The short version: Sow two or three seeds at each station, and when the seedlings have their first true leaf, keep the strongest and cut the others off at soil level with scissors. Do not pull them, because the roots tangle and pulling one disturbs the one you are keeping. At this spacing one plant with room beats two sharing it.
Why sow more than you need
Zucchini, pumpkin and cucumber seed is large and usually germinates well, but a seed can rot in cold wet soil, get eaten, or come up weak. Sowing two or three at each spot and choosing the best is cheaper than a gap and a resow two weeks later. Sow about 2 cm deep. The spacing is between stations, with rows about the same apart, for one plant per station: roughly 60 to 90 cm for zucchini and cucumber, and 1 to 2 metres for pumpkin, wider for the big trailing types.
How to thin
- Wait until the seedlings have one true leaf, the rough, lobed leaf that follows the two smooth seed leaves, usually a week or two after they come up. By then you can see which is strongest, and the keeper has settled in.
- Choose the one to keep: the thickest stem, the largest true leaf, and standing upright, not the tallest and thinnest. If two look the same, keep the one nearer the centre of the station.
- Snip the others at soil level with clean scissors or snips. The cut stump dies back and the roots rot away without disturbing the keeper.
- Water the keeper in afterwards. If cutworms have felled the others, fit a cardboard collar about 10 cm tall, pushed 3 cm into the soil, around the keeper; a collar does not stop snails, so check for them separately.
Do this in the evening or on a cool day, and before the seedlings start to crowd and lean. A week too late is better than not at all.
Why one beats two
Two cucurbits in one spot share the same root zone and the same light. Each tends to grow smaller, the leaves shade each other, air moves less between them, and the still, humid air inside a crowded canopy is where powdery mildew gets its start. Pumpkins left as a pair compete for water and food and tend to crop less, and a zucchini pair makes the picking harder. A spare seedling can fill a station where nothing came up: lift it with a spoon and a ball of soil at the seed-leaf stage, before the roots tangle, and expect it to sulk. For the one you keep, a clean cut and some room is the kindest thing you can do.
Two or three seeds, one survivor, and a plant with room to itself.
Related reading
Sow two or three to a station from the zucchini seeds collection.