At the time of such hearing as provided in RCW 17.04.210, or at such time to which the same may be continued or adjourned, the board of directors shall proceed to examine expenses incurred by the weed inspector in controlling weeds on the parcel of land in question, and shall hear such testimony of such other persons who may have legal interest in the proceedings, and shall enter an order upon its minutes as to what amount, if any, is properly chargeable against the lands for weed control. Cost of serving, mailing and posting shall be added to any amount so found to be due and shall be considered part of the cost of weed control on the land in question. The amount so charged by the directors shall be a tax on the land on which said work was done after the expiration of ten days from the date of entry of said order, unless an appeal be taken as in this chapter provided, in which event the same shall become a tax at the time the amount to be paid shall be determined by the court; and the county treasurer shall enter the same on the tax rolls against the land for the current year and collect it, together with penalty and interest, as other taxes are collected, and when so collected the same shall be paid into the fund for such weed district: PROVIDED, That a failure to serve, mail or post any of the notices or statements provided for in this chapter, shall not invalidate said tax, but in case of such failure the lien of such tax shall be subordinate and inferior to the interests of any mortgagee to whom notice has not been given in accordance with the provisions of this chapter.