⏳ 19 min read · Last updated: September 2026
Disclosure: As an Amazon Associate I earn from qualifying purchases. This post contains affiliate links. If you buy through these links, I may earn a small commission at no extra cost to you. Some of the gear here I have grown with myself. The rest I research thoroughly before it earns a place on this page, and I always tell you which is which. Thank you for supporting Urban Hydro Space!
🌱 Hands-on tested: the KETOTEK pH, EC and temperature meter, the ComSaf wide-mouth mason jars, the Kratky wide-mouth lids and ZeeDix 2-inch net cups. I have used these in my own grows. 🔎 Research pick: everything else in this post, chosen from published specifications and listings. How I test and choose products.
Choosing the best net cups for mason jars is the first real decision in any Kratky setup, and the improvised version goes wrong fast. Thin plastic drinking cups balanced inside wide-mouth jars, with the gaps sealed by crumpled foil, let in enough light that green algae can cover the glass within days while basil roots turn brown and slimy. What the jar needs instead is a cup that fits the opening and seals out light without any DIY patching.
Hardware made for the job stops the algae problem at the source, which is why it is what I use on my countertop jars. This guide covers the two options that work best for apartment Kratky growers: a bulk pack of standard 2-inch net cups and an all-in-one lid-and-basket combo. You’ll also find a step-by-step setup guide, EC and pH targets, and a cleaning routine that keeps your equipment going for years. If you’re new to the Kratky method, start with the apartment hydroponics beginner guide for full system context before reading on.
Will It Fit Your Jar?
Check the jar first: a 2-inch net cup sits on the rim of a wide-mouth mason jar. Regular-mouth jars are too narrow for it.
🏆 1 to 4 jars: Kratky lid kit
🔎 Research pick: the Kratky lids I grew with are out of stock, and this kit is the closest in stock.
- Fits: wide-mouth jars; each 3.38 x 3.38 in lid holds its own net basket.
- In the kit: 6 lids, 6 baskets and 6 rings.
- Best for: a light-tight jar on the first try, with no tape.
- Skip if: you are planning 10 or more jars, where bulk cups cost less per jar.
🛒 Check price on Amazon $14.99
📦 10 or more jars: AC Infinity 2-inch net cups (50-pack)
🔎 Research pick: the ZeeDix cups I grew with are out of stock.
- Fits: the metal band of a wide-mouth jar.
- Best for: a shelf of jars at the lowest cost per jar.
- Skip if: you would rather not block light yourself; the metal band can also rust.
🛒 Check price on Amazon $17.99
🫙 No jars yet: ComSaf 32 oz wide-mouth mason jars (2-pack)
🌱 Hands-on tested: I grow in these jars.
- Fits: one plant per jar; sleeve the glass so no light reaches the water.
- Best for: starting from nothing, then adding the lids or cups above.
🛒 Check price on Amazon $11.79
Every option side by side is in the comparison table below. Prices are pulled from Amazon and were last checked on 8 October 2026 at 8:30 pm. Amazon prices change often, so check the listing before buying.
The Best Mason Jar Net Cups: What Actually Matters
Overall, three things decide whether the best mason jar net cups work in your setup: diameter fit, light blocking, and material durability. Get one wrong and you’re dealing with root rot or algae before your seedlings even establish. Fortunately, once you know the specs, the buying decision is straightforward.
- Wide-mouth mason jars need a 2-inch diameter net cup to sit securely on the rim without falling in.
- Start with the base of the cup sitting in the solution. A 1 to 2 inch air gap forms on its own as the plant drinks, and the roots in it breathe.
- Start seedlings at EC 1.0 to 1.2 and move up to EC 1.5 once plants are established.
- Keep pH at 5.5 to 6.5 for herbs and leafy greens.
- Size the jar to last the crop and let the level fall; if a long crop runs low, add a little nutrient solution, no more than about an inch.
- Block every light path into the reservoir. Algae needs only a pinhole of light to bloom.
Quick Comparison: Which Option Fits Your Setup
🏆 Sarah’s Top Pick
Kratky Hydroponic Lids Wide Mouth 24-Pack
One piece of hard plastic replaces the metal band, the net cup, and all the tape you’d otherwise need. No rust, no light gaps, no algae headaches. The fastest way to get a clean Kratky jar on your first try.
★★★★★ Best for Beginners · 24-Pack
Out of stock on Amazon right now. The button below goes to the closest in-stock kit, a research pick with separate net baskets.
ZeeDix 2-Inch Net Cups: The Budget Bulk Option
Out of stock right now: these cups are currently unavailable on Amazon. Until they are back, the closest in-stock option is these AC Infinity 2-inch net cups (50-pack), Amazon price right now $17.99. They are a research pick, not the cups I grew with, and their wide lip is made to sit in a mason jar mouth.
★★★★☆ Best for Scaling Up / 50-Pack Value50-Pack
⚠️ Watch out for: No included lid, so light leaks around the metal band require a DIY fix; metal screw bands rust within a few months in humid setups
The ZeeDix 2-inch net cups (50-pack) are the cups I grew with, and the standard choice for anyone building more than a handful of jars. Bought fifty at a time, each cup costs very little. A standard 2-inch cup also sits snugly inside a wide-mouth mason jar band, while the open-mesh sides let roots escape the rockwool cube and reach the water below within days.
Why Bulk Cups Work at Scale
Once you grow your first successful jar of basil, you’ll want six more. Having a full pack on hand means you can start a new crop the moment the last one finishes. There’s no waiting for a reorder, and no borrowing from one jar to fix another. Plenty of growers who start with a single jar soon want a full shelf, which is why a bulk pack pays off.
The slotted mesh design also matters for root development. Young roots soon find the gaps and push through into the nutrient water below. That early root-to-water contact is also what drives fast top growth in lettuce, basil, and mint. You can see this in action if you follow the step-by-step Kratky jar setup guide when you build your first jar.
Solving the Light-Leak Problem
The one real limitation of standalone cups is that they don’t include a lid. The metal screw band holds the cup in place, but leaves gaps around the edges. Light slips through those gaps and hits the nutrient water, so algae follows. Algae follows.
You have three solid options for blocking that light:
- Wrap the outside of the jar in a single layer of black electrical tape where the band sits.
- Cut a circle of black craft foam and press it under the band before screwing it down.
- Use a paint marker to blacken the inside of the glass lid area if you’re committed to a permanent setup.
None of these cost more than a few cents per jar, although a purpose-made lid is tidier. The foam circle method is the quickest and leaves no residue when you clean between harvests. For a deeper look at the algae issue, my guide on stopping algae in hydroponic jars covers every gap growers miss.
What to Do When the Metal Band Rusts
The cups themselves are durable plastic and won’t corrode. The metal screw band is the weak point. Moisture from daily evaporation and nutrient spills causes the band to rust within a few months, especially in humid apartments.
Fortunately, the fix is simple: replace the metal bands with plastic canning rings. Many hardware stores and online suppliers sell wide-mouth plastic screw bands that fit standard mason jars. Swap them in before rust appears and you’ll never fight a stuck lid again. When a band does rust shut, run it under very hot water for 30 seconds and grip with a dry rubber mat to get the torque you need.
→ DWC vs Kratky for Apartment Beginners
→ 3 Best Countertop Hydroponic Systems for Apartments
Kratky Hydroponic Lids: The All-in-One Pick
Amazon price right now: $16.99 Prices refresh automatically. Prices are pulled from Amazon and were last checked on 8 October 2026 at 8:30 pm. Amazon prices change often, so check the listing before buying.
★★★★★ Best for Beginners / All-in-One Design24-Pack
Out of stock right now: these lids are currently unavailable on Amazon. Until they are back, the closest in-stock option is this Kratky lid kit with separate net baskets. It is a research pick, not the lid I grew with, and because its basket is a separate piece it does not have the one-piece design described below.
⚠️ Watch out for: Occasionally requires a silicone gasket on older jars with chipped rims to prevent moisture running down the glass
The Kratky Hydroponic Lids Wide Mouth (24-pack) solve the light and rust problems in one part. Each lid is a single piece of hard plastic: a wide, opaque top with a built-in 2-inch basket at the center. It screws onto any standard wide-mouth jar the same way the original metal lid does. As a result, nothing gaps, nothing rusts, and no foil or tape is needed.
Why the Solid Top Makes a Real Difference
The flat opaque cover also blocks light from above and from the sides where the band meets the glass. That’s the single most common algae entry point that the bulk cup method struggles with. When your jars sit near a south-facing window or under a bright grow light, this matters a lot. After all, ambient light from multiple angles is harder to manage with DIY solutions than with a purpose-made opaque lid.
For windowsill growers especially, these lids are worth it. You can read more about setting up a sunny windowsill jar in my guide on growing Kratky lettuce on a sunny windowsill where light control is the deciding factor between a clean setup and a green mess.
No More Rust on the Rim
Because the lid is molded from a single piece of hard plastic, there’s no metal anywhere. The threads grip the glass jar directly. After a full growing season of refills and weekly checks, the lid looks exactly the same as when you bought it. That matters when your jars sit on a kitchen counter or shelf visible from the living room. Rusty rings look bad and also drop iron flakes into your nutrient solution over time.
What to Do if the Lid Leaks
However, plastic threads occasionally don’t seal tightly against smooth glass, especially on older jars where the rim has micro-chips. If you notice moisture running down the outside of the jar when you pick it up, press a thin silicone gasket ring under the lid before screwing it down. Silicone canning rings are cheap and easy to find at most hardware stores. One ring per lid fixes the problem permanently.
How to Set Up Net Cups in Glass Jars
Assembly takes about five minutes per jar once you’ve done it once. In practice, the most common beginner mistake is filling the jar too high on the first fill. Roots drown when they have no air. Here’s the correct sequence from empty jar to planted seedling.
Choosing and Filling Your Jars
Use wide-mouth jars only. Regular-mouth jars are too narrow for a 2-inch cup, and the smaller cup sizes they require restrict root growth. The ComSaf 32oz wide-mouth mason jars are a reliable starting point. The 32oz size holds enough nutrient solution to last a full 2 weeks between changes for most herbs without running critically low.
Mix your nutrient solution before filling. Start at EC 1.0 to 1.2 for seedlings and young transplants. Fill the jar to about 1 inch below the rim of the threads. This leaves room for the net cup base to sit above the water when the lid is on. You can use the pH and nutrient calculator to dial in the exact drop count for your water volume.
Seating the Rockwool or Clay Pebbles
Place a pre-soaked 1-inch rockwool cube into the center of the net cup. Press it gently until the bottom of the cube sits flush with the base of the basket. If there’s space around the rockwool, fill it with a few Halatool clay pebbles to hold the cube upright and stop it from tipping when leaves grow heavy. Don’t pack the clay too tight. Roots need space to push through the gaps.
For the full seed-starting process, the guide on starting hydroponic seeds in rockwool covers pre-soaking, seeding depth, and germination conditions in detail.
Getting the Air Gap Right
The air gap is the most important measurement in a Kratky jar. It’s the empty space between the water surface and the base of the net cup. This void fills with humid air as the plant grows, and the roots develop specialized “air roots” that absorb oxygen directly from it.
Target a gap of 1 to 2 inches when you first plant. As the plant consumes water over the following days, the water level drops naturally and the gap expands. Do not top off to the original level during this phase. The gap is supposed to grow.
Managing Water Levels Week by Week
During the first 5 to 7 days, seedling roots are very short and won’t reach the water from the cup. Keep the water level slightly higher so the bottom of the rockwool just touches the surface. This wicks moisture upward so the germinating seed stays hydrated.
When you see white roots dangling down into the jar, lower the water to create the proper air gap and maintain it from that point forward. If you’re running a grow light above your jars, use the hydroponic grow light calculator to confirm your lamp is hanging at the right height for the crop in each jar. Track your plant’s weekly progress with the seed-to-harvest calculator so you know when to expect the first leaves and when harvest is due.
Nutrients and pH for Mason Jar Kratky Setups
Good hardware only gets you halfway. Specifically, the nutrient solution inside the jar determines whether your herbs grow fast and taste strong or sit there looking pale and tired. Two numbers control everything: EC and pH.
EC Targets for Seedlings and Young Herbs
EC measures how concentrated your nutrient solution is. Start low and build up. Young seedlings in their first 2 weeks need EC 1.0 to 1.2. Push higher than that and you risk burning the delicate root tips before they’ve had a chance to harden off.
When plants are established with visible new leaf growth, raise EC to the crop-specific target:
- Lettuce: EC 1.0 to 1.4
- Basil: EC 1.0 to 1.6
- Mint: EC 2.0 to 2.4
- Thyme: EC 1.2 to 1.6; oregano 1.4 to 1.8; chives 1.8 to 2.0
- Mixed jar with multiple herbs: stay at the safe middle ground of EC 1.5
When you see brown leaf tips on any herb, EC is the first thing to check. In most cases, tip burn means the solution is too strong. Dump 25 percent of the reservoir and top off with plain pH-adjusted water so the EC comes down.
Keeping pH Stable in a Small Jar
Keep your water at pH 5.5 to 6.5 for herbs and leafy greens. Outside that band, nutrients become chemically locked and the plant can’t absorb them no matter how much fertilizer is in the water. A yellowing plant in a correctly stocked jar is usually a pH problem rather than a nutrient deficiency.
The pH Up and Down Kit gives you both adjustment solutions in one kit. Add drops one at a time, stir, and retest. Small jars move quickly, so a single extra drop can overshoot. For measuring, the KETOTEK pH and EC combo pen reads pH, EC, and water temperature in one tool. It’s the most cost-effective starting point for a beginner.
🧮 Free tool
Mix the right solution for your jar
The pH and nutrient calculator gives you the pH range, the EC target and the dose for a single jar. Here it is for lettuce in a 1 liter mason jar.
How to Clean and Reuse Your Net Cups and Lids
A good set of net cups or lids can run for years as long as you clean them properly between harvests. Over time, mineral salt crust builds up on the plastic where water evaporates at the rim. Left alone, that buildup traps bacteria, so your next crop starts out more vulnerable.
Deep Cleaning Between Harvests
When a plant finishes, do a full teardown. Remove the old rockwool block and root mass and bin them. Do not put old roots or rockwool into compost. They can carry pathogens. Then clean the cup or lid in four steps:
- Scrub the inner basket slots with an old toothbrush and warm dish soap to break down root residue.
- Rinse under hot running water until no soap remains.
- Soak in a solution of one part 3-percent hydrogen peroxide to two parts water for 20 to 30 minutes.
- Rinse lightly and air dry on a clean towel before storing.
For the jar itself, the same peroxide soak works on the glass. More detail on cleaning the full system is in my guide on cleaning a small hydroponic system in an apartment.
Your Ongoing Maintenance Schedule
Between deep cleans, stay on top of these three habits:
- Daily: Check the level and let it fall. If a long crop runs low, add a little nutrient solution, no more than about an inch, never back to the brim.
- Every few days: Check pH and EC, since a small jar drifts faster than it looks.
- Every harvest: Full teardown, toothbrush scrub, hydrogen peroxide soak, air dry.
If you notice a bad smell, change the solution at once. Warm apartment temperatures above 75°F (24°C) speed up bacterial growth. More on what drives those smell and timing issues is in my post on how often to change hydroponic water. And if roots already show brown rot, follow the recovery steps in the root rot prevention guide before starting a fresh crop.
→ University of Minnesota Extension: Small-Scale Hydroponics Guide
→ University of New Hampshire Extension: Hydroponics at Home
- pH + EC monitor: KETOTEK pH/EC/Temp Combo Pen. Reads pH, EC, and water temperature from one probe. Essential for catching nutrient drift before it burns roots in a small jar.
- pH adjustment: pH Up and Down Kit. Small jars swing fast. Having both solutions on hand means you can correct a reading in under a minute without waiting for a delivery.
- Grow media: 1-inch Rockwool Cubes and Halatool Clay Pebbles. Pre-soak the rockwool, drop your seed in, fill the gaps with clay pebbles to hold it upright.
- Jars: ComSaf 32oz Wide-Mouth Mason Jars. Wide-mouth only. Regular-mouth jars won’t fit a 2-inch cup and will force you to restart with the right hardware anyway.
Plan your complete apartment Kratky setup with the hydroponic shopping list builder.
A Word From Sarah
The Lesson I Learned Scaling Up
Jar type matters more than it looks once you go past a handful of jars. Mixed glass from around the apartment usually includes a few regular-mouth jars, and 2-inch cups do not fit those narrow openings: they sit crooked, the light gaps are huge, and a regular-mouth jar can turn green within a week while the wide-mouth ones stay clean. So I keep one type only: wide-mouth 32oz, same lid, same cup, across every single jar. Matching hardware sounds like a small thing, yet it saves hours of troubleshooting.
Frequently Asked Questions
What size net cup fits a wide-mouth mason jar?
A 2-inch diameter net cup fits securely inside a standard wide-mouth mason jar band. The cup lip rests on the glass rim and the basket hangs down into the reservoir. Regular-mouth jars are too narrow for a 2-inch cup and restrict root expansion, so wide-mouth jars are the right choice for apartment Kratky setups.
How do I block light in mason jar hydroponics?
The most reliable method is using opaque Kratky hydroponic lids that cover the jar opening with a single piece of hard plastic. If you use separate net cups, wrap the jar exterior with black electrical tape or slide a black sock over the glass. The area under the metal band is the most common gap, so pay special attention to sealing it. Any light that reaches the nutrient water will feed algae.
Can I reuse hydroponic net cups and lids?
Yes, both net cups and Kratky lids can be reused across many harvests. After each crop, remove old plant material, scrub the slots with a toothbrush and dish soap, then soak in diluted hydrogen peroxide for 20 to 30 minutes. Rinse and air dry before your next planting. Avoid scratching the plastic with metal scrubbers, as rough surfaces trap bacteria.
Why is algae growing inside my Kratky jar?
Algae grows when light reaches the nutrient solution. Even a small gap around the net cup rim or a translucent jar wall is enough. Check every point where light could enter: the area under the metal band, the cup-to-jar contact zone, and the jar body itself. Switching to opaque Kratky lids and covering the jar exterior solves most algae problems within the first week of the next crop.
Do I need an air pump for mason jar hydroponics?
No. The Kratky method is a passive system with no pump required. Roots get oxygen from the air gap that forms as water is consumed. That humid void between the water surface and the base of the net cup replaces what an air stone would do in an active system. This is what makes Kratky jars ideal for apartments where noise and electricity use are a concern.
How many seeds should I put in a net cup?
Place 2 to 3 seeds in the rockwool cube when you start. Once the seedlings develop their first true leaves, thin down to the single strongest one by snipping the extras at the base with clean scissors. Leaving multiple plants in one cup crowds the root zone and reduces airflow, which slows growth and raises the risk of damping off in young seedlings.
How often should I change the water in my mason jar setup?
In a Kratky jar, do not top off daily: size the jar to last the crop, let the level fall, and if a long crop runs low, add a little nutrient solution, no more than about an inch. In an aerated jar, top off with plain pH-adjusted water daily and do a full reservoir change every 2 weeks: empty the jar, rinse it, and refill with fresh nutrient solution at the correct EC and pH. Skipping the full change lets mineral salts accumulate and bacteria build up, which leads to root problems and poor growth in the third and fourth week.
How deep should the net cup sit in a mason jar?
At planting, the bottom half inch to one inch of the net cup should sit in the nutrient solution, with the rest of the cup above the water line so the seedling roots get air. Never push the cup deeper to “help it reach”, and never top the jar back up above that first mark. As the level drops, the air gap is what keeps the roots from drowning. The Kratky jar setup guide shows the fill line for wide-mouth jars.
Can I make DIY net cups for a mason jar?
You can, for one experiment. A plastic cup with a few slits cut in the bottom holds a seedling for a week or two. The problems show up later: thin cup walls flex when roots grab them, non-food-safe plastics sit in warm water for months, and the rim rarely seats flush in a wide-mouth band, which lets light leak past. A food-safe wide-mouth net cup costs a couple of dollars and removes all three problems at once.
What can I use instead of net cups?
A small plastic cup with plenty of slits or holes cut in its sides and base works, as long as it holds the growing medium, fits the jar opening snugly and lets the roots out. The catch is light: a homemade cup rarely seals the opening, so cover any gaps to keep algae out. A purpose-made net cup or an all-in-one Kratky lid costs little and avoids both problems.
What growing media works inside a net cup?
Starter plugs and rockwool cubes drop straight in and hold seeds steady. Clay pebbles work for transplants: fill the cup halfway, seat the roots, then fill around them. Whatever you use, rinse it first, and keep the top half inch of the cup dry so the stem never sits in soaked media. The rockwool seed-starting guide covers germination step by step, and used clay pebbles can be cleaned and reused for years.
Happy growing! 🌿
Sarah, Urban Hydro Space
Sarah Whitfield is the founder of Urban Hydro Space and an indoor gardening enthusiast dedicated to helping apartment dwellers grow fresh herbs and vegetables in small spaces. She researches hydroponic systems carefully from real specifications and verified buyer feedback, and shares practical tips and honest, clearly labeled product picks to make indoor gardening accessible for beginners.
