Will God Heal My Plants?

Yes.

Righteousness has been imputed to you, so every blessing that pertained to the person who kept the whole Law under the Old Covenant belongs to you.  The fruit of your ground is declared blessed.

Deuteronomy 28:4:
Blessed shall be the fruit of thy body, and the fruit of thy ground, and the fruit of thy cattle, the increase of thy kine, and the flocks of thy sheep.

Deuteronomy 28:11:
And the LORD shall make thee plenteous in goods, in the fruit of thy body, and in the fruit of thy cattle, and in the fruit of thy ground, in the land which the LORD sware unto thy fathers to give thee.

Another way to look at it is that Christ has redeemed you from the curse of the Law (Galatians 3:13), which includes the following:

Deuteronomy 28:18:
Cursed shall be the fruit of thy body, and the fruit of thy land, the increase of thy kine, and the flocks of thy sheep.

Deuteronomy 28:38-40:
Thou shalt carry much seed out into the field, and shalt gather but little in; for the locust shall consume it.
Thou shalt plant vineyards, and dress them, but shalt neither drink of the wine, nor gather the grapes; for the worms shall eat them.
Thou shalt have olive trees throughout all thy coasts, but thou shalt not anoint thyself with the oil; for thine olive shall cast his fruit. “You shall carry much seed out to the field but gather little in, for the locust shall consume it.

Deuteronomy 28:42:
All thy trees and fruit of thy land shall the locust consume.

God blessed Obed-Edom and all that he had, which would include any plants he had, because the same presence of God that is in you was in the Ark of the Covenant in his house (1 Chronicles 13:14).  You should expect all that you have to be blessed.

This doesn’t mean that the annuals you bought this year will last multiple years, though I admit to being cheap with my annuals; I’ve kept some in my basement in cold weather for years and brought them out again when the weather warmed up in the spring.