Without mods I use floor tiles around my garden plot to give me an idea of space. The plants don't need a full tile of space to grow. If you plant walking left, right, or backwards, you can get plants too close together. Usually the plants that are too close will die before you can harvest the ones nearby and give the others more room.

To plant successfully every time, without the plants being crowded, plant walking forward. The plant will stay red until it is far enough away from the previous plant to have enough space to grow. If you try to plant before that it will give you a need cultivated ground error (even though the ground is cultivated) or if you are planting at too sharp an angle you will get the blocked error. To make sure the first plant has enough room to grow start a tile away from the front edge and stop planting when you are a tile away from the back edge or any walls you may have built.

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