Like any plant (or animal for that matter), Tulasi devi needs water in order to survive. It is important that she does not get too little, and just as important that she does not get too much. Too little water will result in stunted growth and wilting, eventually leading to death. Too much water, while not immediately posing a risk, will result in the development of root rot. This is where overwatering allows for fungus and bacteria to grow on the roots and start to cause the roots to decay. Once a plant, and this is especially true of Tulasi devi, develops root rot, there is very little chance of recovery, and even if it can be put under control, eventually root rot will kill the plant. In my personal experience, generally within a month of a Tulasi plant showing signs of root rot she would leave her body. It is possible to slow this, and I have been able to keep a plant going for a year after she got root rot, but generally all efforts in this regard are unsuccessful.
So then, how should one go about watering? The first necessity as mentioned in the "Planting the Seed" page is a water filter. Tap water and hose water are both full of chemicals that the government puts inside for God knows what reason. These chemicals are good neither for humans and animals nor for Tulasi devi or other plants. If Tulasi devi is being kept indoors, a faucet filter will be needed. I don't have any personal experience with these, so personal research needs to be done. If anyone finds any good filters for faucets then please let me know so I can post it for others! Just send a message on the contact form on the bottom of the home page. The hose filter which I use and have found to work well is the "Gard'n Gro" hose filter by Rainshow'r. Each filter lasts for two years and can be replaced by getting a replacement cartridge rather than a completely new filter. You can also get a "Gard'n Gro Prefiltration Kit" to extend the life of the filter, but this is probably only necessary if the water quality is considerably bad.
Once you have your filter, the next step is to water Tulasi devi. The key is to give her a ton of water. Water her enough so that the water is flowing through the bottom of the pot. And then, give her some more water. After that, don't water her again until she starts drying out, usually three days or so. If she is needing water more often than every three days, it is a sign that she needs to be transplanted because her roots are filling the pot. But that is discussed on another page. Be careful with big Tulasi. The top of the soil will dry out a day or so before the soil where her roots are, and this is especially true if she is being kept outside. A good way to judge this is by letting some weeds grow in the pot with her, such as clovers to help indicate when to water. The clovers will wilt before Tulasi devi does, and you can know from that sign that Tulasi devi will need to be watered soon. Of course, overtime one will be able to tell when it is getting to be time to water Tulasi devi even without these signs.
As stated above, there are ways to go about dealing with root rot, which I have seen to be successful at times and not as successful other times. Take these as last resorts- to do these to healthy plants is not recommended.
The first method I have tried with some success is giving Tulasi devi hydrogen peroxide. Of course, hydrogen peroxide is not good for plants or animals because of its corrosive nature. However, if a plant is in the process of dying due to root rot, hydrogen peroxide can slow this down. The bacteria and fungus that cause root rot are anaerobic- they live where there is no or little air. If you add air to the soil, then this slows them down. Hydrogen peroxide breaks down into a water molecule and an oxygen atom. So if this can be gotten into the soil where the roots are before it breaks down, it will kill some of the root rot. I don't have an exact method for doing this, but be sure to dilute the hydrogen peroxide to at most 1.5%, and don't do this more than once a week.
The second method is to transplant Tulasi devi. The important part of this is to break down as much dirt as possible around the roots and to (*cringe*) shave off roots that are damaged from the root rot. Once this has been done, the diluted hydrogen peroxide mentioned above can be poured through the roots. Then, she should be put into completely new soil in a completely new pot. The old soil should be thrown out and the pot thoroughly washed with bleach. The new soil used needs to be highly aerated- lots of perlite and diatomaceous earth. Then, and most importantly, be sure to not overwater her again.
All plants need nutrition as fuel for their growth. They get this from the soil they are in, and initially all soil should have nutrients in it. However, overtime these nutrients become depleted. So, more need to be added to have a healthy plant. If these nutrients are not added, then the plant will have to take nutrients from older leaves to compensate until either more nutrients are added to the soil or the plant dies. As Vaishnavas we don't want any plant to unnecessarily die, especially Tulasi devi, so nutrients have to be monitored and replenished.
When Tulasi devi is put in new soil, the soil should be good enough to last for a few months as far as nutrients are concerned. After that, some plant food is needed. What I have found to work is called "Hasta Gro 6-12-6". This "6-12-6" is a very important aspect of the plant food used. This indicates the ratio between the nitrogen, phosphorus, and potassium. Different plants need different ratios in their plant food; this ratio works well for Tulasi devi. Hasta Gro can be given every three weeks or so during the high growth season, and every five or six weeks during the winter. A sure sign that Tulasi devi needs more nutrients is that leaves will start turning yellow and then falling off. This is what happens when she starts taking the nutrients out of her leaves. If this is seen to be happening regularly, then you can know that she needs nutrients.
Simply fill a large bucket with the amount of water that you would normally use to water Tulasi devi and add 1oz of Hasta Gro per gallon of water. Be sure to not give too much nutrients, as this can cause problems with Tulasi being able to soak up water through her roots. Plants absorb water through a system of passive diffusion. Diffusion is the tendency of things to move from areas of high concentration to areas of lower concentration until an equilibrium is attained. Plants are able to absorb water when the salt concentration in their cells is higher than the concentration in the soil, which causes the water to move up the plant and go into each of the cells to find an equilibrium. If a plant food high in salts is used, then this system will not function. Hasta Gro is very low is salts, and therefore there is not much risk unless a ton of extra is used, but other brands of plant food may cause problems. In the instance that too much salt accumulates in the soil, the solution is to flush the soil with lots of water.