Find the kind of land you'll be on, and you'll know the rule. This anchors every section below.
Take nothing
State & city parks, preserves
In a Texas state park it's illegal to pick, cut, dig, or remove any plant, rock, fossil, antler, or artifact - take only photos. Collecting is a Class C misdemeanor (a $25-$500 fine). City parks and nature preserves follow the same 'leave it' rule.
Mostly take nothing
National parks & Padre Island Seashore
National parks (Big Bend, Guadalupe Mountains) ban collecting plants, rocks, and artifacts. The friendly exception: at Padre Island National Seashore you may keep up to a 1-gallon container of empty shells and sea beans per person (no commercial collecting, and only those two items).
Foraging allowed
National forests
The big flip. On the four national forests (Sam Houston, Davy Crockett, Angelina, Sabine), personal-use gathering IS allowed - mushrooms, hobby amounts of rocks and petrified wood (never to sell), and some fruit or nuts. Quantities vary by ranger district, so check with the district first. At Big Thicket National Preserve you may gather up to a 1-quart container of certain fruits, nuts, and berries.
Only with permission
Private land
You may collect whatever the landowner agrees to. Without permission, foraging is legally theft and trespassing. Since Texas is about 95% private land, this is the usual path - just ask first. Many landowners are happy to let you pull 'weeds' like dewberries.
Limited
Roadsides & rights-of-way
No Texas law clearly grants a right to pick along roads, and none clearly forbids it. The practical rule: pick above-ground parts only (never dig), don't step onto adjoining private land, don't block traffic, and never on Interstates (stopping there is for emergencies). Skip roadsides that may be sprayed.
Public, with rules
Rivers & beaches
The beds of navigable rivers and the Gulf beaches are public, but collecting rules still apply (see The Beach). Don't assume 'public water' means 'take anything.'