Dendrobium Spectabile - Rotten Roots?

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Older roots on Dens are light brown. Velamen comes off old normal roots when you pull. Velamen is not living tissue much past the green tip. It is a kind of sponge. The more you pull and poke on the roots, the worse off this plant becomes. I am not looking at the original plant, but I suspect it had completely healthy older roots when you got it. The seller is likely accepting a return fearing a bad review.

[Roberta]

You can sort of "comb" the roots with your fingers on the inside part of the plant to remove what is loose, don't worry about the rest. But be kind to the roots on the outside - those coming from the new growth look very good. It is much better to leave a bit of stuff that is on the way out than to damage the good ones by being too aggressive. Losing roots on old pseudobulbs is normal. The new growths are where the good ones develop - and those look fine.

[dianecty]

I really want to keep it. I think it is so healthy and firm and I am so excited, hoping it will flower. Any tips on how to take care of this plant - fertilizer, light - how many footcandles, temperature, how much watering, etc...? It does get hot outside here, sometimes above 100F but my apartment is quite cool.

[estación seca]

I've been to San Jose quite a few times at different times of the year. There are Dendrobiums from all sorts of climates. This Den likes things warm, bright and humid all year. It likes more light than most Dendrobiums, about Cattleya light. It should not go dry but it should not stay soggy wet. It uses a lot of water during the summer. I had trouble keeping it moist in a smallish pot with large-chunk bark. I think it requires more humidity to grow well than many other Dendrobiums seem to need here.

I would think light dappled shade in your climate outdoors would be fine, except for the low humidity. A slat-roof patio with a lot of other plants would probably be fine since the humidity would be higher. If you have a sunny patio, consider putting it under 50% shade cloth. It might be able to take more sun than that, but that would be a good place to start.

How cool is your apartment? Low 70s F / 21-23C year round might not be great for growing this.

I would suggest using the Search function in the top menu to see how people have grown it. Search in the Dendrobium forum. I know it was discussed just a few days ago in this thread:
What Genus are you finding difficult to grow?

[Orchid Whisperer]

FWIW I grow mine in a terracotta orchid pot (big slits on the side), the growing medium is about 10% bark, the other 90% is rock the same size as the bark. The pot is wider than it is tall.

I water 2 or 3 times a week but it drains well, so roots don't get waterlogged. Balanced fertilizer, plus some calcium and magnesium supplement, roughly monthly, more often when actively growing.

Mine gets bright filtered sunlight outdoors: spring, summer and fall. Winter, it is indoors under fluorescent daylight-spectrum lights.

I purchased my plant while it was small, roughly 6 years ago. It is now mature (expect it to be 1.5 to 2+ feet tall at maturity). It will develop flower spikes in the fall that will bloom in late winter to early spring. Mine is a reliable bloomer.