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Do Greenhouse Plants Need More Water? Let’s find out.

Greenhouse gardening has, over the past number of years, becoming extremely popular. Being able to grow plants and fresh produce on your doorstep has, in some ways, revolutionized the food industry.

A contentious issue in greenhouse gardening is its water usage. Some people believe that greenhouse plants need more water. But do they?

Greenhouse plants use less water for four primary reasons.

  • Because of less wind, regulated temperature, and increased humidity, plants, through evapotranspiration, lose less water.
  • Drip irrigation systems improve water usage efficiency.
  • The spacing of crops reduces water wastage.
  • Greenhouse crop cycles are shorter than open-air growing, resulting in less water used over the entire growing season.

As a greenhouse gardener, you exercise specific control over nature where plants are concerned.  Understanding your plant’s needs and knowing when to water them will optimize your water usage.

How often should I water my plants in the greenhouse?

  • A lot will depend on the type of plants you grow as each plant has different needs.
  • Herbs need watering every few days provided the greenhouse is not too hot.
  • Container plants, in hot sunny weather, need watering daily. In contrast, mature shrubs might only need water in extreme drought.
  • As a general rule, wait until the first couple of inches of soil are dry before you water.

How to water greenhouse plants?

  • When it’s hot and sunny in summer, water and hosepipe restrictions are in force; water only when you need to.
  • Watering plants in the morning discourages both fungal growths like mildew and insects like slugs.
  • Morning watering allows both the plants and compost surface to dry out faster. But remember, even in hot conditions, it is easy to overwater plants.
  • Check to see if any of the plants have any leaf adaptations, such as silvery foliage. If they do, this means they can retain water for longer and don’t need watering.
  • Check if any of the plants are wilting or swollen. If they are, they’ll need watering.
  • Also, another indication the plant needs water is when the compost is dry.  
  • Water a greenhouse plant at the roots and try to avoid getting the leaves wet.
  • Thoroughly watering a plant means it needs water less frequently than slightly wetting it each day.
  • Give plants the equivalent amount of water to 10% of their container volume (for example, ½ Lt of water for a 5 Lt container).
  • For the compost to absorb the water, you need to pour the water slowly rather than letting the water run out over the top of the pot.
  • Place saucers under containers to catch any excess water and leave it to be absorbed by the plant when it’s ready.
  • It is better for the compost to dry out between watering than to stay wet and soggy.

Here’s a helpful video with some tips for watering in a greenhouse:

Seven factors that affect water usage in a greenhouse

These seven factors can decrease or increase the amount of water needed in a greenhouse.

1. Solar radiation. 

Because of glazing materials used in greenhouses, the rate of transpiration is reduced, reducing radiation levels reaching the plants by up to 40 percent.

2. Shading. 

Adding shade to both the inside and outside of the greenhouse will reduce the radiation level on the plants.

Also, depending on the shade provided, the evapotranspiration of moisture will decrease, as will the plants’ water needs.

3. Air movement. 

Horizontal-airflow systems and fan ventilation increase evapotranspiration.

A slight, five mph breeze can increase evapotranspiration by up to 20 percent.

The location of the greenhouse can also influence evapotranspiration.

4. Size and types of plants. 

Small potted plants and seedlings require less water than a full-grown crop. And heavy leaf canopy plants increase water needs.

5. Types of irrigation systems. 

Only twenty percent of the irrigation water applied with an overhead sprinkler system reaches a potted plant crop with heavy foliage.

Flooded floors, ebb-and-flood systems, and hydroponics conserve water by reusing and recycling excess water.

6. Leaching. 

It’s recommended that at least 10 percent of the water applied to greenhouse plants be allowed to filter out.  Leaching removes excess fertilizer salts and increases water usage significantly.

The growing mix also affects the watering frequency and the amount of water-holding capacity a plant has.

7. Other factors that affect water usage. 

In addition to plant needs, water is used for pesticide application, evaporative cooling, and clean-up.  

Here’s another video with some tips about watering plants:

Eight questions about greenhouse plants and water usage

1. How often should I mist my greenhouse?

Most misting systems come with electronic timers that can be programmed to mist between one and fourteen times a day. Each misting session can range between 1 minute and 24 minutes. Some misting systems can be programmed to mist for shorter periods but more often.  Misting system nozzles are usually made from brass or plastic material.

2. What is the best time to water a greenhouse?

As with plants grown in a garden, the best time to water a greenhouse plant is first thing in the morning giving, allowing them time to dry out.

Watering plants in the evening will reduce evaporation. However, late afternoon watering means your plants aren’t totally dry before sunset.

3. How much water does a greenhouse use?

A greenhouse peak-use rate on a hot day is 0.3 to 0.4 gallons of water per square foot of growing area.

For example, a greenhouse with 2400square feet of bench area would require a peak-use rate of 720-960 gallons of water per day.

4. How long can plants go without water in a greenhouse?

Most houseplants will survive without water for two to three weeks.

However, plant watering needs will vary depending on the types of plants you have and the weather.

5. What are the most reliable methods of watering greenhouse plants?

  • For medium-sized structures, like hobby greenhouses, the best irrigation is drip tubing.
  • Place one drip tube into each potted plant and connect each tube to the main water supply for constant moisture.
  • When the plants need watering, turn the main supply on and fill the tubes. 
  • When the tubes are full, they’ll emit a fine spray directly at the pots’ topsoil.

6. Do plants grow faster in a greenhouse?

Greenhouse plants grow faster for two reasons.

  • You can control the temperature in a greenhouse ecosystem.
  • And the carbon dioxide content in a greenhouse is higher as compared to the outdoors.

Both these factors contribute to plant growth, making greenhouse plants grow faster.

7. Should I water my greenhouse every day?

  • No rule that says one must water a greenhouse plant every day as every plant has different needs.
  • Some may need watering every day, while others may not.
  • Whatever plant you are cultivating, please do some research regarding its water needs. If you do this, you can’t go wrong and over water or underwater. 

8. Why are my plants wilting in the greenhouse?

  • The lack of sunlight and plant disease will cause leaves to wilt or turn yellow.
  • If the greenhouse is in the sun for long periods and watering schedules are not adhered to, plants will wilt.
  • Poor drainage and too much fertilizer can cause greenhouse plants to wilt.
  • Greenhouse plants will wilt if they receive insufficient light.
  • If you give plants too much water, they’ll wilt.
  • Plants that are overfed but don’t have adequate drainage will wilt.

Final thoughts on – do greenhouse plants need more water?

  • If managed correctly, greenhouse plants will use less water.
  • Because a greenhouse is a controlled environment, the gardener has control over water usage.
  • To optimize water usage, install a greenhouse irrigation system.
  • Remember, overwatering and excessive moisture can lead to fungal diseases and create an environment for certain pests to thrive.
  • Plants with similar water needs need to be grouped.
  • Don’t use an overhead watering system if you have different plants in your greenhouse. Instead, save on water costs by installing a collection tank for recycled water.
  • For small and hobby-sized greenhouses, use a drip system. Drip systems provide the precise amount of water needed each day throughout the year.
  • Remember, not all plants need the same amount of water.

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