Re: ほど, X ほど Y だ is a frequent idiom to make a parallelism, meaning “the more X is, the more (or less) Y is”. As you explained, ほど (or 程 in Kanji) means “degree” or “extent”. If X increases, Y proportionately increases (or decreases). That’s the function of ほど in this phrase.
Re: もの (物 = things; ones), it is quite straightforward. There are a variety of plants in the world. Among them. the speaker picks up the ones that have low values/qualities/performance = 価値が低いもの.
If you assess the values of all plants in the world on a 1-10 scale, you will see a certain negative correlation between their values and growth speed.
X ほど Y だ has several variations such as X すればするほど Y になる.
歳をとればとるほど、人は頑固になる。= The older one gets, the more stubborn he becomes.
母親が叱れば叱るほど、息子は勉強を嫌がるようになった。= The more harshly the mother scolded her son, the more frequently he tried to escape from studying.
声の大きい人ほど、その発言が通りやすくなりがちだ。= The louder one speaks, the more easily the one’s opinion tend to be accepted by others. (lit. If you pick up more people who speak louder, the average acceptant rate of their opinions can be higher.)
==> You can also rephrase this as 人は声が大きくなればなるほど、その発言が通りやすくなりがちだ.
By the way, the original Japanese sentence, particularly 植物は価値が低い and 盛んである part, is understandable but sounds very unnatural. This is another junk imported from Tanaka Corpus… My alternative translation is:
雑草 literally means “a variety of grasses”. 雑草 as a group noun, therefore, refers to “ugly and annoying weeds that are growing everywhere, are not worth to be named individually, and consume our precious time to get rid of them.” We don’t say 価値が低い植物. It’s just a bad collocation.
勢い means “momentum”. 雑草が勢いよく成長する includes 1) not only the growth speed of one stalk of weed is so strong that its length becomes longer, 2) but also the reproduction ability is so strong that they are rampant across a certain area quickly.
生長 and 成長 are homophones, but we use 成長 mainly for human beings and organizations while 生長 is for plants and animals. 成長 means both physical and mental growth while 生長 often refers to physical growth only. However, some people use 成長 for all kinds of growth.
We say 成長が速い, but don’t say 成長が盛んだ. This is also a terrible collocation. Maybe 成長が著しい works (pron. いちじるしい), but this phrase is usually for human beings or organizations.