How to Water Seedlings in Trays: Bottom Watering vs Top Watering - Linkwin

How should you water seedlings in trays? For most seedlings, the goal is not simply to add as much water as possible, but to keep the growing medium evenly moist while allowing excess water to drain away. Two common approaches are bottom watering and top watering. Bottom watering supplies water from underneath through the drainage holes, while top watering applies water directly to the surface of the growing medium.

If you are unsure which method to use, how often to water, or how to avoid keeping your seedling trays too wet, this guide will walk you through both methods step by step. You will learn how bottom watering and top watering work, when each method is useful, how to apply water without disturbing young seedlings, and how to recognize the signs of overwatering or underwatering. By the end, you should have a clearer idea of which seedling tray watering method fits your seedlings, growing medium, and growing conditions.

Bottom Watering

Definition

Bottom watering is a method of watering seedlings by supplying water from underneath the seedling tray rather than pouring water directly onto the surface of the growing medium. A tray with drainage holes is placed in a shallow container of water, allowing the growing medium to absorb moisture upward through the drainage holes and into the root zone. This method can reduce disturbance to seeds and young seedlings while helping moisten the growing medium from below, but the tray should be removed from standing water after the growing medium has absorbed sufficient moisture.

Working Principle

        Seedlings
           ↑
     ┌───────────┐
     │ Growing   │
     │  Media    │
     │     ↑     │
     │     ↑     │
     └─────↑─────┘
       Drainage Holes
     ───────────────
          Water

Suitable Applications: Bottom watering is suitable for newly germinated seedlings, young seedlings with fragile stems, seed trays with drainage holes, lightweight seed-starting mixes, established seedlings in cell trays, indoor seedling trays, greenhouse seedling trays, propagation trays, nursery trays, and plug trays.

Watering Process

Prepare the Tray: Check that the seedling tray has open drainage holes.
Add Water: Pour a shallow amount of clean water into a larger tray or container.
Place the Seedling Tray: Set the seedling tray into the water so that the drainage holes can contact the water.
Allow Absorption: Let the growing medium absorb water upward through the drainage holes.
Check Moisture: Check whether the growing medium has become evenly moist.
Remove the Tray: Take the seedling tray out of the water once sufficient moisture has been absorbed.
Drain Excess Water: Allow excess water to drain before returning the tray to its growing area.

Top Watering

Definition

Top watering is a method of watering seedlings by applying water directly to the upper surface of the growing medium and allowing it to move downward toward the root zone. Water can be applied with a watering can fitted with a fine rose, spray bottle, squeeze bottle, or another gentle watering tool. When performed correctly, top watering can provide direct control over individual cells and different areas of a seedling tray, but the water should be applied gently to prevent seeds or lightweight growing media from being displaced.

Working Principle

          Water
        ↓ ↓ ↓ ↓ ↓
       ↓  ↓  ↓  ↓
     ┌─────────────┐
     │  Seedlings  │
     │      ↓      │
     │  Growing    │
     │    Media    │
     │      ↓      │
     │      ↓      │
     └──────┬──────┘
        Drainage Holes

Suitable Applications: Top watering is suitable for newly planted seeds, individual seedling cells, small seedling trays, seedlings requiring targeted watering, cells with uneven moisture, seedling trays with easy access from above, indoor seed starting, greenhouse seedling production, propagation trays, and nursery trays.

Watering Process

Prepare the Watering Tool: Select a watering can with a fine rose, spray bottle, squeeze bottle, or gentle nozzle.
Check the Growing Medium: Check the surface and moisture condition of the seedling cells.
Apply Water Gently: Direct a gentle flow of water onto the surface of the growing medium.
Water the Root Zone: Allow the water to move downward through the growing medium.
Avoid Disturbing Seeds: Keep the water pressure low enough to prevent seeds or growing media from being displaced.
Check Drainage: Make sure excess water can leave through the drainage holes.
Stop When Adequately Moist: Stop watering once the growing medium is evenly moist and avoid leaving excess water around the tray.

Bottom Watering vs Top Watering

The main difference between bottom watering and top watering seedlings is how water enters the growing medium. Bottom watering supplies water through the drainage holes and allows the growing medium to wick moisture upward, while top watering applies water directly to the surface and allows it to move downward through the root zone.

Factor	Bottom Watering	Top Watering
Water application	Place the seedling tray in approximately 1–2 inches (2.5–5 cm) of water so moisture enters through the drainage holes.	Apply water directly to the growing medium from above using a fine spray, gentle stream, or fine-nozzle watering tool.
Water movement	Water moves from the bottom upward through the drainage holes and growing medium by capillary action. The medium must have good wicking ability.	Water moves from the surface downward through the growing medium and eventually exits through the drainage holes.
Surface disturbance	Minimal. Because water does not need to be poured directly onto the growing medium, it is less likely to displace lightweight media or uncovered seeds.	Depends on water pressure. A fine mist or gentle stream can minimize disturbance, while a strong stream can wash away seeds or expose roots.
Individual-cell watering	Less targeted. Water enters multiple cells through their drainage holes, so it is difficult to give substantially different amounts to individual cells during one watering cycle.	More targeted. You can direct water toward individual cells that are visibly drier than neighboring cells.
Standing-water control	Do not leave the tray sitting in water for days. After the growing medium has absorbed sufficient moisture, remove the tray and discard remaining drainage water.	Allow excess water to drain through the holes. Do not allow drained water to remain underneath the tray and be continuously reabsorbed.
Drainage requirement	Required. Bottom watering depends on drainage holes because water must enter the container from underneath.	Required for controlled irrigation. Drainage holes allow excess water to leave the growing medium and help prevent continuously saturated conditions.

For either method, the goal is not to keep the growing medium continuously saturated. Utah State University Extension recommends keeping seedling media damp but not perpetually soggy and advises checking moisture based on factors such as tray weight and media color. University of Minnesota Extension also recommends draining excess water from the tray to maintain healthy roots.

Frequently Asked Questions

1. How can I water seedlings without washing away the seeds or growing medium?

Use a fine spray, gentle stream, or watering can fitted with a fine rose rather than pouring a strong stream directly onto the cell. For very small seeds or newly planted trays, bottom watering is another option because the water enters through the drainage holes instead of striking the seedbed from above. If you top water, aim the water at the growing medium rather than directly at the stem or exposed seed. Utah State University Extension lists both spray-bottle watering and bottom watering as methods that can prevent seeds from being accidentally washed away, while the University of Minnesota recommends gentle surface watering during early seed development.

2. How much water should I use when bottom watering a seedling tray?

There is no single volume in milliliters that works for every seedling tray because cell size, tray depth, growing medium, and tray design vary considerably. A more useful measurement is the depth of water surrounding the tray. Utah State University Extension recommends approximately 1–2 inches (2.5–5 cm) of water for sub-irrigating seedling packs or containers. The tray should then be removed once the growing medium has absorbed sufficient moisture. Do not assume that using more water will hydrate the seedlings better; the important result is evenly moist growing media with adequate drainage, not a specific amount of water remaining in the tray.

3. Should I bottom water or top water newly germinated seedlings?

For newly germinated seedlings, either method can work, but the important issue is keeping the seed zone evenly moist without disturbing the young plants. If you top water, use a very gentle stream or fine spray so you do not wash away the growing medium or expose the roots. Bottom watering can reduce disturbance because water enters through the drainage holes instead of hitting the surface directly. However, bottom watering should not be treated as an automatic solution for every tray; the growing medium must be able to wick moisture upward, and excess water should be drained afterward. University of Minnesota Extension recommends gentle surface watering during germination and also describes adding water to the tray so moisture can move upward through the growing medium.

4. How long should seedlings sit in water when bottom watering?

Do not use a long soaking period as a universal rule. The goal is to allow the growing medium to absorb enough water to become evenly moist, then remove the tray from standing water. For reference, the RHS recommends placing a seed tray in approximately 3 cm of water and removing it once the compost has become wet, while specifically advising not to leave plants or seeds standing in water for more than 4 hours. Other extension guidance similarly emphasizes draining excess water rather than leaving containers continuously submerged. For routine tray watering, check the medium rather than assuming that a longer soaking time is better.

5. Why is the bottom of my seedling tray wet while the top is still dry?

This usually means the water has entered the lower part of the growing medium but has not moved evenly through the entire root zone. Check whether the growing medium has become excessively dry before bottom watering, because very dry media may not wick water uniformly. Also check the depth of the cells and the composition of the seed-starting mix. If the upper layer remains dry after bottom watering, you can gently water from the top to re-moisten the surface rather than repeatedly leaving the tray in standing water. University of Minnesota Extension specifically warns that it is possible to mist the surface while the rest of the container remains dry, which is why the moisture of the entire root zone needs to be checked.

Conclusion

Successful seedling watering is less about choosing one method and more about maintaining the right moisture balance. Bottom watering can reduce surface disturbance and moisten the growing medium through drainage holes, while top watering provides more direct control over individual cells when applied gently. The right choice depends on the growing medium, tray drainage, seedling stage, and surrounding conditions.

Whether you are watering seedlings in trays indoors, in a greenhouse, or in a nursery, check the moisture of the root zone rather than following a rigid watering schedule. Keep the medium moist but not continuously saturated, drain excess water after bottom watering, and adjust watering as seedlings grow and their water needs change.