What the photo shows is the stage most growers judge a tray by: seedlings stood up in the cells, leaves just meeting across the row, root visible through the drainage slot at the base of each cell. Getting a tray to that point is not about the tray alone. It is about holding the seedlings in the same cell long enough for the root to fill it, then moving the whole lot onto the hardening bench before they set back in the field.

This is the same multi-cell tray used across our planting tools range, and it is the stage of growth above that decides how the trays are used in practice.

Seedlings raised under glass or film come out soft. The leaf cuticle is thin and the plant has never met wind. The fix is a graduated move over roughly seven to fourteen days: reduce the temperature by two to three degrees each day, cut irrigation slightly so the medium is damp rather than wet, and increase ventilation before any move outdoors. A tray with tall cells makes that easier to manage because the root ball holds together when the tray is carried out and back each day.

The work is usually done in the tray rather than on the bench. Trays are carried out in the morning and back in under cover at night, so the seedlings experience real wind and cooler air at the time of day when the sun is kindest.

Two signs say the tray is ready to plant out. The first is a root mat that holds the medium together when a cell is turned out; the second is a leaf canopy that covers the tray surface so the medium below is shaded. If the medium still crumbles away from the roots, the seedling is not ready, and moving it early costs more than the days saved. If the canopy is already touching across the rows, the seedlings have been held too long and will stretch.

Cell volume is what buys that window. The models in the range run from 32 to 288 cells on the same 540 x 280 mm tray, so the hold time before planting out is chosen by picking the cell count, not by changing the bench.

Propagation practice, including the move from raising to hardening, is covered in FAO horticultural guidance, and the commercial side is reported monthly by Greenhouse Grower. Cell depth and wall thickness are the two figures that matter most: depth sets the root volume and the hold time, wall thickness sets how many seasons the tray survives washing and stacking. Both are listed per model in the table below.

Q: How long should seedlings stay in the tray before planting out?
A: Until the root holds the cell together and the leaves shade the medium. That is roughly four to six weeks for most vegetable crops, longer in larger cells. Holding them past that point stretches the stems.

Q: What is hardening off, in practice?
A: A graduated move over seven to fourteen days: a small drop in temperature each day, less water so the medium is damp not wet, more ventilation, then carrying the trays outdoors by day and back under cover at night.

Q: Does cell size change how I harden the plants?
A: Yes. Larger cells hold more medium and dry more slowly, so the watering step in the sequence is gentler. Smaller cells need closer attention, which is why they are usually kept for shorter holding periods.

Q: Can the same trays be reused after a crop?
A: Yes. Wash and stack them between crops. Service life follows the wall thickness ordered, from 0.5 mm to 1.1 mm, and thicker walls stand up better to repeated handling.

Q: Are these trays only for vegetables?
A: No. The same cells are used for flower plugs, herbs and tree and shrub liners. What changes between crops is the cell count, not the tray footprint.