Monsters
Botanical Lion (1)
Copy Plant (2)
Gigantic Cephalotus (1)
Gigaplant (3)
Lonefire Blossom (3)
Lord Poison (3)
Nettles (3)
Twilight Rose Knight (1)
Tytannial, Princess of Camellias (1)
Violet Witch (2)
-1 Copy Plant
-1 Lonefire Blossom
-3 Nettles
-2 Violet Witch
+1 Dandylion
+1 Morphing Jar
+1 Sangan
+1 Tytannial, Princess of Camellias
+X Botanical Lion / Gigantic Cephalotus
Copy Plant is useless outside of your small window of Synchros and it's easily searchable and revivable with a Plant-type backbone. Lonefire Blossom is semi-limited to 2. Nettles is near useless in this type of deck (and Copy Plant is much more versatile). Violet Witch won't really benefit you because it's a Spellcaster and the things you'll be searching for will 100% of the time be Lonefire Blossom anyway (or it should be); I replaced them with Sangan and Morphing Jar. Dandylion is a phenomenal card (especially with Tytannial). Tytannial is always good in twos.
Other monsters you may consider: UFO Turtle, Naturia Rosewhip
Synchro Monsters
Black Rose Dragon (1)
Queen of Thorns (1)
+1 Brionac, Dragon of the Ice Barrier
+1 Goyo Guardian
+1 Thought Ruler Archfiend
+1 Red Archfiend Dragon
+1 Stardust Dragon
(just fill this up to 15 at least)
Spells
Fragrance Storm (2)
Heavy Storm (3)
Lightning Vortex (1)
Mark of the Rose (3)
Miracle Fertilizer (1)
Mystical Space Typhoon (3)
Smashing Ground (2)
Swords of Revealing Light (1)
The World Tree (1)
-2 Fragrance Storm
-2 Heavy Storm
-3 Mark of the Rose
-2 Mystical Space Typhoon
+1 Giant Trunade
+1 Miracle Fertalizer
Traps
Bottomless Trap Hole (1)
Call of the Haunted (1)
Dark Bribe (1)
Mirror Force (1)
Plant Food Chain (3)
Pollinosis (1)
Solemn Judgment (1)
Wall of Thorns (2)
Wall of Thorns isn't really necessary in any capacity, but I won't say to remove it. You may want to run additional copies of Pollinosis, though.
I didn't count the cards, so the suggestions I made may drop you below 40; you'll just need to fill those up :3
You may also want to side-deck Imperial Iron Wall.
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For the record, I run this. You may like it better since it feels a lot more beatdown than a normal plant deck: