’t set fruit in late summer for early fall production because the hours of daylight won’t be right.)

If you live in a location that has naturally low humidity, Albion will help you avoid problems with tip burn. Albion produces good Brix numbers and reliable fruit quality. For most growers, the yield from Albion won’t be as high as it is with Portola and San Andreas, but it will be good enough to turn a profit.

Portola is the new standard for field production in California, and it is sold out fast from nurseries even in other parts of the country. But it is such a powerful growing plant that it is hard to maintain for optimal yields on hydroponic nourishment. The problem is that this cultivar grows a little too well in hydroponic conditions.

The Portola’s root system is constantly pumping water and nutrients into the plant, keeping it vigorous and giving growers fantastic yields, but the high rate of growth fills the fruit with just a little too much water. It’s hard to keep the fruit as sweet as consumers would like it.

If you don’t use overhead lighting, you can try a variety called Camino Real for short-day, winter production. Be forewarned that this cultivar is extremely sensitive to dry air, and subject to tip burn when humidity levels are low.

When you are starting your greenhouse production, you will want to try a number of cultivars. There is no single cultivar that works best in every greenhouse. What works in your neighbor’s greenhouse may not work in yours. Keep exact records of temperature, humidity, and nutrient mix, and always plant more than one cultivar to make sure you have a crop even with unusual conditions.

What is tip burn?

Tip burn is a calcium deficiency that occurs when humidity levels are low. In dry air, the leaves of the strawberry plant are constantly releasing water vapor into greenhouse air. They are unable to pump calcium into the edges or their leaves and these edges appeared burned.

But it isn’t the damage to leaves of the plants that is the marketability problem caused by tip burn. The calyx of leaves around the strawberry is also burned. You have a beautiful fruit crowned with dead leaves that makes the strawberry unmarketable.

You may be able to fine-tune your fertilizer to prevent tip burn but doing this successfully requires small adjustments made on an almost hourly basis. There is a tradeoff between your time and the number of berries you have to discard because of tip burn.

What are appropriate containers for growing strawberry plants?

We have already covered the concept of going with what you have for containers for growing strawberries. But sooner or later you will probably grow strawberries in a gutter system.

You can think of the gutter growing method as a protective cocoon for your plants. The gutter itself is pressed Styrofoam. It’s light but strong enough to support plants. The Styrofoam also keeps the growing medium that substitutes for soil at a more nearly constant temperature.

Into the gutter goes a plastic liner to protect the Styrofoam gutter from the nutrient solution. Over the plastic liner, there is a drainage pipe. This protects plants from becoming waterlogged, but it also gives you a way to see how much of your nutrient mix is being taken up by your plants. You can test the water draining away from your plants as well as the water into which you mix nutrients.

Over the drainage pipe goes a plastic grid. The plastic grid allows air to circulate below the roots of the strawberries. And over the wire mesh you place weed cloth. This keeps strawberry roots from growing into their feeding pipes and clogging them.

Onto the weed cloth go your plants. You will probably want to grow them out in small pots before you plant them in the gutter. This allows you to weed out defective plants. (You could also plant dormant runners or fully grown plants.) Place the plants against the edge of the gutter so their foliage and flowers will hang over the side. This keeps fruit from coming in contact with moisture. and gives the plants the benefits of more light exposure and air movement. Alternate plants on either side so there is a plant about every 6 inches (25 cm) on either side of the gutter.

Once you have set out your plants, you will fill them in with growing medium. Chances are that you will want to use some combination of coconut core, peat moss, and perlite. Coconut core is an excellent absorbent for nutrients for slow release to the plant, but its pH is a little too high, so it can be mixed with peat moss. A 1:1 ratio of coconut core and peat moss keeps pH of the growing medium between 5.5 and 6.5.

The next step is to water in the substrate, using tap water. Wetting the growing medium allows you to find places where you need to add more to fill out the trough, although it is important not to bury the crown of any plant under the soil. The plant is watered from overhead while it is settling in. At this stage, the plants are not stressed by giving them nutrient

After two or three days of overhead watering, the next step is laying irrigation line over the growing medium in the trough between the plants. The line is laid down the middle of the gutter, on top of the growing medium, with one emitter per plant, typically 2 liter per hour emitters. The emitter needs to be near the plant but pointed up so roots do not grow up from the soil into the emitter.

The final step of preparing the gutter is laying white-on-black plastic cloth over the growing medium. This protective layer keeps fruit off the bare moist growing medium so it does not degrade in quality. The black side of the cloth keeps algae from growing on top of the growing medium. The white side of the cloth reflects sunlight up and out to the leaves of the plant.

Most greenhouse growers of strawberries use the gutter system, but there is also a container system for growing strawberries known as the Dutch or Bato Bucket. We will discuss that method in detail in a future article.

What changes in nutrient medium affect the quality of my strawberries?

We have discussed this topic in much greater depth in Changes in Hydroponic Water Additives That Enhance Fruiting and Flavor of Strawberries but if you’re considering getting into hydroponic strawberry production, here are some of the kinds of concepts you need to consider

  • Strawberries need calcium to mature, but calcium cannot be mixed with phosphates and sulfates. If you store the chemicals together, they will precipitate down into the bottom of the barrel where they can never reach your plants.
  • Strawberries need more nitrogen when they are growing leaves. You can add more nitrogen to their nutrient medium, but you will have to lower its electroconductivity level to make sure their roots can absorb it.
  • Strawberries need more potassium when they are setting fruit to keep sugars forming in the fruit from flowing back into the plant. But you will have to raise the electroconductivity level of the nutrient solution to make sure the potassium is absorbed through their roots.
  • Strawberries have to have sulfur in their nutrient medium to attain their distinctive "strawberry" aroma.
  • Silicon helps strawberries absorb other micronutrients.
  • Boron and molybdenum are critical for the formation of strawberry flavor compounds.

And there is no way you can be sure your plants are getting all of these nutrients if you are unable to maintain a constant pH of your growing medium and your nutrient solution. You don’t have to be a chemist to get nutrient chemicals right, but creating the right nutrient blend for your irrigation water will take careful planning and frequent monitoring.

What kinds of pest control problems are there in greenhouse production of strawberries?

The good news about greenhouse production of strawberries is that you can grow strawberries with toxic chemicals. But if you don’t use agrichemicals, you will have to pursue integrated pest management, controlling insects with insects.

If you grow strawberries indoors, you are almost guaranteed to get spider mites. Spider mites tend to go dormant in winter production, but they will a major a problem during the warmest periods of indoor production. One of the ways you can control this pest is by introducing predator mites. One of the predator mites, californicus, can be put on the crop before you see any evidence of spider mites, because that particular mite can survive on the pollen on the flower. Spider mites always breed faster than predator mites, so if you see disease on your leaves, it’s best to use a more aggressive predator mite such as persimilis. The persimilis is an aggressive feeder on spider mites, but if you put in on your plants before spider mites arrive, the persimilis mites will feed on each other.

Release two or three californicus predator mites per plant if you don’t see any evidence of spider mites, but five persimilis predator mites per plant if there is evidence of spider mite damage.

Spider mites can also be controlled with applications of Safer Soap every three days. Repeated applications of this safe miticide are necessary because it only kills adult mites. You will want to use Safer Soap only on those plants with severe infestation because it ruins the taste of the fruit.

The confusing nomenclature of predator mites.

Predator mites are referred to by their species names, for example, californicus and persimilis, but they are listed in catalogs by their full scientific names

  • Phytoseiulus persimilis is the scientific name for persimilis mites.
  • Neoseiulus (Amblyseius) californicus is the scientific name for californicus mites.
  • There are also Galendromus occidentalis and Neoseiulus fallacis for spider mite control.

Lacewings, big-eyed bugs, damsel bugs, and sixspotted thrips are also natural enemies of spider mites, but they are not typically used in greenhouses for strawberry production.

Whiteflies are not particularly fond of strawberries, but when they appear in your greenhouse they can be kept under control with Eretmocerus wasps. Use yellow sticky cards to monitor the whitefly population in your greenhouse. As soon as you detect white flies, release the wasps.

Thrips are common in indoor production of strawberries, but they do not do a lot of damage to the fruit or to the plant. If you want to treat them, you can release miniature parrot bugs (Oreus, such as those in the commercial product Oriline). Parrot bugs will eat everything that isn’t bigger than they are, so they are useful to have in your crop.

Learn more about keeping birds, bugs, and other pests away from your strawberries.

What kinds of diseases affect indoor strawberry crops?

In conditions of controlled humidity, you won’t have a lot of problems with bacterial and fungal disease. It may be possible to control powdery mildew just by removing affected leaves and treating the plants with potassium silicate salts.

A problem that can sneak up on you even in controlled conditions is botrytis, also known as fruit rot. A tiny spot appears on a young fruit. The botrytis fungus will go dormant and the spot won’t be noticeable until the strawberry begins to accumulate sugar. Then the fungus will rapidly multiply, covering the fruit with “fur,” and creating spores that can land on nearby plants.

The key to disease control is starting with a clean growing space, only bringing in disease-free plants, and maintaining the sanitation of your house. You may want to establish multiple, smaller growing spaces rather than a single, large growing space to keep your crop pest- and disease-free.

When do you harvest your fruit?

Understanding when to harvest your strawberries is an essential factor in marketing your crop. The tie to harvest strawberries depends on their cultivar. For instance, if you grow the cultivar Albion, you are looking for the berry to be almost entirely ruby red, with a little bit of whiteness on the shoulders. You will never get a completely red fruit, but 90 to 95 percent of the berry will be obviously ripe.

If you grow the cultivar Portola, the fruit will start out with an orangey color. It won’t turn a deep red like Albion, but it does redden up. You will be looking for the oranginess to go away to be replaced with a lighter, pinker red to tell you it is time to pic those fruit

Some cultivars will let you leave the fruit on the plant a little longer for a higher sugar level. Some cultivars have to be harvested. Color is a reliable criterion for harvest under greenhouse conditions, but one of the peculiarities of greenhouse production is that fruit may be completely ripe with white shoulders. You can use a Brix refractometer to help you choose fruit at the peak of its sugar content, but you will always need to record your experience with your cultivars in your greenhouse to choose the perfect time for harvest.

Final Words

Hydroponic operations in greenhouses shield strawberry growers from losses due to heat, cold, drought, birds, insects, and wind-borne diseases. They give growers a chance to manage a single planting so that it yields crops every other week for up to six months. With the latest understanding of physiologocial needs of their plants hydroponic growers can easily produce larger crops of strawberries. Producers can grow strawberries of high-quality when field crops are not available.

Indoor production of strawberries is the wave of the future, and hydroponics are the surest way to deliver the nutrients strawberries need. Make sure you are prepared for the technical challenges of raising strawberries with the information on this site, but start with what you have.

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