He was twice captured by Confederate forces, and twice exchanged. A bullet once pierced his boot and he had his horse shot from under him. On 2 June 1864 he was commissioned as lieutenant colonel of the 56th Massachusetts Regiment and was promoted to colonel in command of the regiment on 31 May 1864. He was mustered out of the Army on 12 July 1865. On January 13, 1866, Weld was nominated by President Andrew Johnson to be breveted (i.e. an honorary promotion) to the rank of brevet brigadier general, U.S. Volunteers, to rank from March 13, 1865 (when he was 23 years old), for gallant and meritorious services. The U.S. Senate confirmed the brevet on March 12, 1866. Even today, Weld family members remember him as "the General." In 1866 he was elected a member of the Ancient and Honorable Artillery Company of Massachusetts. He was also a member of the Military Order of the Loyal Legion of the United States. After the Civil War, Weld returned home to misfortune. In 1867, his father died suddenly, prompting his uncle William Fletcher Weld to erect Harvard's Weld Hall in his honor. Soon after, a felting mill in which he had inherited about $14,000 failed. He made a new start by borrowing $25,000 from friends to buy a cotton mill that was connected with the first venture. That mill, however, was swept away by a flood when a pond two miles (3 km) away broke its dam. To pay his debts, Weld became a cotton broker and eventually built a fortune. Despite setbacks such as the embezzlement of $326,000 by a friend and business partner, the downtown Boston firm of "S.M. Weld & Company" prospered and opened branches in India and Japan. This made "the General' another very wealthy Weld. Weld had a great interest in botany and became president of the Massachusetts Horticultural Society in 1906; he also helped Charles Sprague Sargent build the collection at the Arnold Arboretum. But his greatest horticultural achievement was his own estate: Rockweld in Dedham, Massachusetts. Weld lived on a estate he named "Rockweld" in Dedham, most of which was surrounded by a high stone wall. He created one of the finest rock gardens in the country and employed eight gardeners. His estate had a water tower and a dozen greenhouses, one of which was dedicated to growing grapes year round.