Seeds, live plants, cut flowers, and especially soil are among the most tightly controlled things you can put in luggage anywhere in the world, because they can carry pests and plant diseases. Soil is outright banned in many countries, and plants with roots almost always need permits.
Where import is allowed at all, you generally need a phytosanitary certificate issued by the plant health authority of the exporting country, sometimes an import permit obtained in advance, and the items must be declared for inspection on arrival. Commercially packaged seeds are treated more leniently in some places, but plenty of countries prohibit even those without paperwork.
Honestly, unless you have a strong reason and time to do the permits properly, it's simpler to leave plant material out of your bags. If you must bring something, declare it every time. Undeclared plant items are a classic trigger for fines, and biosecurity-strict countries take it very seriously. Check the destination's agriculture department website for specifics.