Starting seeds for hydroponic towers vs soil: rockwool, plugs and sponges
By FarmerTested · a network of working growers based in Waxhaw, NC
Links marked "on Amazon" in this guide are affiliate links. As an Amazon Associate I earn from qualifying purchases, at no extra cost to you. True Leaf Market and Botanical Interests links are affiliate links through Awin. Links to other stores are coming as those programs approve us. Commissions never change what I recommend, and I haven't tested these specific products. Full disclosure
I've started seeds in trays of soilless mix for years. If you've done the same and you're eyeing a hydroponic or aeroponic tower, here's the good news: most of what you know carries over. Seeds still want moisture, warmth and light. What changes is the material the seed sits in and what happens when it moves into the system.
I don't own a tower yet. I'm buying my first test system this winter and will start seed for it alongside my usual trays. This guide covers how tower seed-starting works and what each option costs, using published prices. My own results will follow, here and in the newsletter.
The big difference: no soil in the system
In a tower, roots hang in moving water or nutrient mist. Anything loose around them, like soil, peat crumbs or bits of mix, can wash into the reservoir and clog pumps or spray nozzles. It can also carry disease into water that every plant shares. So tower seedlings start in a plug: a small block of material that holds its shape, holds the seed, and drops straight into the tower's net cup or slot.
That's also why I wouldn't take a soil-grown transplant, wash its roots and stick it in a tower. You can find people online who do it, but you're tearing roots and still risk bringing soil into the reservoir. For a tower, start in a plug from day one.
The four common plug types
Rockwool cubes
Rockwool is spun mineral fiber, the most common hydroponic starting cube. It holds water and air well and doesn't break down in the system.
- Soak it first. New rockwool tends to raise pH. Growers commonly soak cubes in water adjusted to around pH 5.5 before sowing.
- Handle it with care. The fibers can irritate skin and lungs when dry, so wet it before handling and consider gloves.
- Downside: it doesn't compost, so used cubes go in the trash.
(see rockwool starter cubes on Amazon)
Peat or composted-bark plugs
These are pre-formed plugs made of organic material held together with a binder. They come ready to use with no pH soak, hold moisture well and are compostable afterward. They can stay wetter than rockwool, so go easy on watering. (see peat and bark starter plugs on Amazon)
Coco coir plugs
Coir is coconut husk fiber. It's renewable and holds air and water well. Like peat plugs, it can shed fine particles, so let it root through fully before it goes into a misting system.
Foam or "grow sponges"
Many countertop systems use small sponges sized for their pods. LetPot, for example, sells a pack of 100 grow sponges for $24.99, about $0.25 each (LetPot, price checked Oct 26, 2026). They're clean and simple, and they fit the unit exactly.
What each major system expects
The system you buy decides some of this for you:
- Gardyn is built around its own yCubes. If you want to use your own seed, Gardyn sells a seedless yCube for $4.99, so bringing your own seed doesn't save money on the pod itself (our Gardyn math).
- Lettuce Grow ships live seedlings but also accepts your own plugs, which makes it much cheaper to run if you start seed (our cost breakdown).
- Tower Garden is an open system. You start your own seedlings in starter cubes and move them into the tower, and we estimate own-seed costs at around $0.35 a plant.
- Open countertop systems like LetPot take sponges and your own seed. Pod systems like Click & Grow and AeroGarden are designed around pre-seeded pods (countertop cost per plant).
Check your system's manual for plug size before you buy anything. A plug that's too big won't seat, and one that's too small falls through.
Step by step: starting plugs for a tower
- Prep the plugs. Soak rockwool in pH-adjusted water. Peat and coir plugs just need moistening. Let excess water drain. Plugs should be damp, not dripping.
- Sow 1–2 seeds per plug in the hole, at about the depth on the packet. Lettuce needs light to germinate, so barely cover it or don't cover it at all.
- Cover and keep warm. Use a dome or a loose lid over the tray until most seeds sprout, then remove it.
- Light right away. Same as soil seedlings: 14–16 hours, close enough that they don't stretch. See our grow light guide.
- Thin to one seedling per plug. Snip the extra with scissors instead of pulling it, so you don't disturb the keeper's roots.
- Feed lightly. Seedlings live on the seed's stored food at first. A common approach is plain pH-adjusted water until true leaves form, then nutrients at a fraction of full strength, working up gradually.
- Transplant when roots show. Once white roots poke out the bottom and sides of the plug, it's ready for the tower. For fast greens like lettuce, that's often a couple of weeks after sowing, depending on temperature and light.
pH: the new habit for soil growers
In soil, the soil buffers pH for you. In water it doesn't, and nutrient uptake depends on it. Most leafy greens and herbs in hydroponics are grown in a slightly acidic range, roughly 5.5–6.5, and most systems publish a target in their manual. A basic pH test kit or meter and a bottle of pH-down are the extra tools a soil grower needs. (see pH test kits and meters for hydroponics on Amazon)
Cost per plug, done honestly
Seed is the cheap part. A 200-seed packet of organic Genovese basil was $5.25 at Johnny's when we checked, about 5 cents for the two seeds you'd sow per plug. Add a $0.25 LetPot sponge and you're at about $0.30 per plant before nutrients and power. Compare that with $2 to $5 for a pre-seeded pod in most pod systems. That gap is the main reason open systems come out cheapest per plant on our comparison table.
The trade-off is your time, plus a little more room for error: germination trays, thinning, and getting pH and feeding right. Pod systems sell you convenience. Whether that's worth the markup depends on how much you enjoy starting seed.
For lettuce, basil and microgreen seed in bulk-friendly sizes, try True Leaf Market lettuce seed, basil and microgreen seed. Botanical Interests also carries lettuce, basil and microgreen seed.
Tower vs soil: what I'll be testing
This winter I plan to sow the same lettuce varieties in plugs for a tower and in my usual soilless trays, then grow them side by side. I'll track germination, days to transplant, and what each one costs. I don't know yet how it'll come out, and that's the point. Follow along in the newsletter and you'll see the results as they come in.