When laying out your plants in your garden plan, you should lock any plants that will remain in your garden for longer than one growing season, including perennials. When you create a garden plan for the next growing season, you have the option to copy any locked plants from your current garden into your new garden. So if you've locked plants in your current garden, they will remain in the same place in your PLAN in the new garden your are creating, and those plants will retain their original planting dates in your garden JOURNAL.
Also, if you're using the Recommend Plan feature, you may want to lock certain plants that are already planted before having Smart Gardener recommend a layout for your plants, so that Smart Gardener knows not to layout another plant where your locked plant is.
Here are the steps for locking plants in your Smart Garden Plan:
1. Go to the PLAN page (located on the main menu).
2. Click on the Recommend Plan layer on the left side of the plan (the layer will turn orange and open to reveal the Lock button).
3. Select the plant or group of plants that you want to lock with your cursor. You can select multiple plants or multiple groups of plants by clicking and dragging your cursor over the entire area of those plants.
4. Click the green Lock icon 
to lock the selected plants, and the lock will turn orange to indicate that the plant is locked.