By Marcia Kirinus, NC State Extension Master GardenerSM volunteer of Durham County
This article is part two of a three-part series on starting seeds indoors using a light cart. In part one, we focused on lighting and environmental setup, the foundation for growing compact, healthy seedlings indoors. Find it here.
In this installment we will cover watering, fertility, and pest management, including strategies to prevent common issues such as fungus gnats and nutrient stress. In part three we will go over hardening off and transplanting into the garden.
If seedlings could talk, most would say the same thing: “I need less water than you think – and more patience than you expect.”
Chlorosis and stunted growth resulting from oxygen deprivation and root stress caused by excess moisture. (Image credit: Marcia kirinus)
The Art of Consistent Watering
Inconsistent moisture is the single biggest reason seedlings fail under otherwise excellent growing conditions. Too wet, and roots suffocate, diseases thrive, and fungus gnats move in. Too dry, and growth stalls, stress builds, and plants struggle to recover. On a light cart, where plants are grown intensively in small containers, water management becomes both an art and a discipline.
How Hard Can it be to Water a Plant?
The concept is simple in theory but complex in practice, because good watering requires judgment, not rules. Think of it as managing moisture rather than watering a plant. You’re not watering just a plant, you’re watering an entire system, most of which is hidden below the surface.
Many factors influence how quickly soil dries and how often watering is needed:
- Soil type (peaty, sandy, clay, compost-heavy)
- Pot size and material
- Drainage quality
- Root size and development
- Temperature
- Humidity
- Airflow
- Light levels
- Growth stage
All of these variables make it hard to prescribe watering rules. Most people want fixed instructions like “water every third day” rather than learning to read plants and soil. But watering is a skill, not a schedule. Skills require observation, pattern recognition, and experience. The more you practice, the more intuitive it becomes. And yes most gardeners have a long history of plant “murders” while learning this deceptively simple task.
Why Roots Need Oxygen as Much as Water
If you’re unsure whether to water, err on the side of staying slightly dry. Why? Because roots need oxygen just as much as they need water.
Roots are not straws sucking up water. They are living tissue, absorbing water and nutrients while constantly respiring. When soil stays saturated, air spaces fill with water, oxygen disappears, and roots literally suffocate.
When we say, “this soil needs better drainage,” what we really mean is, “roots need more access to oxygen.” Good drainage creates air pockets, allowing gas exchange and supporting healthy microbial life.
Master the concept of creating a bed of moisture rather than simply watering a plant, and your seedlings will reward you with sturdy stems, healthy root systems, and uniform growth.
Monitoring Moisture on a Light Cart
Seedlings grow best when roots experience consistent moisture combined with ample oxygen. That balance minimizes stress, promotes strong root development, and supports even growth across trays.
Because plug trays and soil blocks hold limited water, daily monitoring is essential, especially under warm lights and circulating air.
Signs It’s Time to Water
- Soil surface looks dull, matte, or slightly lighter in color
- Containers feel noticeably lighter when lifted
- Soil blocks feel firm rather than spongy
- Early signs of leaf softening or slight wilt (loss of turgor pressure)
The goal is to water before seedlings show stress – not after.
Examples of the potting media moisture levels 1-5. Dry (Level 1) is on the left and Saturation (Level 5) is on the right. We allow most plants to dry down to Level 2 and water to Level 4. (Image credit: Iowa State University Extention and Outreach)
Why Bottom Watering Works Best
All watering in a light cart system should be done from the bottom. Dry soil readily absorbs water through drainage holes, pulling moisture upward evenly through capillary action.
Benefits of Bottom Watering
- Keeps foliage dry, reducing disease pressure
- Prevents soil splash and cross-contamination
- Encourages deeper, stronger root systems
- Produces more uniform moisture across trays
How to Bottom Water
- Pour room-temperature water into the solid bottom tray until it reaches ¼–½ inch deep.
- Allow trays to absorb moisture for 15–20 minutes.
- Drain any remaining water – never leave roots sitting in standing water.
Consistency matters. Developing a predictable daily monitoring and watering rhythm makes it easier to spot subtle changes in plant health and respond before problems develop.
Remove one plant to add ¼–½ inch of water to the solid bottom tray. Replace plant, allow trays to absorb moisture for 15–30 minutes, then drain any remaining water. (Image credit: Marcia Kirinus)
Transition: From Watering to Fertilizing
It’s tempting to jump straight to fertilizing when seedlings appear slow or pale, but feeding only works when watering is already mastered. Healthy roots can easily draw nutrients where as waterlogged plants can not. If moisture levels are inconsistent or soil stays waterlogged, fertilizers can’t fix the problem and may even make it worse. When roots are healthy, hydrated, and well-oxygenated, fertility becomes a gentle support system rather than a corrective measure.
Less is More When it Comes to Fertilizer
When it comes to fertilizing seedlings, less is always more. Young plants have modest nutritional needs, and their tender roots are easily damaged by excess salts and nutrients. Over fertilizing leads to rapid, weak growth, burned root tips, nutrient imbalances, and increased susceptibility to pests and disease. Our goal is not fast growth, but steady, balanced development. Light, consistent feeding supports healthy leaf color, sturdy stems, and strong root systems, setting seedlings up for long-term success once transplanted into the garden.
Fertilizing: When and How Much
Seed-starting mixes sold at most nurseries and big-box stores contain limited nutrients. Once seedlings develop true leaves, they need supplemental feeding to sustain healthy growth.
Fertilizer Guidelines
- Use a water-soluble fertilizer (such as 20-20-20 or similar)
- Apply at ¼–½ the labeled rate
- Begin 10–14 days after germination – after true leaves appear
- Feed once per week, replacing one normal watering
Fungus Gnats: A Common Indoor Pest
Even with excellent watering and fertilizing practices, one persistent pest often finds its way into indoor seed-starting systems: fungus gnats (Bradysia spp.). These tiny flies thrive in moist environments and organic growing media, making light carts and seed trays especially inviting. Understanding their life cycle and how our watering habits influence their presence is key to preventing minor annoyances from becoming major infestations.
How Fungus Gnats First Arrive
Fungus gnats don’t appear out of nowhere, but they can certainly feel like they do. Their life cycle is fast – often as short as 2–3 weeks from egg to adult. That means: eggs you never saw quickly become larvae you don’t notice, then suddenly turn into flying adults that are annoying. That quick turn around time feels like a sudden invasion, when in fact they’ve been quietly developing underground. In reality, they arrive in one of three common ways:
- In potting mix: Eggs and larvae often hitchhike in bagged potting soil, compost, and seed-starting mixes. Even high-quality commercial products can occasionally contain fungus gnat eggs, especially if the mix was stored warm or moist before packaging.
- Through open doors and windows: Adult fungus gnats are tiny and easily slip indoors, especially during warm weather. Once inside, moist soil provides the perfect place to lay eggs
- On plants brought indoors: Houseplants, new seedlings, and nursery transplants frequently carry eggs or larvae in the soil.
Understanding Fungus Gnat damage
Only one stage of the fungus gnat life cycle causes direct harm: the larva.
Left: Life stages of the fungus gnat. Bradysia spp. Adults lay eggs near the surface of moist soil. Eggs hatch in a few days and larva feed on tender young roots of new seedlings. larva turns to pupa and emerge as adults. Right: Larva stage of fungus gnat. Note small black pin head. (Image credit: UMass Extension Vegetable Program, UC Extention bulletin)
Damage Symptoms From Fungus Gnats:
- Wilting or stunted growth
- Yellowing leaves
- Roots appear chewed or damaged
- Slimy or overly wet soil
- Silvery “snail trails” on the soil surface, indicating high larval populations
Cultural Controls: The First Line of Defense
Since fungus gnats thrive in moist, nutrient-rich soil and warm, conditions. By adjusting how you grow seedlings, you can make the environment inhospitable for them. Here are some cultural practices that create an environment where fungus gnats do not thrive:
Effective Cultural Controls
- Let soil surfaces dry slightly between waterings
- Avoid constant moisture; allow the top ¼ inch to dry.
- Bottom watering helps maintain dry surfaces.
- Use a well-draining growing medium
- Light, soilless mixes promote air pockets and faster drying.
- Avoid over-fertilizing
- Excess nutrients in wet soil feed larvae.
- Provide good air circulation
- Fans and spacing dry the soil surface and discourage adult gnats.
- Maintain sanitation
- Remove dead leaves, spilled soil, and algae.
- Wash trays between plantings with mild soap or a 10% bleach solution.
- Quarantine new plants and soil
- Inspect and isolate new materials before introducing them.
- Use physical barriers and monitoring tools
- Yellow sticky traps catch adults and monitor populations.
- Apply thin layers of horticultural sand or perlite to soil surfaces.
Biological Control of Fungus Gnats:
When cultural controls aren’t enough, biological control offers a safe, effective way to reduce fungus gnat populations without chemicals. Two of the most commonly used biological agents are Bacillus thuringiensis var. israelensis (Bt israelensis) and beneficial nematodes (Steinernema spp.). Both target larvae in the soil, the stage responsible for most plant damage, while leaving seedlings, people, and pets unharmed.
How they work:
- Bti: A naturally occurring bacterium applied as a soil drench. Larvae ingest it while feeding and die within days. Use weekly in place of one watering. Most effective on young larvae.
- Beneficial nematodes: Microscopic worms that seek out and infect larvae. Effective against all larval stages and can be applied alongside Bti.
Tips for success:
- Apply to moist soil, as both Bt and nematodes rely on soil contact.
- Avoid overwatering immediately after application; larvae need to encounter the biological agents.
- Repeat applications as recommended on the product label to maintain control over multiple generations.
- Combine with cultural controls – dry soil surfaces, clean trays, and proper watering – for the best results.
Key point: Biological controls don’t replace good growing practices; they supplement prevention and help keep larvae populations in check while maintaining healthy seedlings.
Final Takeaway
Spend a few minutes each day visually inspecting plants during watering. Healthy, well-managed seedlings are less attractive to pests and better able to tolerate occasional stress. Prevention always beats cure, and early action prevents population explosions.
Daily care transforms a light cart from a shelf of plants into a precision growing system. Thoughtful watering, simple fertilizer routines, and proactive pest prevention dramatically reduce problems and produce healthier, stronger seedlings.
Get these fundamentals right, and your plants will grow predictably, evenly, and vigorously – setting the stage for successful transplanting and beautiful final crops.
Resources and Additional Information
- Past Blog Post: Starting Seeds Indoors with a Light Cart: A Science-Based Guide (Part 1 of 3) https://wp.me/p2nIr1-6UX
- Past Blog Post: What I love about starting seeds using a soil block: https://wp.me/p2nIr1-4rv
- UMass Vegetable Extention Program on Fungus Gnats and Shore Flies: https://www.umass.edu/agriculture-food-environment/vegetable/fact-sheets/fungus-gnats-shore-flies
- University of California, Davis, Integrated Pest Management Program Bulletin on Fungus Gnat: https://ipm.ucanr.edu/legacy_assets/PDF/PESTNOTES/pnfungusgnats.pdf#:~:text=Fungus%20gnats%20are%20small%20flies%20that%20in%2D,greenhouses%2C%20nurseries%2C%20potted%20plants%20and%20interior%20plantscapes.
- NC Gardening Handbook Chapter on Plants Grown in Containers : https://content.ces.ncsu.edu/extension-gardener-handbook/18-plants-grown-in-containers
- Iowa State University Extension and Outreach, Watering Plants in Containers And Plug Trays: https://yardandgarden.extension.iastate.edu/article/2025/04/watering-plants-containers-and-plug-trays
- Michigan State University, Dampening Off, Plant and Pest Diagnostics: https://www.canr.msu.edu/resources/damping-off
Article Short Link: https://wp.me/p2nIr1-73H
