Your California Black Walnut will not be too picky about how you choose to water it. As such, you can use just about any common watering tool to moisten this plant’s soil. Watering cans, hoses, and even cups will work just fine when it is time to water your California Black Walnut. Regardless of which watering tool you use, you should typically apply the water directly to the soil. In doing so, you should ensure that you moisten all soil areas equally to give all parts of the root system the water it needs. It can help to use filtered water, as tap water can contain particles that are harmful to plants. It is also beneficial to use water that is at or slightly above room temperature, as colder or hotter water can be somewhat shocking to the California Black Walnut. However, the California Black Walnut usually responds well to any kind of water you give it.

For outdoor plants, especially newly planted plants or plant seedlings, they can be prone to lack of watering. Remember that you need to keep watering enough for a few months when the tree is small or just planted. This is because once the roots are established, California Black Walnut can rely on rain most of the time. When your California Black Walnut is planted in pots, overwatering is often more likely to.When you accidentally overwater your California Black Walnut, you should be prepared to remedy the situation immediately. First, you should stop watering your plant right away to minimize the effect of your overwatering. After, you should consider removing your California Black Walnut from its pot to inspect its roots. If you find that none of the roots have developed root rot, it may be permissible to return your plant to its container. If you do discover signs of root rot, then you should trim away any roots that have been affected. You may also want to apply a fungicide to prevent further damage. Lastly, you should repot your California Black Walnut in soil that is well-draining. In the case of an underwatered California Black Walnut, simply water this plant more frequently. Underwatering is often an easy fix. If you underwater, the plant's leaves will tend to droop and dry out and fall off, and the leaves will quickly return to fullness after sufficient watering. Please correct your watering frequency as soon as underwatering occurs.

Most plants that grow naturally outdoors can be allowed to grow normally with rainfall. If your area lacks rainfall, consider giving your plants adequate watering every 2 weeks during the spring and fall. More frequent watering is needed in summer. In winter, when growth becomes slower and plants need less water, water more sparingly. Throughout the winter, you may not give it additional watering at all. If your California Black Walnut is young or newly planted, then you should water more frequently to help it establish, and mature and grow up to have more adaptable and drought tolerant plants. For potted plants, there are two main ways that you can determine how often to water your California Black Walnut. The first way is to set a predetermined watering schedule. If you choose this route, you should plan to water this plant about once every week or once every other week. However, this approach may not always work as it does not consider the unique conditions of the growing environment for your California Black Walnut . Your watering frequency can also change depending on the season. For instance, a predetermined watering schedule will likely not suffice during summer when this plant's water needs are highest. An alternative route is to set your watering frequency based on soil moisture. Typically, it is best to wait until the first two to four inches of soil, usually ⅓ to ½ depth of the pots, have dried out entirely before you give more water.

When it comes time to water your California Black Walnut, you may be surprised to find that this plant does not always need a high volume of water. Instead, if only a few inches of soil have dried since your last watering, you can support healthy growth in the California Black Walnut by giving it about five to ten ounces of water every time you water. You can also decide your water volume based on soil moisture. As mentioned above, you should note how many inches of soil have dried out between waterings. A surefire way to make sure your California Black Walnut gets the moisture it needs is to supply enough water to moisten all the soil layers that became dry since the last time you watered. If more than half of the soil has become dry, you should consider giving more water than usual. In those cases, continue adding water until you see excess water draining from your pot’s drainage holes. If your California Black Walnut is planted in an area that gets plenty of rain outdoors, it may not need additional watering. When the California Black Walnut is young or just getting established, make sure it gets 1-2 inches of rain per week. As it continues to grow and establish, it can survive entirely on rainwater and only when the weather is hot and there is no rainfall at all for 2-3 weeks, then consider giving your California Black Walnut a full watering to prevent them from suffering stress.

Overwatering is a far more common problem for the California Black Walnut, and there are several signs you should look for when this occurs. Generally, an overwatered California Black Walnut will have yellowing leaves and may even drop some leaves. Also, overwatering can cause the overall structure of your plant to shrivel and may also promote root rot. On the other hand, an underwatered California Black Walnut will also begin to wilt. It may also display leaves that are brown or brittle to the touch. Whether you see signs of overwatering or underwatering, you should be prepared to intervene and restore the health of your California Black Walnut.

When the California Black Walnut is very young, such as when it is in a seedling stage, you will need to give it more water than you would if it were at a mature age. During the early stages of this plant’s life, it is important to keep the soil consistently moist to encourage root development. The same is true for any California Black Walnut that you have transplanted to a new growing location. Also, the California Black Walnut can develop showy flowers and fruits when you give them the correct care. If your California Black Walnut is in a flowering or fruiting phase, you will likely need to give a bit more water than you usually would to support these plant structures.

The seasonal changes will affect how often you water your California Black Walnut. Mainly, during the hottest summer months, you will likely need to increase how much you water this plant, especially if it grows in an area that receives ample sunlight. Strong summer sunlight can cause soil to dry out much faster than usual, meaning that you’ll need to water more frequently. By contrast, your California Black Walnut will need much less water during the winter, as it will not be in an active growing phase. During winter, you can get by with watering once every 2 to 3 weeks or sometimes not at all. For those growing this plant indoors, you should be somewhat wary of appliances such as air conditioners, which can cause your plant to dry out more quickly, which also calls for more frequent watering.

In some cases, your California Black Walnut may not need any supplemental watering when it grows outside and will survive on rainwater alone. However, if you live in an area of little to no rain, you should water this plant about every two weeks. If you belong to the group of people who live out of this plant's natural hardiness zone, you should grow it indoors. In an indoor setting, you should monitor your plant's soil as it can dry out more quickly when it is in a container or when it is exposed to HVAC units such as air conditioners. Those drying factors will lead you to water this plant a bit more often than if you grew it outdoors.

For healthy growth, make sure that California Black Walnut receives at least 3–6 hours of sun each day. This is actually a minimum requirement—most plants that can handle part sun can also thrive in full sun, but because they require less light for photosynthesis, they are more flexible than plants that require full sun or part shade.

California Black Walnut does best with exposure to full or part sun. They will perform best with direct morning light, but in summer they need protection from the strong afternoon sun. In temperate environments, too much hot afternoon sun can burn the leaves, damaging the plant's appearance and health.

California Black Walnut planted indoors can easily be damaged by direct sunlight when it's moved outdoors. The best way to prevent sunburns from overexposure is to move pots gradually from a shaded area to a brighter spot, gradually. But even plants that are acclimated to the summer sun can be damaged by extreme heat. In a heatwave, it is important to keep the soil consistently moist so that plants can cope with excessive levels of heat. Moving plants in containers to areas with afternoon shade or erecting a shade cloth over them can protect sensitive California Black Walnut during extreme weather events.

While bright morning sun and some full sun exposure can be highly beneficial for California Black Walnut, the harsh, hot midday sun of summer can be too much to handle. If planted in the ground, the summer sun will usually ramp up slowly enough through the season for California Black Walnut to gradually adapt to its intensity. But a potted plant that has been indoors or in a protected location will often suffer injury when placed suddenly into a location where the direct summer sun reaches it in the hottest part of the day. To protect this plant from the brutal afternoon summer sun, plant or place it in an understory location where it is shaded at midday by taller trees and plants or by a building or landscape feature.

When California Black Walnut receives too little sun, they may become pale green or display drooping, yellow leaves. While some leaf drop is normal, if leaves are dropping but no new ones are growing in to replace them, it is a sign that something is wrong. If California Black Walnut receiving inadequate light does manage to grow, the new growth is often spindly, pale, and prone to insect infestation. Paying attention to these signs and changing the lighting conditions of the plant will make a significant difference.

Tender, new leaves are especially sensitive to sunburn. Bearing this in mind, very young California Black Walnut and when it's in a strong growth phase, such as in late spring and early summer, will be more sensitive to harsh sun and heat than the mature one or those in a more dormant fall growth stage. California Black Walnut fresh from a nursery is also usually not prepared for strong full sunlight and must be introduced to it slowly.

Recently transplanted California Black Walnut will often experience a bit of shock and will need to be cared for carefully, either shaded from bright afternoon sun or placed in a protected area. On very hot days, you may see the leaves of California Black Walnut drooping—this is usually nothing to worry about. Plants will send the water in their leaves down into their roots to protect them from burning. However, if the leaves are still drooping in the evening or the next morning, the plant needs water. Always avoid watering during the hottest times of day, as sunlight can hit wet leaves and scorch them easily. California Black Walnut that has been underwatered will be weaker than that with consistently moist soil. This can leave it with weak roots that are unable to protect the leaves on hot, sunny summer days by diverting water away from the leaves. Care for an underwatered plant by giving it a long, deep watering and then allowing the top two inches of soil to dry out before the next watering. Even if it loses its leaves, if cared for properly it will grow new ones.