Seed Trays vs Module Trays: Choosing Cell Size and Depth

Illustration of seedlings in module trays on a bright windowsill

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Choosing between seed trays and module trays depends primarily on the size of the seed, the root sensitivity of the plant, and whether you intend to transplant the seedlings. A seed tray is a shallow container with a base and drainage holes used for sowing many seeds closely together, which are later ‘pricked out’ into individual pots. In contrast, module trays (or cell trays) consist of individual cells that allow each plant to grow its own root system, enabling transplantation with minimal root disturbance.

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Distinguishing seed trays from module trays

The fundamental difference between these two options is the structure of the growing space and the subsequent handling of the plant. Seed trays are designed for the initial germination phase of smaller seeds or crops that can tolerate being moved. Because they lack internal dividers, they are ideal for scattering seeds thinly and evenly over a large surface area.

Module trays provide a separate root space for each seedling or deliberately sown cluster. This is particularly beneficial for plants with delicate root systems that may suffer transplant shock if handled during pricking out. By growing a plant in a module, the root ball remains intact when the seedling is pushed out of the cell and placed into the ground or a larger pot.

Choosing cell size based on seed type

The size of the seed is a primary indicator of which container to use. Smaller seeds, such as those for many herbs or salad leaves, are best suited to shallow seed trays where they can be sown in bulk. Larger seeds require more space and depth to accommodate their initial root growth and the energy reserves stored within the seed.

Small seeds: Shallow seed trays are useful for thin sowings that will later be pricked out; suitable small seeds can also be sown in modules. These are often sown by scattering a pinch of seeds across the surface and covering them with a light layer of sieved compost.
Large seeds: Use modules or individual pots. Examples include sunflowers or broad beans, which benefit from the dedicated space of a cell to prevent overcrowding from the start.
Root-sensitive crops: Individual modules can reduce disturbance where transplanting is suitable. Long-rooted carrots and parsnips are usually sown directly in their final growing position instead.

Determining the required depth

Depth requirements vary significantly based on the intended use of the plant and the growth habit of the species. If a plant is only intended to stay in the container for a short period before being moved, a shallow tray suffices. However, certain crops require deeper environments to support their early development.

For ‘cut and come again’ crops, such as certain lettuces or kale, where the plant remains in the container for a longer period to be harvested repeatedly, a depth of 10–15cm is recommended. For very large seeds that require deep burial, deeper individual containers or cardboard tubes may be more appropriate than a standard shallow tray.

Diagnosis: When to switch from trays to modules

Many gardeners use a two-stage process: starting in a seed tray and moving to modules. You can diagnose the need to move your seedlings based on the following signs:

Overcrowding: When seedlings are so close that their leaves touch, they compete for light and water, which can lead to stunted growth.
Leaf Development: Prick out once seedlings are large enough to handle safely, often around the first true-leaf stage. Do not wait for two pairs of true leaves if a tray is already crowded.
Fungal Risk: If air circulation is inadequate and the soil is too soggy, seedlings in dense seed trays are more susceptible to damping off. Prevent this through spacing, clean equipment and careful watering. Do not transplant collapsed or visibly diseased seedlings as a supposed cure.

Using modules for limited space

For those with limited greenhouse or windowsill space, ‘multiblock sowing’ in module trays is an efficient technique. This involves sowing a cluster of 3–5 seeds in a single cell rather than one. This is particularly effective for root, bulb, or stem vegetables such as leeks, beetroot, or spring onions.

In this method, the seedlings are not thinned out within the cell. Instead, the entire cluster is planted out together at the correct spacing for that vegetable once plants are ready, the roots hold the compost together and outdoor conditions are suitable; harden off indoor-raised plants first. This maximizes the number of plants produced per square centimetre of propagation space.

Summary of selection criteria

To simplify the choice, refer to the following criteria based on your plant’s needs:

Plant Characteristic	Recommended Container	Reasoning
Small seeds / Bulk sowing	Seed Tray	Easier to scatter and sow in quantity.
Large seeds (e.g. Sunflowers)	Module Tray	Prevents early overcrowding.
Delicate root systems	Module Tray	Minimises root disturbance during transplant.
Cut-and-come-again crops	Deep Containers (10-15cm)	Provides space for prolonged growth.
High-volume baby veg	Module Tray (Multiblock)	Efficient use of limited space.

Useful products for this job

The links below open Amazon UK product searches. Check the exact item, seller, current price and compatibility before buying.

24-cell seed module tray: Offers separate root space for crops suited to transplanting; the actual cell volume matters more than the tray’s cell count. Check the depth of the cells to ensure they are sufficient for the specific seed type (e.g., deeper cells for broad beans). Search Amazon for 24-cell seed module tray.
Shallow seed tray: Best for scattering small seeds and starting crops that will be pricked out later. Check drainage holes, overall dimensions and sufficient strength for lifting when filled; drainage alone does not prevent seedling disease. Search Amazon for Shallow seed tray.

Related garden guides

Sources and further guidance

Guidance researched in September 2026. The feature image is an AI-generated illustration. Product guidance is based on published information, not hands-on testing.