I won't reiterate what the other professionals have said. What I will recommend is to get another appraisal if you aren't satisfied with what you got. I've two friends in the past 3 months that got appraisals that were $5,000 less than what they "needed" it to be. In both circumstances the followup appraisal worked for them. If your appraisal is off by a lot, I don't think the landscaping would make up the difference anyway, but if you are close to where you need to be, try another company. Email me if you want me to get the mortgage company name.
Best of luck! Hopefully it all works out.
You are correct to have concerns about an appraisal being done in the winter but not necessarily because of the yard issue. Often times appraisers aren't able to access sold comps as easily if ice and snow are a factor as they would be today.
The fact is that many pieces go into the appraisal process the most important being the comparable sales.
If you have spring/summer/fall photos that don't contain snow and your yard is unusually valuable provide them to the appraiser.
If you review the appraisal and feel as though there isn't enough supporting data meaning photos of the sold comps aren't included then contact the appraiser and voice your feelings.
After reviewing many appraisals I can tell you confidently that the yard isn't the most important piece.Size, location,amenity,condition,age and any defects hold more weight than the quality of yard.
Best of luck!
Tierra Antigua Realty