If you plant side to side or walking backwards you can get your plants too close together and they will wither because they don't have enough space BUT if you plant walking forward the plant in your "hand" stays red (just like it is in your screen shot) until there is enough space between it and the previous plant (provided you don't deselect the cultivator). If you try to plant when its still red you will get a "need cultivated soil" error (even thought the soil is cultivated). As long as you keep the plants in a straight row (you can use wooden floor tiles or stone tiles on the edge of your garden as a guide) you can plant extremely long rows.

You can then use the distance between the plants you planted walking forward as an estimate of how far away to plant your next row. Carrots, turnips, and onions all need about .7 meters of space to be healthy.
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