Using Fertiliser During Winter Dormancy in Hydroponics

Hydroponic growers often assume that because plants are grown without soil, fertiliser should be applied year-round. In reality, this can be a serious mistake; especially when growing perennial crops that require winter dormancy.

In this blog it explains why fertiliser use during cold or dormant periods can actively damage plants, even in hydroponic systems, and how nitrogen chemistry plays a critical role.

Dormancy Still Matters in Hydroponics

If you are growing perennial crops such as bonsai, overwintered chillies, or mother plants, dormancy still applies, even in hydroponics.

During winter or cold storage:

• Plant metabolic activity drops sharply
• Root nutrient uptake slows to near zero
• Growth is intentionally paused

This means fertiliser applied during dormancy is not used by the plant and is either wasted or becomes harmful.

Why Fertiliser Is Wasted in Cold Conditions

Plants in dormancy:

• Take up very little water
• Take up virtually no nutrients

This is why dormant trees are often sold bare-root, without soil or fertiliser. In hydroponics, continuing to feed during dormancy simply leads to nutrient runoff and salt accumulation, but the bigger problem is chemical, not economic.

The Real Danger: Toxic Nitrite Accumulation

The most important reason to stop fertilising during cold periods is the risk of toxic nitrite (NO₂⁻) formation.

Understanding nitrogen forms

Nitrogen typically exists in three forms:

• Ammonium (NH₄⁺)
• Nitrite (NO₂⁻)
• Nitrate (NO₃⁻)

In healthy, warm systems, microbial activity rapidly converts ammonium → nitrite → nitrate.
However, cold temperatures inhibit the bacteria responsible for the final step.

What happens in winter

• Ammonium can still convert into nitrite
• Conversion of nitrite into nitrate slows or stops
• Nitrites accumulate

Nitrites are toxic to plants, even at relatively low concentrations.

How Nitrites Damage Plants

Nitrites enter plant roots via nitrate transporters and cause multiple forms of damage:

• Disrupt cellular respiration by interfering with mitochondria
• Increase oxidative stress, overwhelming plant antioxidant systems
• Inhibit key enzymes
• Reduce chlorophyll synthesis
• Suppress photosynthesis when plants exit dormancy
• Inhibit seed germination

The damage may not be immediately obvious. Plants often decline slowly, leading growers to incorrectly apply more fertiliser, making the problem worse.

Increased Risk of Root Disease

Nitrites and excess nitrogen also weaken root tissues, making plants more susceptible to:

• Pythium
• Rhizoctonia
• Cold-related root rots

This is particularly common in overwintered chillies, mother plants, and other sensitive perennials.

Human Health Context (Brief but Relevant)

Nitrites are not just harmful to plants. They are also toxic to humans in excess, which is why their use as preservatives (for example in processed meats) is tightly regulated.

Avoiding nitrites is good practice; for crops and consumers alike.

Practical Winter Feeding Advice for Hydroponic Growers

During winter or dormancy:

• Reduce fertiliser to near zero
• Avoid ammonium-based nitrogen entirely
• Minimise all nitrogen inputs where possible
• Keep growing media clean and low in residual nutrients
• Resume feeding only once temperatures and active growth return

This advice is especially important for:

• Bonsai
• Overwintered chillies
• Mother plants
• Any perennial crop undergoing cold treatment or dormancy

When Fertiliser Is Appropriate

If you are growing in:

• A heated grow room
• A greenhouse with stable temperatures
• Continuous production systems with no dormancy

…then fertiliser use can continue as normal, provided dosing is accurate.

Over-application is one of the most common causes of poor plant performance. Applying more than recommended is often worse than under-feeding.

Key Takeaway

In winter, less is more.
Feeding dormant plants, especially with nitrogen, can lead to toxic nitrite build-up, poor plant health, and long-term damage that persists even after growth resumes.

Correct seasonal nutrient management is just as important in hydroponics as it is in soil.

And if you are looking for an excellent fertilizer to help out with growing in the winter months, check out Liquid Gold!

Article by Dr Russell Sharp

If you would like to keep up to date with subjects just like this, you can listen to both our podcasts! Links can be found bellow:

Hydroponics Daily Podcast: https://podcasts.apple.com/us/podcast/hydroponics-daily/id1788172771

Cereal Killers Podcast: https://podcasts.apple.com/us/podcast/cereal-killers/id1695783663