How to Grow Healthy Plants in Hydroponic Systems

How to Grow Healthy Plants in Hydroponic Systems

Hydroponics is a type of controlled-environment agriculture (CEA) that feeds plants through water rather than soil. Unlike some field crops that can go long stretches with little care, hydroponic crops need attention every day.

Below are straightforward steps for starting seeds hydroponically, followed by an overview of the key environmental factors to watch closely for healthy seedlings.

10 Steps for Successful Hydroponic Seed-Starting

Good seed-starting practice is much the same whether you grow in water or soil. See our indoor seed-starting guide for tips and troubleshooting. Beyond that, these steps will help you raise healthy hydroponic seedlings:

Some crops and varieties are better suited to hydroponics than others. For ideas, look through our hydroponic-friendly varieties and our hydroponic head lettuce selection

1. Choose varieties. Some crops and varieties are better suited to hydroponics than others. For ideas, look through our hydroponic-friendly varieties and our hydroponic head lettuce selection.
2. Pick a medium. Options include flats of soilless mix, germination plugs or other media. See our guide to hydroponic grow media.
3. Moisten the medium Water or soak the medium until it's evenly moist throughout with no dry spots, adjusting the water's pH to your target if needed. For more on media, see our guide to hydroponic grow media.
4. Sow the seeds. Use the seeding depth and spacing recommended for each variety.
5. Keep seeds moist until they sprout. Hold in moisture by covering seeds with a thin layer of vermiculite, a clear humidity dome, or both. A dome traps heat, so watch that it doesn't get too warm underneath, and take it off as soon as seeds germinate.
6. Water often with plain water. Seeds must stay moist while germinating. Water gently by hand, mist from above, or use subirrigation (bottom watering). Hold off on full-strength hydroponic nutrients until after germination; the salts can make it harder for seeds to absorb water and get going, and can weaken early root growth.
7. Keep germination temperatures in range. Raise or lower temperatures as needed. A heat mat helps germination come up more evenly.
8. Feed correctly. After seedlings emerge, water with a diluted hydroponic nutrient solution, following the label.
9. Add light as needed. A few crops need light to germinate, though most don't. After germination, light needs depend on the crop, your setup and the environment.
10. Transplant on time. Seedlings are usually ready to transplant once they have true leaves and, in propagation plugs, roots showing through the sides. Hydroponic plants don't compete for nutrients, so they can usually be set closer together than in soil.

What to Monitor & Why: The Primary Environmental Variables of Hydroponic Systems

Monitoring Tools

A few monitoring tools are worth keeping on hand. Most are inexpensive and easy to find at hydroponic suppliers.

• A thermometer
• EC (electrical conductivity) meter
• A light meter
• pH test strips or kit

Temperature

Temperatures that are too high or too low lead to poor germination and more disease. The best range depends on the crop and its growth stage, so follow the growing information for each crop.

After establishment, many crops grow best when nights are about 10°F (roughly 6°C) cooler than days, for example around 75°F (24°C) by day and 60–65°F (16–18°C) at night. Above about 85–90°F (29–32°C), photosynthesis slows in most non-tropical crops.

In protected culture, supplemental lights can raise air temperature noticeably. To protect roots, keep the nutrient solution no warmer than the surrounding air.

pH

Tap and well water are often neutral to slightly alkaline. Most nutrient solutions are acidic and lower the pH once mixed in.

Solution pH strongly influences how well plants take up nutrients. For most crops and systems, aim for 5.8–6.2 (a little higher in organic and aquaponic systems).

You can measure pH with:

• a digital pH meter
• litmus test strips
• liquid indicator drops

To learn how pH shifts in your own setup, consider checking it regularly in:

• your source water
• the mixed nutrient solution
• the growing medium after adding nutrient solution

If pH is out of range, correct it with a commercial pH adjuster ("pH Up" or "pH Down").

With regular checks you'll notice pH creeping up as plants use nutrients, then dropping when you top up the solution. Sudden or large swings can signal a problem such as root rot or algae growth, so investigate if you see them.

Light

• DLI: Many growers, especially in protected culture, measure light as the daily light integral (DLI), the total photosynthetically active radiation (photons) plants get in a day. Much as a rain gauge totals rainfall, a light meter can total DLI, usually given in moles of photons (mol) per square meter (m²) per day.
• Light targets: Most vegetables need about 14 hours of light a day and at least 12 mol per m² per day. Greenhouse hydroponic strawberries do best at roughly 15–25 mol, with 12 as a minimum, measured at the canopy. Glazing and framing typically cut greenhouse DLI by a quarter to a half compared with outdoors, so depending on the crop, season and location, you may need supplemental light for part or all of the year.
• Spectrum: With supplemental lighting, color matters too. Plants use visible light from 400 to 700 nanometers (nm). Herbs and leafy greens respond well to lights richer in blue (around 450–495 nm), which favors leafy growth. Tomatoes and other fruiting crops benefit from more red (around 620–700 nm), which supports flowering and fruiting. UV light helps color develop in fruits such as strawberries. LEDs are the most energy-efficient choice, and some let you tune the spectrum to the crop.

Hydroponic Nutrient Solutions & Concentrations

Home and beginning growers should start with a ready-made nutrient solution; each comes with its own mixing and feeding directions.

To track nutrient strength, measure the electroconductivity (EC) of your solution over time. An EC meter reads total dissolved nutrients, expressed in millisiemens per centimeter (mS/cm). The label will list target EC levels. EC isn't a dependable measure with organic fertilizer solutions, however.

Note the EC when you mix the solution, then check it daily. EC falls as plants feed; when it drops below your target range, add nutrient solution in proportion to how far it has fallen.

Taking readings from both the solution and spots in the growing medium can show whether nutrients are building up in the medium.

• Nutrient targets: The best EC depends on the crop. Plants generally need lower EC in warm weather and higher EC in cool weather and during fruiting. Table 2 gives starting ranges, but expect to experiment to find what works for your crop, season and system.
• Changing the solution: EC shows total nutrient strength, not which nutrients are present. Plants use nutrients at different rates, so the solution can drift out of balance even while EC looks fine. That's why the whole solution should be replaced regularly. Advice varies; a good starting point is every 3–4 weeks, or sooner if plants show deficiency or toxicity symptoms.
• Organic fertilizers: Organic fertilizers are harder to manage in hydroponics than synthetic ones. Their makeup is less exact, so deficiencies show up more easily. They can also be high in carbon, which in excess can feed fungi and bacteria. And since EC doesn't reliably reflect nutrient levels with organic solutions, monitoring is harder. If you go organic, choose a product made for hydroponics unless you can run your own trials.

Water

Testing your water shows how its natural minerals might affect your plants. With hard water (high in calcium and magnesium), consider a nutrient solution formulated for it. Water softened with sodium or other softening chemicals can harm plants, and high salt levels can limit calcium uptake, leading to disorders and lower yields.

Carbon Dioxide

Plants need enough carbon dioxide (CO₂) for photosynthesis; when CO₂ runs low, growth slows and flowers and fruit may drop, cutting yields.

Indoor spaces can run short of CO₂ , especially a closed greenhouse on a cold, sunny winter morning when fans are off and plants are busy photosynthesizing and drawing down CO₂. In a sealed greenhouse, plants can use up the available CO₂ surprisingly quickly.

Good ventilation keeps CO₂ at adequate levels.

Air Circulation

Good airflow lowers disease pressure, evens out hot and cold spots, and, as noted above, keeps CO₂levels up. Moving air also helps seedlings build thicker stems for shorter, sturdier, less leggy plants, and can reduce tipburn in leafy crops.

Oxygen Levels

Roots need oxygen. Overwatering and compacted media both starve roots of oxygen and can kill them, so use a medium that stays well aerated.

Warm solution holds less dissolved oxygen, so a nutrient solution that's too warm limits what roots can get. Some growers aerate the solution with an air pump and use a chiller to keep it at a good temperature.