I talked to my neighbor, and visited the Kampheng Phet Road area today.

According to my neighbor who, depending on incoming orders, visits this market each week, there is a wholesale operation meant for stores and shops around the city. This wholesale portion of the market does operate roughly on the hours referenced, overnight Tuesday through Wednesday. This is not unusual for wholesale markets as it makes transport a bit easier at off-hours. She buys on the spot and has a van for transport, but she also can request special deliveries from wholesale suppliers, which get delivered in boxes teetering off the back of motorbike taxies, or Vespas.

Most wholesale operations also have retail shops, both inside and outside the market complex. Most of these retail shops inside are open Wed. and Thu. There are dozens of shops outside which are open every day, all were open today (Friday).

There are also dozens of retail shops both in this complex, some of which are open 7 days a week at varying hours, and all along Kampheng Phet Road (south side, use exit 3 from MRT station at Kampheng Road), hook a left u-turn at the top of the escalator and walk west on Kampheng Phet road. From the flyover to the over-street walkway, ~ 1 Km, there are dozens of shops, most also have large seed displays. There is also a massive complex called the Siam Orchid Center which has a lot of seeds and supplies. You can use Google Maps and Streetview to get a feel for the many, many, many, many retail shops, selling all sorts of seeds, plants, supplies, planters, etc.

Also, be sure to visit Or Tor Kor which is an impressive fresh/wet market, with ready to eat and sit-down options as well.

Finally, a portion of the Chatuchuk Weekend Market is dedicated to gardening, and this portion is close to the same MRT station, use exit 2. See the map below, the sections in the lower left in brown...