Plant is pronounced with a final /t/, and planned is pronounced with something between /t/ and /d/, but usually closer to /d/. Note also that in British English the middle vowel is pronounced differently.
When a final -ed follows an unvoiced consonant, as in looked, laughed and helped, the -ed becomes /t/.
When it follows a voiced consonant, as in longed, loved and barbed, the -ed is pronounced /d/, but it is quite a soft /d/: heading toward a /t/ compared to an initial /d/, as in dark. To a native English speaker, it is seems perfectly clear as a /d/, because that's the way all native English speakers say it, but to English learners, it might sound more like a /t/.
When a final voiced consonant is completely converted to an unvoiced consonant, this is called final obstruent devoicing. It is well documented in other languages. In German, the word Bild (photograph) is always pronounced /bilt/, and the word Zug (train) is pronounced /tsuk/.
Because it is only slight in English, most native English speakers don't notice it.