The "Potion of Sturdiness" is called the Potion of Hardiness in the English game. To make a high quality one, learn the recipe in Potions 101 on Spring 4, practice it until your best score reaches 70 points (3 stars), then brew it with Twigs and 2 Cut Grass. Every potion you brew from then on comes out High quality. Martha, the hedge blocking the path, will only let you through for a high quality one.

What do you need?
| Requirement | Where to get it |
|---|---|
| The recipe | Potions 101 class at the university, Spring 4 |
| Twigs | Chop dead trees with the Chant of the Woodcutters |
| 2 Cut Grass | Cut grass in your garden with a scythe (Kavita, 65 Gold Petals); see Cut Grass |
| A cauldron | The Potions Lab in the Etchery (always open), a Portable Cauldron (600 Gold Petals at Newt's Eye), or a Potion Station for your shed (350 Gold Petals) |
The class gives you the recipe right away; you'll see "Potion of Hardiness recipe acquired".

Zoddronite's playthrough shows the whole potions class and the first practice round:
How do you get High quality?
Quality comes from your best practice score for the recipe, not from the ingredients. Practicing is free and uses no ingredients, so retry as often as you like.
| Stars | Quality | Score needed |
|---|---|---|
| 1 | Poor | 50 |
| 2 | Average | 60 |
| 3 | High | 70 |
| 4 | Exceptional | 80 |
| 5 | Legendary | 95 |
Open a cauldron and press Practice
Pick Potion of Hardiness. The practice round lasts 60 seconds.
Fill cauldrons with the right ingredients
Each finished cauldron is worth 5 points, so 70 points means 14 cauldrons in 60 seconds. With all three Potionwork Proficiency abilities you need fewer.

Use the side bowl carefully
Park ingredients you don't need yet in the side bowl (right-click to switch bowls). Every ingredient still in it when the round ends costs you a point.

Brew
Once your best score is 70 or more, press Brew. It's instant: the Twigs and Cut Grass disappear and a High quality potion lands in your inventory.
Struggling to hit 70? Turn on Potion Mastery Help in the options. Each step (Slight, Extra, Double Extra) lowers every target by another 20%, and your stars are recalculated from your best score, so potions you brew afterwards can come out at a higher quality straight away.
Where do you use it?
Give it to Martha, the hedge in the Sentinel Shrub who won't let you pass. A low quality one won't do; the villagers warn you not to "even think about getting through with that low quality stuff."

In the garden, pour it on a crop so the plot stays wet overnight. Higher quality keeps it wet longer: Poor gives a 50% chance of one night, and each quality step adds half a night. See How to Plant Seeds for watering.
For other recipes and cooking, see Potions and Cooking.