Top Watering vs Bottom Watering

Are you watering your houseplants correctly? With the explosion of indoor plant popularity and the plethora of conflicting advice from the latest 'influencer' trying to get you to click their latest affiliate offering, watering methods have left many plant parents confused. The internet is full of unsupported claims that bottom watering is "superior" or that self-watering pots will solve all your plant problems - but what does science actually say?

This Unlikely Gardener guide takes a deep dive into the true differences between top watering and bottom watering, exposes common myths about self-watering systems, and reveals why the direction of water might matter far less than you've been led to believe.

The Great Watering Debate: What Everyone Gets Wrong

Before examining the science, let's address some popular claims circulating on social media and various plant forums:

Common Bottom Watering Claims vs. Reality

These misconceptions persist because they oversimplify the complex relationship between plants, water, and your favourite grow-mix. To truly understand what's happening when you water your hoard (PHA Members should get this reference), we need to examine the science.

The Science of Water in Soilless Media: What Actually Matters

Understanding True Soil vs. Soilless Growing Media

To understand the differences in water behaviour, it's important for unlikely gardeners to distinguish between soil and soilless growing media:

Soil is a natural material found in the earth's surface composed of:

• Mineral components (sand, silt, and clay particles) in various proportions
• Organic matter from decomposed plant and animal residues (humus)
• Living organisms (bacteria, fungi, protozoa, nematodes, arthropods, etc.)
• Water in films and pores
• Air in pore spaces

Soil develops over hundreds or thousands of years through weathering processes and biological activity. Its mineral composition typically includes 45-50% minerals by volume, 1-5% organic matter, and the remainder split between water and air-filled pores. Garden soil, topsoil, and field soil are all examples of actual soil.

Soilless Growing Media (Soilless Grow Mix) – often incorrectly but innocuously called "soil" in the houseplant context, are engineered substrates specifically designed for indoor and container cultivation and typically contain:

• Little to no actual mineral soil components
• Manufactured or processed materials (perlite, vermiculite, rockwool)
• Minimally decomposed organic materials (peat moss, coconut coir, bark)
• Inorganic amendments (perlite, pumice, LECA, PON)
• Precisely calibrated proportions to create specific air and water relationships

Common commercial potting "soils" are actually soilless growing media, designed to overcome the limitations of true soil when used indoors and in containers (poor drainage, compaction, lack of aeration). These engineered media generally provide 10-30% more total porosity than mineral soils, with greater air-filled porosity at field capacity.

Understanding Porosity in Soilless Growing Media

Porosity refers to the amount of space between particles in your substrate—spaces that can hold either air or water.

In indoor soilless systems, we focus on two specific properties:

• Air-Filled Porosity (AFP): The portion of pore space that remains filled with air after excess water has drained. Roots need this oxygen for respiration.
• Water-Holding Capacity (WHC): The portion of the substrate that holds onto water after drainage. This is what your plant draws from between waterings.

Field Capacity vs. WHC: Clearing Up the Confusion

In traditional soil science, Field Capacity (FC) is defined as the amount of water soil holds after gravity has pulled the excess away—typically measured a day or two after watering. In soilless growing, we use the term Water-Holding Capacity (WHC) to describe the same general concept: how much water the mix retains after it drains (30-60 minutes).

The difference is mostly in terminology—WHC is more common in horticulture and container-based systems, while FC is traditionally used in outdoor soil science. For indoor plants in soilless media, we typically target a WHC of 40-60%, which provides adequate moisture while maintaining enough air space for healthy root function.

These concepts are critical because they directly determine the all-important air-to-water ratio in your growing medium:

The Critical Air:Water Balance in Soilless Media

The space between particles in any soilless growing medium consists of pores that can contain either water or air— like time travel, they can never both simultaneously exist in the same space. This creates a zero-sum relationship: more water means less air, and vice versa.

Research in the early 1970s established that optimal growing conditions for most indoor and containerized plants in soilless media occur when:

• 45-65% of the total pore space contains water (water-filled porosity)
• 35-55% contains air (air-filled porosity)

However, these ratios vary by plant species. Succulents, cacti, and epiphytes typically require higher air-filled porosity, while moisture-loving tropical plants like Alocasia and Monstera can thrive with higher water-filled porosity.

How Soilless Media Properties Determine Water Relations

Several physical properties of soilless grow-mix media determine how water behaves regardless of watering method:

1. Particle Size Distribution: Finer particles create smaller pores that hold water more tightly against gravity, while larger particles create larger pores that drain more freely.
2. Bulk Density: The weight of the dry media per unit volume affects both water retention and drainage properties.
3. Porosity Types:
   • Macropores (>75μm): Allow rapid drainage and contain air at field capacity
   • Mesopores (30-75μm): Hold plant-available water
   • Micropores (<30μm): Hold water so tightly that it's often unavailable to plants
4. Wettability: Hydrophobic materials like peat or coconut coir can initially resist water uptake, affecting how water moves through the medium.
5. Media Stability: Modern soilless growing media for indoor plants are primarily composed of inert or slow-decomposing materials (perlite, pumice, LECA, coco coir, etc.) specifically designed to maintain consistent physical properties over time. Unlike traditional garden soil, quality soilless grow mixes experience minimal structural changes, though some components like peat or coconut coir may still gradually break down, albeit at a much slower rate due to lower microbial populations in indoor and container environments.

Top Watering vs. Bottom Watering: A Scientific Comparison

Now that all of us plant hoarders understand the fundamental principles, let's objectively compare these watering methods:

Top Watering: Advantages and Limitations

Advantages of Top Watering

1. Flushes Accumulated Salts: Regular top watering helps flush accumulated salts and minerals from fertilizers out of the growing medium, preventing potential toxicity issues.
2. Visual Feedback: You can directly observe how the growing medium responds to watering, making it easier to gauge when you've provided enough moisture.
3. Adaptability: The frequency and volume can be easily adjusted based on seasonal changes, plant growth stages, and environmental conditions.
4. Soil Environment Monitoring: Proper top watering techniques allow for visual assessment of the growing medium's response to irrigation, helping growers develop appropriate watering schedules based on actual observation rather than fixed calendars.
5. Variable Moisture Gradients: Creates moisture gradients within the pot that may influence root distribution, though root development is determined by multiple factors including genetics, plant hormones, and overall growing conditions.

Limitations of Top Watering

1. Time-Consuming: Requires consistent attention and manual intervention, especially for larger collections.
2. Inconsistent Moisture: Water may channel through soilless media unevenly, potentially missing portions of the root system.
3. Potential Substrate Displacement: In very lightweight soilless mixes, the force of watering might displace finer particles, though this is minimal in well-formulated indoor grow mixes with proper structure. Most grow mixes contain components that maintain their structure well when watered.
4. Leaf Spotting: Water on leaves of sensitive plants can cause cosmetic issues or fungal problems in some species.
5. Overflow Mess: Excess water can spill over pot edges, creating cleanup issues.

Bottom Watering: Advantages and Limitations

Advantages of Bottom Watering

1. Potential for More Uniform Saturation: Bottom watering can help achieve more uniform water holding capacity (WHC) in some gor mix types, though this depends primarily on the physical properties and structure of the growing medium rather than the watering direction itself.
2. Controlled Saturation Level: Bottom watering typically saturates soil to Maximum WHC without exceeding it, though this is determined by how long plants remain in water, not by plants "choosing" their water intake.
3. Cleaner Process: No water splashing on leaves or spilling onto furniture.
4. Different Moisture Gradient: Creates a different moisture gradient in the pot compared to top watering, though root growth patterns depend on multiple factors including genetics, hormones, and overall environmental conditions—not just watering method.
5. Surface Condition Management: The soil surface typically remains drier between bottom waterings, which can be beneficial for certain plant types sensitive to pests and crown rot.

Limitations of Bottom Watering

1. Salt Buildup: Without periodic top watering to flush the system, minerals and salts can accumulate in the growing medium.
2. Time-Intensive: Requires monitoring to ensure plants don't sit in water too long.
3. Limited Media Information: Harder to assess the moisture level throughout the growing medium profile.
4. Potential Root Issues: Can encourage excess root growth at the bottom of the pot, potentially leading to increases in root-bound conditions.
5. May Not Fully Saturate: In some grow mixes or tall containers, the upper layers may remain dry risking root dehydration.

Self-Watering Systems: Beyond the Marketing Hype

Self-watering pots have gained immense popularity, marketed as convenient solutions by influencers for busy plant owners. These systems typically consist of a water reservoir at the bottom, separated from the growing medium by a barrier, with some form of wicking mechanism to draw water up into the soilless mix through capillary action.

Marketing Claims vs. Scientific Reality

Why Self-Watering Systems Often Fail

1. The Porosity Problem

The effectiveness of self-watering systems hinges entirely on the porosity of the grow mix composition:

• Variable Capillary Performance: The effectiveness of wicking action depends on the specific physical properties of the soilless medium used, including particle size distribution, pore connectivity, and hydrophilic/hydrophobic characteristics. These properties can vary significantly between different commercial soilless mixes and even more so with DIY grow mix solutions without consistent measurement and quality assurance.
• Unable to Adapt: Unlike manual watering, these systems can't adjust to changing plant needs.
• Substrate Specificity: Many self-watering pots work well only with specific soil types, not necessarily ideal for the particular plant.

2. The Aeration Challenge

A significant consideration with self-watering systems is soil aeration, which is determined by soil structure, porosity, and moisture content:

• Moisture-Oxygen Balance: Many plants benefit from a fluctuation in soil moisture that allows for periodic increased oxygen availability in the root zone. This cycle depends on soil properties and isn't automatically achieved with any particular watering method.
• Moisture Gradient: Self-watering systems typically maintain higher moisture levels in the lower portion of the container. Whether this causes problems depends on the specific porosity and drainage properties of the growing medium used.
• Potential for Reduced Gas Exchange: In poorly designed systems or inappropriate growing media, consistently saturated areas can experience reduced gas exchange, potentially leading to hypoxic conditions that favor anaerobic microorganisms which cause root rot.

3. Wicking Material Limitations

The wicking mechanism itself presents several challenges:

• Material Degradation: Organic wicks (cotton, rope, soil columns) break down over time, becoming less effective or failing completely.
• Clogging: Mineral buildup from fertilizers and tap water can obstruct wicking materials.
• Inconsistent Performance: Changes in temperature and humidity affect capillary action efficiency.
• One-Size-Fits-All Design: Different plants have vastly different water needs, but most self-watering systems offer limited customization.

4. Root Health Concerns

Self-watering systems can impact root development:

• Altered Root Distribution: The consistent moisture gradient may lead to different root distribution patterns compared to top watering, though this is primarily a response to moisture availability rather than roots being "lazy."
• Moisture-Driven Distribution: Root density tends to be higher in areas of optimal moisture and oxygen availability, regardless of watering method used.
• Rot-Prone Conditions: The constant moisture at the bottom creates ideal conditions for root rot pathogens.
• Hypoxic Stress: Insufficient oxygen in perpetually wet soil stresses roots, making plants more susceptible to disease.

5. Salt and Mineral Accumulation

Without periodic flushing that comes with top watering:

• Fertilizer Buildup: Nutrients concentrate in the soil as water evaporates but salts remain.
• pH Shifts: Mineral accumulation can alter soil pH over time, potentially causing nutrient lockout.
• Toxic Conditions: Eventually, salt levels can reach harmful concentrations for sensitive plants.
• White Crust Formation: Visible mineral deposits on soil surface or pot rims.

Making Evidence-Based Watering Decisions

Matching Methods to Plants

Instead of seeking a universal solution, consider these plant-specific factors:

Plants That Generally Prefer Top Watering:

• Epiphytes: Orchids, bromeliads, and many aroids evolved to receive water from above
• Drought-Adapted Species: Cacti and succulents benefit from complete soil drying between waterings
• Salt-Sensitive Plants: Species vulnerable to mineral buildup need regular flushing
• Surface Rooting Plants: Specimens with shallow root systems

Plants That May Do Well With Bottom Watering:

• Moisture-Loving Species: Ferns, peace lilies, and calatheas often appreciate consistent moisture
• Leaf-Sensitive Plants: African violets and other plants whose foliage sometimes reacts poorly to water contact
• Fine Root Systems: Plants with delicate, fibrous roots that benefit from gentle watering

Plants That Might Succeed in Self-Watering Systems (With Careful Monitoring):

• Consistent Water Users: Spider plants, pothos, and philodendrons can adapt to steady moisture
• Bog-Native Species: Plants naturally found in consistently wet environments
• Water-Loving Herbs: Basil, mint, and other herbs that consume water rapidly

Substrate Considerations for Different Watering Methods

The growing medium significantly impacts watering success:

For Top Watering:

• Well-Structured Soils: Media with perlite, pumice, or bark that resist compaction
• Moisture-Retentive Amendments: Coco coir or peat to hold appropriate moisture
• Graduated Particle Size: Mix of components that create both water retention and drainage

For Bottom Watering:

• Capillary-Friendly Media: Substrates that effectively wick moisture upward
• Consistent Texture: Uniform particle size for even water distribution
• Less Hydrophobic Components: Materials that readily accept water after drying

For Self-Watering Systems (If You Must Use Them):

• Highly Aerated Base Layer: LECA, pumice, or lava rock at the bottom for increased air circulation
• Appropriate Porosity Throughout: Media specifically formulated for self-watering applications
• Minimal Organic Content: Lower percentages of peat or coco to reduce decomposition and compaction
• pH Stable Components: Materials resistant to pH fluctuation from mineral buildup

Practical Solutions: A Science-Based Approach

The Evidence-Based Method

Rather than focusing on watering direction as the primary variable, consider these scientifically sound principles:

1. Match Water Frequency to Plant Needs and Media Properties: Different species have evolved different optimal moisture cycles based on their native habitats.
2. Monitor Actual Moisture Content: Use weight, lab grade moisture meters, or visual/tactile assessment rather than rigid schedules.
3. Regular Leaching: Periodically apply sufficient water to create 10-20% leachate to manage salt accumulation, regardless of your primary watering method.
4. Understand Environmental Influence: Adjust practices based on temperature, humidity, light intensity and air movement, which all affect evapotranspiration rates.

Improving Self-Watering Systems (If You Choose to Use Them)

If you're committed to self-watering pots, these modifications can mitigate common issues:

1. Install Aeration Columns: Create vertical channels filled with perlite or LECA to increase oxygen flow to lower roots.
2. Use Semi-Hydroponics Principles: Incorporate LECA or PON as a reservoir buffer to maintain air pockets.
3. Implement Periodic Dry Periods: Empty the reservoir completely every few weeks to allow soil to dry partially.
4. Create a Custom Substrate: Formulate a mix with higher aeration components specifically for your self-watering system.
5. Monitor EC Levels: Regularly check electrical conductivity of your reservoir water to detect salt buildup.
6. Alternative Wicking Materials: Experiment with synthetic, rot-resistant wicks that maintain consistent performance.

Warning Signs: When to Intervene in Any System

Regardless of your chosen watering method, watch for these indicators of problems:

• Yellow Leaves: Often a sign of overwatering or nutrient issues
• Media Moisture Assessment: Visual examination of the growing medium may not accurately represent actual moisture conditions throughout the container. Using weight, lab grade moisture meters, or systematic monitoring provides more reliable indicators of when intervention is needed.
• Stunted Growth: May signal root problems from improper watering
• Soil Pulling Away From Pot Edges: Could indicate hydrophobic soil that's not accepting water properly
• White Crust Formation: Shows mineral buildup requiring flushing

Conclusion: Beyond the Watering Method Myth

After examining the scientific evidence, all of us unlikely gardeners can draw several evidence-based conclusions:

1. No Universal "Best" Method: Neither top watering nor bottom watering is inherently superior—both can be effective or problematic depending on context.
2. Media Properties Trump Water Direction: The physical properties of your growing medium have far more impact on plant outcomes than whether water is applied from top or bottom.
3. Plant Needs Vary Widely: Different species have evolved in specific conditions that influence their optimal moisture levels, making blanket statements about watering methods inappropriate.
4. Self-Watering Is Not Self-Thinking: These systems don't intelligently adjust to plant needs—they simply maintain whatever moisture gradient is created by their design and the media used.
5. The Bottom Line: Successful plant care requires matching your watering approach to your specific plants, growing media, and environmental conditions—not blindly following trends about watering direction.

Rather than asking "Is bottom watering better than top watering?" the more productive question is: "What watering approach best matches this specific plant's needs, my growing medium's properties, and my environmental conditions?"

By focusing on these fundamentals rather than on myths about watering direction, you'll develop a more scientifically sound approach to plant care that leads to consistently healthier plants.