Looking at the market's tremendous bull run since late March, it might be hard to believe we are still in the middle of a pandemic and recession. Investors can remain defensive by sticking to companies with recession-proof business models to perform well in this tough economic environment.

Here's why Costco Wholesale (NASDAQ:COST) and Dollar General (NYSE:DG) fit the bill and could potentially make great buys in September.

a pile of $100 bills

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Costco Wholesale: A defensive business model 

With a price-to-earnings (P/E) multiple of 41, Costco is expensive from a valuation perspective. But investors get what they pay for. The discount wholesale retailer's stock has jumped almost 18% this year through Wednesday's close, and it looks poised for continued success because of the company's defensive business model and rapidly expanding online footprint.

Costco uses its ultra-streamlined operations to pass cost savings on to customers, who must pay an annual membership fee to shop at its warehouses. The company aims to undercut competitors' prices by selling items in bulk -- making the warehouses an attractive place to shop when money is tight. Costco deserves its rich valuation because of how well suited this business model is to this uncertain economic environment.

Costco reported earnings for its fiscal third quarter on June 3, and the results demonstrate compelling, pandemic-proof growth. Net sales jumped 7.3% to $36.45 billion while operating income increased 5% to $1.16 billion. The company did see net income fall by 8% to $906 million -- due partially to weakness in its travel booking business. But overall, coronavirus pandemic has created an opportunity for Costco to drive expansion in its fast-growing online platform.

Costco's e-commerce sales rose 101.6% in August as consumers elected to shop online to reduce their risk of COVID-19 infection.

Dollar General: Strong, recession-proof growth 

Dollar General is the perfect stock for investors who want to try a safe consumer staple company with respectable sales growth. Shares in the discount retail chain have risen almost 27% year to date and look poised for continued market-beating performance thanks to the company's strong financials and recession-proof business model. Here's why.

The U.S. economy is in tough shape: GDP plunged 31.7% in the second quarter, and unemployment still stands at 8.4%, according to August data. In times like these, consumers are more likely to turn to discount retailers like Dollar General to buy essentials like food, soap, and household supplies. The company's fiscal second-quarter earnings reflect strong demand for its products in this uncertain economic environment. Net sales jumped 24% to $8.7 billion, driven by strong comparable sales and the opening of 500 new stores in the first half the year.

Management plans to drive continued top-line growth through several strategic initiatives, including DG Pickup and DG Fresh -- a shift toward self-distribution of frozen and refrigerated goods expected to boost sales and improve margins in this important consumables category.

Dollar General generated a net income of $787.6 million in the second quarter of fiscal 2021 (up 85% from the prior-year period). And earnings per share (EPS) has grown at a compound annual growth rate (CAGR) of 11% since fiscal 2015 to stand at $6.68 in fiscal 2020 and $9.25 over the trailing 12 months. The company trades at a P/E multiple of 21.5, which is a little below the S&P 500 average.