Well, that certainly makes things a lot more difficult to only have a 7-8 month work season. Here we are able to work 12 months out of the year. All our maintenance customers pay us the same flat rate, all year long. It would be tough to have to just shut down operations for 4-5 months.
But I do have a few ideas. These are things I'd do if I were in your position:
1) Get your clients on autopay. You need to have a reliable, predictable source of income. It's not that difficult to do but it does take some time to get all your customers on board with autopay. I have it down pat. We've been doing it for over 10 years now. Now we never get a late payer and we always know exactly how much money from maintenance is coming in every week. Furthermore, we never have to mail invoices. PM me with your email address and we can discuss it more via email. But this will help solve a lot of your cash-flow problems.
2) Offer a 5% discount on the entire season if the client pre-pays for the whole season by a certain date (say, April 1st) And you send those letters out Feb. 28th. So they arrive in their mailbox March 1st. This will help get the season started a little better. We find about 5% of our customers choose to pre-pay. In your case, that would only equate to 2 customers. But still, that's an influx of money that would help.
3) Get into sprinkler system blow-outs. Every state differs on what kid of license is required to do sprinkler work. But if you can get licensed to do irrigation work (if a license is even needed) doing winterizations/blow-outs can be BIG money. It can help you make a ton of money right before winter. It takes several years to build up a lot of customers for this. But these days we bring in about $50,000 in one month in just winterization income. I know guys that bring in 10x that amount.
4) Figure out a way to make more money during the winter months with snow plowing. Rather than just getting the occasional call for snow plowing, get contracts! Spend some time in the snow plow forum and figure out how those guys do it. We don't get much snow these parts. So it's not an option for me. But if we did, I'd be all over it. If you're set up right, you can make some really good money and be working 24 hours a day (between you and your other workers) plowing snow.
5) Consider advertising/marketing more so that you can charge more. That way you can capitalize on the growing season even better. Most people think the amount you charge is totally dependent on what the going rate is from your competitors. That's not really true. That's only part of the equation. What your price really depends on is supply and demand. The more people who are demanding your services, the more you can charge. Let's say you're getting 20 new calls every week. And let's say you raise your rates 25% over what they are now. You may turn off 80% of those you give a bid to with those kind of rates. But if the other 20% say yes, now you're making 25% more money! With the increase in income you can afford the additional advertising/marketing plus save more money for the winter.