Subtle, but worth a try
Plant material mainly consists of water held in a cell whose wall is strengthened by pectin, with the walls of adjacent cells bonded together by pectin and hemicellulose. Cooking involves softening the cell walls, and breaking down the glue between cells. This occurs across a range of temperatures, and softening progresses with time. Temperature is less important for plant foods, and one temperature almost suits all - 85C. Time is probably more important.
Conventionally, plants are cooked in boiling water (100C) or fried at higher temperatures in butter or oil. Delicate plants are often steamed (which is not dissimilar to boiling because of condensation on the surface of the food). Each approach uses temperatures around or above 100C. Plant cells begin to soften at lower temperatures, typically 85C. Cooking at this lower temperature broadens the time-window for doneness in many cases, and so reduces the importance of time.
A nuisance for cooking plants sous vide is that at temperatures of 85–90C, the bags usually inflate. While these temperatures are below boiling point, they are high enough for dissolved gasses, or gasses trapped in cells, to come out.
A similar phenomenon occurs as a pot of water comes to the boil. As the temperature approaches (but is still below) boiling point, small bubbles of gas can be seen on the bottom of the pot. This gas (air typically) had naturally dissolved in the water when it was cool — the water is not boiling. Gasses escape as temperature increases, which is why a warm beer is flat.
If sous vide bags inflate enough, the food is no longer sufficiently in contact with the water in the bath to cook efficiently. The bags may float.
Some water-baths come with a wire rack to hold bags submerged. Even then, it is a good idea to arrange the bag so that as it inflates the air goes to one end of the bag that is held higher than the end containing the food. Alternatively, use the water displacement method but don’t seal the bag, rather clip it to the side of the container above the waterline so that the food sinks and air can escape. The bag can be weighted with a plate. This problem is greatest when a liquid is added to the bag with the vegetables.
Some random thoughts (not all sous-vide)
Mineral content in the cooking medium will modify the breakdown of chlorophyll, and so is mostly important for green vegetables. Chlorophyll is green only for so long as it holds onto a magnesium ion that sits in the middle of the molecule. Hard water contains a higher than usual concentration of calcium, and this ion displaces magnesium and takes its place, causing a loss of greenness. If cooking in a pot, consider cooking in deionized water (it’s often sold for car batteries in service stations). It tastes awful, because it is the subtle mineral content that makes water agreeable to drink, but it is fine to cook with.
Avoid seasoning deionised water with sea salt (which contains minerals). I think even iodised salt might not be a good idea either, but I am less certain of that. Potassium iodide has been added to table salt since the 1920s, when iodine deficiency (thyroid) was a concern. In modern times this is no longer relevant in developed countries. Many commercial ground salts have anti-caking agents added to promote pouring, and other additives. So, even something that should be simple, salt, has commercial layers of complexity.
Dislodging magnesium, which occurs anyway just by heating, takes a little time. This is why green vegetables are often plunged into ice water to halt cooking.
Plants with water-soluble flavours (carrot, asparagus etc) are usually better cooked in fat/oil, and the converse, also to avoid leeching of nutrients and flavours. Because flavours are complex, it’s not always obvious which cooking medium to use.
Sometimes, it can be a good idea to leech out flavours, such as bitter flavours or even toxic ones (e.g. the cynanide from cassava), or unpleasant sulphorous chemicals such as in cruciferous vegetables (cabbage, brussels sprouts etc). Conventional cooking in water might be a preferred option in that case, or cook sous vide and leave the bag open for a while after cooking to allow flavours to escape.
Acidity can modify colour. It is relevant to vegetables that get their colour from anthocyanins (usually responsible for red-blue coloration). For example, red cabbage will hold its colour better if cooked with a little vinegar.
If the water is slightly alkaline, the breakdown in pectin is accelerated and the vegetable will soften more quickly. Vichy carrots use this approach, as the Vichy mineral water is slightly alkaline.
A further consideration is starch. Plants use starch as their energy source, much as animals use fats for that purpose. Starch is fixed in granules inside the plant cells. Retrogradation is a process in which starch granules are cooked at low temperatures (60–70C) to gelatinize and swell, then cooled to stabilize, then the vegetable is heated to higher temperatures (85–90C) to separate the cells.
Sous-vide suggestions for vegetables
As a sous vide starting point, I suggest trying nearly all vegetables and fruits at 85C. Cooking times for vegetables are usually 45–75m (longer times for harder vegetables such as root vegetables and for thicker portions). For fruits (such as apples and pears) try shorter cooking: 20–30m. The bag can be removed and the doneness of the food tested with a light squeeze.
Unless two water baths are available, there are logistical problems cooking sous vide vegetables and meat proteins for a meal because of the different temperatures. A work around is to individually bag a range of vegetables (e.g. to last a few days), and cook them all at once sous vide at 85C. ‘Shock’ them in an ice water slurry, and store the bags in the fridge until needed. When cooking the meat protein, drop selected vegetables in with the meat to warm through. They won’t cook further because meat cooking temperatures are too low. If the meat had already been bagged also, then a weekday meal can be cooked with next to no effort or mess in the kitchen — the ultimate ‘fast’ slow food.