Thank you for reaching out. The answer is dependent on the specific circumstances of your situation. Under Colorado law, your landlord/management company cannot legally withhold any portion of your security deposit for work that has not been completed after you have moved out unless they meet specific statutory requirements.
According to the Colorado Security Deposits Act, a landlord must provide a written statement listing the exact reasons for retaining any portion of the security deposit, and this statement must be delivered within one month after the termination of the lease or surrender of the premises, unless the lease specifies a longer period -- up to 60 days. The landlord must also return the remaining deposit balance at the same time. If your landlord fails to provide this written statement within the required timeframe, they forfeit their right to withhold any portion of the deposit. C.R.S. 38-12-103.
Furthermore, your landlord can only retain money from your security deposit for specific reasons. These include nonpayment of rent, utility charges, or necessary repair work for damages that exceed "normal wear and tear". "Normal wear and tear" means deterioration, damage, or uncleanliness that occurs through ordinary use of a rental unit or mobile home space, without negligence, accident, abuse, or misuse by the tenant, homeowner, household members, or their guests.
To find your answer, you will need to first look at your lease. If there was a provision requiring you to maintain the lawn and you did not, your landlord may be entitled to the amount needed to replace it as long as they have complied with the written notice requirement. Additionally, if any damage or defective condition preexisted the tenancy, a landlord cannot retain a deposit to cover that preexisting damage or defective condition. I recommend speaking with a local landlord-tenant attorney to discuss your options.