Grow Veg A non-open source, paid desktop/iOS app for hand-designing a garden and planning your crops. Has a very nice looking calendar. Seems to be the best software with garden planning, companioning, rotation, harvesting dates and tasks as of today. So sad It's not free or even better open source.
Tended app (Offline) A non-open source but free and discontinued since January 2020 web app to design your garden, track your activities and harvests, and much more. The visual garden design feature is very enjoyable as It is fluid, easy to use and have precise positionning, beds size at centimeter (different measurements available) scale. There are many other features like record tasks, harvest dates, calendar, reminders, journal, etc. But there's no companioning or rotation feature. It's intended to be a permaculture app. But there are a lot of generic features to take inspiration from.
Crop Planning Open-source Java program to plan crops. Not too well-designed or easily usable. Does not appear to have any data on time until harvest, or take conditions into account, or include any companion crop information. However, it's open-source, so it could theoretically be expanded. At first glance it does not seem like a great base upon which to expand.
Smigo Single page application. Angular front, Spring framework Java back end. Claims to offer advice on companion planting, harvesting dates! No longer hosted online, and have been tried locally, but it definitely does not do what it claims to. For the simple reason It can't be used at It is since the database isn't populated with companion/harvesting dates data. The tables are created but we have to fill all the data by ourselves. So It could be a good idea to incorporate some user experience and interface features from It. A single page application could be a good format for the companion planner due to its cross-platform compatibility. Has 970 commits by a single person so a lot of work has gone into it. But the very complex code base and software design quality isn't worth the time spent learning someone else's code/system.
Garden Planner Open-source planting calendar, this time using Meteor.js. Much more simple, but maybe this means easier to build upon. No companion features but they could be created. After tests, seems to be a very simple interactive calendar (basic legend info on hover) with a search bar and a basic favorites feature who allow you to pin vegetables on the first rows.
Kitchen Garden Aid Gamified plot planner in Java. Has extensive companion data.
List of companion plants Wikipedia's list of companion plants. Quite comprehensive, covers some things not in the database yet. Even if you don't want to trust a wikipedia article, it still has links to 50+ useful sources.
https://waldenlabs.com/the-ultimate-companion-planting-guide-chart An extensive, well-structured list with plant companions.
Companion Plant Spacing Research done by Kentucky State University on the spacing of intermixed companion plants in a hexagonal formation. The gist is that the spacing is a weighted average of the recommended spacing of the two crops based on the proportions of the crops.
Paper by M.K. Bomford Further development on the KYSU research above. Has a few key conclusions, including:
https://github.com/ehsankhoshkhoy/intercropping.pe Little-documented script to calculate intercropping spacings.