If you're wanting to set individual tax rates for specific products, then tax overrides can be used. You can create a manual collection that includes the products you want the tax rate to apply to and then apply the override to that collection. However, this cannot be done at the variant-level as the override will apply to product and all of its variants by default.
If you're needing to charge different tax rates on your plants and seeds, then they will need to be listed as individual products rather than different variants within the same product.
If you're wanting customers to view the plants and seeds together, then I'd recommend arranging them so they appear next to each other in the collections they both feature in for increased visibility. You can also add a recommended products section to your product pages, either through some edits to your theme's code—as explained here on the Shopify Blog—or through an app, such as Frequently Bought Together PRO or Also Bought - Related Products. Adding this to your store will allow you to recommend customers also buy the seeds while browsing the product page for the plant, and vice-versa.
I hope this explains things but please let me know if you've any further questions about this.
Victor | Social Care @ Shopify - Was my reply helpful? Click Like to let me know! - Was your question answered? Mark it as an Accepted Solution - To learn more visit the Shopify Help Center or the Shopify Blog