I think this race is mostly balanced, but I have a few suggestions. Let's take each trait in turn from your stat block.

Ability Score Improvement: +2 Wis, +1 Con, +1 Str, -1 Dex

+2 to Wis and +1 to Con and Str would be balanced without needing the -1 Dex to counter-balance it, if we compare with Half-Elves (PHB, p. 39), who get +2 to Cha and +1 to two other abilities (thanks to András for reminding me).

Most other races have +2 and +1 to something (with the exception of Mountain Dwarves), and Triton (from Volo's Guide to Monsters, see p. 117), who has +1 to three different stats, so it's worth bearing in mind that this does make the race a little stronger, but I don't think the rest of the race's traits would make this overpowered.

Speed: 25 feet

Ok, slower than most other medium races, but the same as most small races; namely, Gnomes, Halflings and Dwarves all use this speed, so this is already something that exists, although it does make the race slightly weaker, but not by much.

I'm ignoring Age and Height, although I hope 8 feet means "as big as a Medium creature can get" (like a Goliath) because there are currently no Large playable races; in fact, so far the designers seem to have gone out of their way to avoid Large races, such as the playable Centaur and Minotaur races in Guildmaster's Guide to Ravnica being Medium, despite their monster counterparts (i.e. from the Monster Manual) being Large.

Natural Armour: You have natural armour which is equal to 10 + your proficiency bonus + your Constitution modifier.

There are a few other races that have natural armour traits, namely the Lizardfolk (Volo's, p. 113) or Loxodon (GGtR, p. 18), but they do not include the proficiency bonus. The Warforged from Wayfinder's Guide to Eberron have AC calculations that do include proficiency, so there is a precedent for it, but this has been changed in Eberron: Rising from the Last War to not include proficiency bonus, implying that it wasn't balanced after all. It might be best to go with the Loxodon's calculation, which is a flat 12 + your Constitution modifier.

Accustomed to the Forest: You gain advantage on all nature, survival checks concerning things in the forest.

On reflection, this might be quite overpowered, certainly at lower levels; it's more typical to give proficiency in a skill or two instead of advantage with those skills, given that advantage is equivalent to +5 (proficiency at lower levels is only +2, +4 with double proficiency, so having advantage instead is a bit much).

Spellcasting: you are able to cast the following spells:

• You learn the cantrip thorn whip.

• At 3rd level, you may cast entangle once per day.

• At 5th level, you may cast speak with plants once per day.

This seems fine, although I assume this is using Wisdom as the spellcasting modifier? It's best to state that. The only one that may be atypical is that you can cast a 3rd level spell, speak with plants, but it's not a combat spell, and it is something that a full-spellcaster (i.e. a druid) could cast at 5th level, so I don't think this will be a problem or needs to be changed. Triton have already set a precedent for that in VGtM (they can cast wall of water at 5th level, which is a 3rd level spell, but it's not that strong-a-spell). It is otherwise in line with other races with innate spellcasting such as Tieflings (PHB, p. 43) or Drow (PHB, p. 24).

Languages: You know Common, Sylvan, and one other language of your choice.

Three languages? Most races only learn two, although there are some exceptions, such as the half elf (PHB, p. 39); high elves (PHB, p. 24) also gain an extra language. Overall, I don't think there's anything that makes this race overpowered, so I don't see an extra language tipping the balance.

All in all, this has some balance issues, but I think can be easily balanced with my recommended suggestions such as to the Natural Armour and Accustomed to the Forest traits. That said, it is on the stronger side of balanced, so adding anything else would very likely overpower it.