Oncidium Orchid Care
Oncidium orchids, which are pronounced on-SID-dee-um, are an extremely varied and large New World genus with over 300 varieties that come from numerous habitats. You may see some that come from tropical areas and others from areas like the Andes mountains. This care guide is a generalized one dealing with the basics of Oncidium care, which means that you may need more specialized information. Thankfully, you can visit the group to ask as many questions as you have!
Humidity
Oncidiums should be fine with a normal greenhouse, or household, humidity level as long as it is between 30-60%. If you cannot provide humidity at those levels, then you can provide your orchid with a pebble tray. This is a tray filled with pebbles and enough water to cover the bottom but not go over the top of the pebbles, so when you place your plant on top it doesn’t soak up the water but evaporation of the water will raise the humidity level in that small area.
Water
This varies depending of numerous factors such as pot size, potting medium, weather, and type of orchid. If it has thinner leaves or thinner roots, it will need watered more often than an orchid that has “fleshier” (a fancy way of saying thicker) leaves or roots. Wait until the potting medium is drying out but not completely dry. When it comes time to water, do so thoroughly. The best way is to place your oncidium in a sink and let water run through the potting medium so it and the roots are fully watered. If you get some water on the leaves, wipe it off as that can cause black spots to form. I’ve seen people suggest taking a pencil, putting it in the potting medium pointy side down a few inches in for a few minutes, then checking to see if the wood is damp or not. If it is damp, the plant doesn’t need watered, but if it is fully dry, then water.
Light
This depends on the species, as some prefer hours of bright light and others basically full sun. You can place these in south, east, or west facing windows — north too, if you give them some supplemental lighting. Remember, their leaves should be a bright almost lime green color, and if they are a dark green then they aren’t getting enough light.
Temperature
Oncidiums like to have a variation between daytime and nighttime temperatures. At night try to give them temps between 55-60° F (12-15° C), and during the day provide them with temperatures ranging from 80-85 F (26-29 C). While they can also withstand temperatures as high as 95-100° F (35-37° C), that is really only if there is really good air flow and much higher humidity levels.
Potting
The best time to re-pot is during the spring (generally speaking) when half of the new growth is mature. You should also keep in mind that these will only need to be re-potted every one or two years, depending on how quickly the potting medium breaks down. If your oncidium has thinner roots, then the potting medium that you should use should be a finer mix, as opposed to a course mix for the thicker rooted varieties. Remove all of the old potting mix from the roots and check them for any signs of illness. Remove any roots that are mushy, brown, or dried out and papery. The roots that are white/tan and firm are normal healthy roots and should be left alone. When placing the plant in the pot, make sure to put the oldest growth as close to the back as possible as to leave plenty of room for the new growth that will appear. Spread the roots out as much as you can, fill in with potting medium making sure to use your hands to firm it so that the plant stays in place.
Oncidiums can be divided when there are at least six pseudobulbs, keeping three per new pot.
Fertilizing
Do this regularly as the plant is actively growing. For the oncidiums that are grown in potting medium give them a good quality 30-10-10 orchid fertilizer twice a month during the summer months, and the same fertilizer but every third week during the winter.
Facts/Troubleshooting
The name oncidium comes from the Greek word for mass or pads, onkos.
Oncidium are what is called sympodial, which means that they grow from a rhizome instead of a single stem — a good example of monopodial orchid would be a Phalaenopsis.
Except for a few exceptions, oncidiums are epiphytes, or a plant that grows on another plant, which is one reason they can tolerate indoor lighting so well — in the wild they get dappled sunlight due to the trees that they grow on.
While they have longer and thinner leaves, you will notice that those come from a thick, kind of short, stalk-like thing. Those are called pseudobulbs, which is where the plant stores moisture. They should be smooth and firm if they are watered well, but some shriveling is also normal. If the pseudobulb is badly shriveled, it needs more water. It is also normal to see some oncidiums grow stacks of pseudobulbs, and roots from these will still grow down into the potting medium. Never trim the live roots from any type of orchid, just make sure to trim off the obviously dead roots — those that are papery or mushy/brown (I know this has been mentioned before, but it really is important).
These like really good air flow, so a closed terrarium that doesn’t have a fan is not a good idea. Also, when watering try not to get water on the leaves, as this can cause black spots to form. Just wipe off the water, but once again, a good air flow will also help prevent black spots on the leaves.
While these do not need to overwinter like some types, they do appreciate a cooler period. During this time, when active growth stops, make sure to water less.
Many types of oncidium produce scented flowers, so they aren’t just beautiful, but can also fill a room with a pleasant scent ranging from chocolate, to vanilla, to citrus.
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